Pressure Temperature Phase Diagram of LiA1H4

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Pressure-Temperature Phase Diagram of LiAlH

4

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science

in Materials Science and Engineering

by

Juan Carlos Fallas Chinchilla

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We recommend that the thesis prepared under our supervision by

JUAN CARLOS FALLAS CHINCHILLA

entitled

Pressure-Temperature Phase Diagram of LiAlH4

be accepted in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

Dhanesh Chandra, Advisor

Qizhen Li, Committee Member

Aaron Covington, Graduate School Representative

Marsha H. Read, Ph. D., Associate Dean, Graduate School

May, 2009

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Abstract

High pressure behavior of complex hydrides is investigated in this thesis. Raman spectroscopy was performed at different temperatures and pressures to determine structural changes in LiAlH4.In situ high-pressure/high-temperature Raman spectroscopy

experiments were carried out using resistively heated diamond anvil cells up to 150°C and 7 GPa. High pressure experiments performed at room temperature showed transformation of the monoclinic α-LiAlH4 to δ-LiAlH4 phase at ~3.25 GPa. As the

temperature is increased to ~100°C both the α and δ phases transform to β-LiAlH4 and

remain stable up to 5.5 GPa. A new γ-LiAlH4 phase forms at temperatures greater than

300oC. The data obtained in this thesis was used along with that of Konovalov and Bulychev (1995) to construct the PT phase diagram of LiAlH4 and to define β and γ

-LiAlH4 phase boundaries. Decomposition data of Block and Gray (1965) was also

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Acknowledgments

I would like to express my immense gratitude to my advisor Dr. Dhanesh Chandra, who was always supportive since my first day in his research group. His humility, professionalism and passion for his work make him an example to emulate. I appreciate all the confidence and opportunities he gave to me. It has definitely been an honor to work with Dr. Aaron Covington and Dr. Erik Emmons. I owe them a huge portion of my knowledge about Materials Science and Engineering. Their ethics, values and patience were crucial for me to learn how to perform my experiments and polish my research techniques. I will never forget my experience at University of Nevada, Reno. On campus services, graduate school and everywhere else, the staff of the university always provides help and fraternity.

During my educational process in Costa Rica, I have to recognize the support of many friends and professors, which made possible my experience at the University of Nevada. There is not enough space for all of them, but I would like to thank Professor Glenn Dewey and Professor Manuel Fallas, for teaching first of all values and ethics with their daily work. My family and old friends in Costa Rica were an incredible source of support, and they were crucial on this journey. Also, I want to thank The National Council for Science and Technology Research of Costa Rica (CONICIT) for their excellent program of scholarships.

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University of Geneva for their support and help with the data analysis. Their kindness and collaboration in this project are greatly appreciated.

I would like to dedicate this thesis to the Costa Ricans who fought in favor of opportunities for education and for better quality of living for people in need, and for future generations. In many ways, they made it so that a person like me could aspire to reach an achievement like a M.S. degree. I hope his work and example stays forever as a guide for all Costa Ricans.

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List of Tables

Table 1.1. Selected hydrides for hydrogen storage research. ... 3

Table 1.2. Different approaches for unit-cell parameters of Li3AlH6. ... 14

Table 4.1. Comparisons of the vibrational mode assignment of LiAlH4 with experimental Raman data... 46

Table 4.2. Raman spectra of the internal modes of AlH4- in crystals. ... 48

Table A-2.1. Pressure-Temperature experimental data (α and δ-LiAlH4 phases). ... 76

Table A-2.2. Pressure-Temperature experimental data (various LiAlH4 phases). ... 77

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List of Figures

Figure 1.1. Crystalline structure of Mg(BH4)2 at ambient conditions. ... 5

Figure 1.2. PT Phase diagram of LiBH4. ... 6

Figure 1.3. Representation of the anions of BH4 in the different structures of LiBH4. ... 6

Figure 1.4. Molecular structure of lithium aluminohydride. ... 12

Figure 1.5. Molecular representation of AlD6 ... 14

Figure 1.6. Structural representation of α-Li3AlH6. ... 15

Figure 1.7. Crystalline structure representation of α-LiNH2. ... 18

Figure 2.1. DAC concept and pictures ... 20

Figure 2.2. Open DAC and gasket ... 21

Figure 2.3. DAC culets. ... 23

Figure 2.4. Merril and Bassett cell and compact cylinder cell... 23

Figure 2.5. Representation of loaded DAC. ... 26

Figure 2.6. Ruby fluorescence spectrum under high pressure. ... 27

Figure 2.7. DAC loaded and Glove bag with microscope ... 28

Figure 2.8. DAC heater system. ... 29

Figure 2.9. Diagram of different types of vibrational modes. ... 33

Figure 3.1. Representation of Gibbs free energy versus pressure curves of H2O at temperatures above, at and below the triple point temperature. ... 40

Figure 4.1. Mode assignments for LiAlH4 from this study (as-loaded sample, ~0 GPa), showing the various vibrational modes... 45

Figure 4.2. Mode assignments for LiAlH4 from this study (as-loaded sample, ~6.6 GPa), showing the various vibrational modes... 47

Figure 4.3. Temperature-dependent Raman spectra for LiAlH4 at an initial pressure of ~1.3 GPa ... 50

Figure 4.4. Temperature-dependent Raman spectra for LiAlH4 initially pressurized to ~3.0 GPa ... 51

Figure 4.5. Temperature-dependent Raman spectra for LiAlH4 at an initial pressure of ~3.3 GPa ... 53

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Figure 4.7. Temperature-dependent Raman spectra for LiAlH4 at an initial pressure of

~6.6 GPa. ... 56 Figure 4.8.Raman spectra of Li3AlH6 at room temperature. ... 60

Figure 4.9. Experimental pressure-temperature phase diagram of LiAlH4.. ... 62

Figure A1.1. X-ray diffraction pattern of LiAlH4 at room temperature and variable

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Chapter 1 Introduction to complex hydride storage

materials

1.1 Materials Under Extreme Conditions

High pressure and temperature studies of complex hydrides are an important scientific research area. High Pressure - Temperature studies have been performed on almost every element in the periodic table1. However, many complex hydrides with potential applications remain to be investigated. This chapter contains a brief introduction of high pressure and temperature science, and its relation with hydrogen storage materials, which are the focus of this thesis.

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It is desirable to observe how arrangements of atoms can change under extreme pressures, during transitions between solid, liquid and gas phases. The scientific community is becoming increasingly interested in solid-solid phase transitions, which have been reported for many elements and compounds. There are excellent publications regarding high pressure effects; papers written by Hemley and Ashcroft3, and the review by Block and Piermarini4 are important. Experimental techniques such as Raman spectroscopy, x-ray diffraction and infrared spectroscopy are generally used to characterize materials under extreme pressures, and give us information on molecular structure behavior.

Other methods examine thermodynamic properties at the macroscopic level, which can be used to determine the equation of state (pVT relations). Both types of measurements are complementary, providing information for a given system at the molecular and macroscopic level, respectively. More recently, high pressure studies related to energy and hydrogen storage materials have attracted considerable attention because of their promise in potential applications related to reduced carbon emissions.

1.2 Hydrogen Storage Materials Under High Pressure

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of hydrogen respectively, making them strong candidates for such applications6,7. It is important to learn about the behavior of these kinds of materials as a function of temperature and pressure in order to determine their suitability for these applications.

Table 1.1 provides a list of relevant hydrides with excellent hydrogen storage characteristics. There are limited data available on these materials at pressures > 1 GPa. It is a priority in society to stimulate the use of cleaner sources of energy. An increase in concern about air pollution and the greenhouse effect are factors that promote this kind of research, since more effective on-board hydrogen storage systems for vehicles8 might reduce our dependence on fossil fuels.

Table 1.1. Selected hydrides for hydrogen storage research.8

Hydride H2 (wt%)

LiAlH4 10.5

NaAlH4 7.5

KAlH4 5.8

Mg(AlH4)2 9.3

Ca(AlH4)2 7.7

LiBH4 18.5

NaBH4 10.6

Mg(BH4)2 14.9

Ca(BH4)2 11.4

Al(BH4)3 16.9

LiAlH2(BH4)2 15.2

Li2NH 6.5

LiNH2 5.74

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Prototype vehicles using hydrogen technologies have been available since the 1980s9. Recent studies on hydrogen storage materials are reported by Sundqvist et al.10 He reviewed the evolution and the current state of research associated with hydrogen storage materials (borohydrides and alanates). Chandra et al.8 have also reviewed research and developments on metal hydrides.

Specific objectives of the research of hydrogen storage materials involve the quantification of parameters that provide a better understanding of their molecular configuration. Information such as lattice parameters, molecular structure, phase transitions and conditions for volume collapse are needed for a through characterization of a compound and a prediction of its behavior when variations of pressure and temperature are induced. Dehydrogenation, rehydrogenation and decomposition of the material are also critical features which must be taken into account, since the development of possible applications depend on these factors.

Characterization methods and their primary role on these types of studies will be explained in further sections. The study of one particular alanate (LiAlH4) under high

pressure and temperature is the main purpose of this thesis. However, it is helpful to review other important previous investigations of high pressure research in hydrogen storage materials first. George et al.11 reported high pressure molecular structure and behavior of Mg(BH4)2. An irreversible phase transition at ~2.4 GPa and a reversible

transition at ~14.4 GPa was reported in the material11. Mg(BH4)2 has a hexagonal

structure at room pressure (P61), with unit cell parameters of a = 10.047 Å, c = 36.34 Å,

and V = 3176 Å3 at 0.2 GPa. Figure 1.1 shows the molecular distribution of Mg(BH4)2 at

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Figure 1.1. Crystalline structure of Mg(BH4)2 at ambient conditions.11

Dmitriev et al.12 developed a pressure temperature phase diagram (PT phase diagram) on LiBH4 using a DAC at elevated temperature. They collected data up to 10

GPa and 500 K, identifying four phases of the material. A phase transition from an ambient Pnma phase (called II), to a 6% denser phase (pseudotetragonal structure with Ama2 symmetry called III) was obtained at 0.6 GPa. At higher pressures, another phase

transition occurred at 8 GPa, with a cubic structure (Fm3 m), named V. A low pressure and high temperature phase known as I was found in the material, with a space group P63mc, appearing at 381 K.

Figure 1.2 shows the PT phase diagram obtained in this research12, and Figure 1.3 illustrates the manner in which the BH4 anions behave in the different structures of

LiBH4. Finally, it is important to comment that computational models (ab initio

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Figure 1.2. PT Phase diagram of LiBH4.12

Figure 1.3. Representation of the anions of BH4 in the different structures of LiBH4.

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1.3 Important Properties of Lithium Hydride Compounds

In this section, a review of important properties and characteristics about pure compounds lithium aluminohydride (LiAlH4), lithium amide (LiNH2) and lithium

hexahydroaluminate (Li3AlH6) has been compiled.

1.3.1 Lithium Aluminum Hydride (LiAlH

4

)

High pressure and temperature studies of LiAlH4 have been performed since the

1970s. One of the first high-pressure studies on that material was carried out by Bulychev

et al.13, who observed a phase transition around 7 GPa and between 250ºC to 300ºC using

a belt-type apparatus to apply high pressure and temperature. These were ex-situ measurements on quenched samples at ambient temperature and pressures. Bastide et al.14 also studied LiAlH4 using the same kind of device, and both studies observed a

variation in coordination number of Al3+ from 4 to 6 in a new high pressure phase. Chellappa et al.6 from our group, performed high pressure studies on LiAlH4 and

observed a phase transition from an orientationally ordered to disordered state near ~3 GPa. Theoretical predictions were first made by Vajeeston et al.15, using the ab initio projected augmented plane-wave method, calculating the density of states in LiAlH4 and

identifying a transition near ~2.6 GPa with a α-NaAlH4-type structure for the new phase.

In the present experiments, to be described later, the phase transition from α-LiAlH4 to δ

-LiAlH4 was detected near 3 GPa at room temperature. This is consistent with the prior

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between 2.2 and 3.5 GPa. In the study of Talyzin et al.16, a 4:1 methanol-ethanol pressure medium was used, which may have reacted with the sample. In addition, the sample was loaded without an inert protective atmosphere, which could have caused sample degradation because the high reactivity of the sample with moisture.

The differences in pressure medium and loading procedure may explain the differences in the range of pressures measured by Talyzin et al.16 and those of Chellappa

et al.6 Based on the Raman spectra from the experiments of Talyzin16, it was determined

that the δ-LiAlH4 structure did not correspond with the structure predicted theoretically

by Vajeeston et al.15 This conclusion was corroborated by Pitt et al.17 in a neutron scattering study at pressures up to 7 GPa and temperatures up to 65ºC on LiAlD4. It is

pointed out that the experimental conditions and loading procedure in the present study are similar to that of Chellappa et al.6, and the Raman spectra are also consistent with a room temperature phase transition from α-LiAlH4 to δ-LiAlH4 detected at ~3 GPa.

Majzoub et al.18 performed a temperature dependent Raman spectroscopy study on LiAlH4 to diagnose the reaction kinetics and release of hydrogen.

It is known that at ambient pressure, LiAlH4 decomposes at temperatures above

145ºC, producing Li3AlH6, aluminum, and hydrogen. Majzoub et al.18 did not report any

vibrational bands of Li3AlH6 in the Raman spectrum at 145ºC. The presence of micro or

nanocrystalline aluminum may have led to excessive Rayleigh scattering, making it difficult to observe the Raman spectra. Majzoub et al.19 also performed a temperature dependent Raman spectroscopy study on the related material NaAlH4. In related studies

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and is capable of reversibly releasing 5.6 wt% of hydrogen in the presence of a Ti-based catalyst20. In fact, the dehydrogenation in both compounds occurs according to the following reactions (the term M corresponds to Na or Li)6:

3MAlH4 →M3AlH6 +2Al+3H2 (1.1) ₃ ₆ ₂

2 3

3MH Al H

AlH

M → + + (1.2)

Bogdanovic et al.21 performed a study which demonstrated that the addition of Ti-based catalysts to NaAlH4 improves the compound chemical kinetics, decreasing its

decomposition temperature. Addition of these catalysts was also shown to promote reversibility. Majzoub et al.19 mention that the catalytic action of Ti should involve an interaction with the AlH4− anion, leading to a weakening of the Al-H bond strength, and

disintegration of the structural unit. In a high-pressure Raman spectroscopy study, Talyzin et al.20 found a structural phase transition from α'-NaAlH4 to β-NaAlH4 between

14 – 15 GPa, which was completely reversible.

Yukawa et al.22 proposed from experimental and theoretical studies on NaAlH4 a

correlation between the highest Raman frequency and the Al-H bond length. Their study demonstrates that for the normal mode of the Al–H stretching vibration, higher frequencies correlate with the shorter Al–H bond length. LiAlH4 must be handled in a

careful manner, to avoid its decomposition. Ke et al.23 demonstrated by a computational simulation method that LiAlH4 decomposes under all the temperature and pressure

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also demonstrated that the sample decomposed into a mixture of hydrides at room pressure and temperature. The white crystals of the compound turn gray upon the release of metallic aluminum, which gives a good indication of the condition of the specimen (also noted in the samples used for this study).

One factor that motivates the present project is to understand the effects of ball milling, which is performed on many of the alkali aluminohydrides during the production process. Ball milling consists of mixing and pulverizing particles in a drum with ceramic balls. Advantages like reduction in particle size and addition of catalysts are obtained with this process, and it has been observed that ball milling can affect the behavior and stability of LiAlH4. In their ball milling system, Balema et al.25 determined that ball

milling treatment for a short time (10 minutes approximately) did not cause significant changes in the LiAlH4 sample, but that after 35 hours of treatment the process can cause

minor changes. The decomposition of the sample was more significant after 75 hours of treatment.

Andreasen et al.26 concluded that ball milled LiAlH4 has faster dehydrogenation

kinetics than unmilled material. There is a proportional relation between the periods of milling time and the rate of dehydrogenation, being faster in samples with longer milling time. Chellappa et al.6 recognized that due to stress induced during ball milling, the sample could undergo a phase transformation, or even react in presence of a catalyst. It is necessary to take into account that experiments performed with a ball milled sample of LiAlH4 may yield different results than those performed with unmilled samples, and it

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phenomena previously described are not well understood, and a more thorough study is needed. When studying the behavior of LiAlH4 at high pressures, the volume reduction is

an important aspect to take into account.

Vajeeston et al.15 performed an ab initio study to characterize this aspect in LiAlH4, noting a 17% volume collapse during α→δ phase transition in the material, due

to electronic transition from Al s- to p-states. In a similar ab initio study, Vajeeston et al.27 calculated a volume contraction of 4% for the α-NaAlH4 →β-NaAlH4, transition. In

situ neutron diffraction research on LiAlD4 carried out by Pitt et al,17 showed a unit cell

volume reduction of 26.9% reducing the volume to 200.9 Å3 between ambient pressure and 7.15 GPa and 25-60°C. Gomes et al.28 reported a unit cell volume of NaAlH4 of

279.12 Å3 as compared to LiAlH4,which has a unit cell volume of 277.10 Å3 at room

temperature.

Sandrock et al.29 mentioned that a nominal value for volume changes in metallic hydrides is approximately 25% during a hydrogenation→ dehydrogenation transition. For complete dehydrogenation of NaAlH4, a 16.2% volume change is expected, but 14.7%

was measured. The importance of the pressure dependence in these materials motivated the studies already cited in this thesis, and shows several interesting results. The present experimental work extends the focus to the effect of the combination of conditions of high pressure and temperature, for a better understanding of phase behavior of LiAlH4. In

this research, the phase transitions of LiAlH4 are examined by increasing both the

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configuration. Li+ atoms are surrounded by five AlH4- tetrahedra. Figure 1.4 illustrates the

structure of LiAlH4 generated by computer projections.

Figure 1.4. Molecular structure of lithium aluminohydride.6

Lovvik et al.30 determined this structure as P21c (z = 4). The unit cell parameters at 300 K are: a = 4.82 Å, b = 7.80 Å and c = 7.89 Å, β = 112.27°. The data was determined by density-functional theory (DFT). The results matched with the crystal structure of the deuteride analog, LiAlD4, found by neutron powder diffraction at 9 K.

The enthalpy of formation at 298 K was predicted at -113.42 KJ/mol. The melting point of LiAlH4 is 125°C, its density is 0.92g/cm3 and its molecular weight is 37.95 g/mol.31

1.3.2 Lithium Hexahydroaluminate (Li

3

AlH

6

)

Lithium hexahydroaluminate has not been as researched as other hydrogen storage materials. The decomposition reaction of Li3AlH6 liberates approximately 2.6

wt% of hydrogen at 160 – 210°C, and is presented in the next equation32:

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In comparison with complex alkali aluminohydrides, 2.6 wt% of hydrogen is relatively a small contribution. However, recent studies have excellent arguments to describe this material and discover its usefulness. Investigations show an enhancement of hydrogen storage properties in Li3AlH6, by mixing it with another compound, like

amides33. Lower activation energy of dehydrogenation can be achieved with this method. Lu et al.34 mention that a combination of Li3AlH6 with LiNH2 has approximately 7.0 wt%

of reversible hydrogen storage capacity under 300°C.

Temperature plays a significant role on those complex reactions. Studies conducted by Chen et al.32 demonstrated interesting capabilities of rehydrogenation/dehydrogenation of Ti-doped Li3AlH6, releasing the hydrogen at lower

temperatures than the ordinary compound. Lee et al.35 in their study comment that a decrease on the decomposition temperature of lithium hexahydroaluiminate from 190 to 160°C has been achieved with the addition of Ti5S3. Shim et al.36,37 have reported an

improvement on the absorption kinetics by mixing Li3AlH6 with TiAl3 and TiCl3.

In order to perform the mixing of Li3AlH6 with a catalyst or another hydride, ball

milling is executed. In this process, the compound goes under certain chemical reactions at high temperature and pressure in a milling chamber, leading into mechanical and chemical reactions and structural transformations. That reason that Kumar et al.33 points, and the possibility to find other phases of Li3AlH6 with better hydrogen storage

properties, makes important the research on Li3AlH6 under high pressure and

temperature. Some structural studies and approaches have been performed on Li3AlH6.

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synchrotron x-ray diffraction and neutron scattering. They found a space group 3R , and the structure correspond to an isolated AlD₆3−octahedral, with six coordinated Li. Figure 1.5 shows a structural representation. Kumar et al.33 performed a study comparing high pressure/temperature synchrotron x-ray data and DFT calculations.

Figure 1.5. Molecular representation of AlD63-.38

The structure obtained for that specific phase is rhombohedral with R space 3 group. A β-phase was found at 10.6 GPa, with cubic structure (Ia ). The calculations of 3 Vaajeston et al.39,40 agree with this result. The previous crystalline structure studies in addition with the performed by Loovik et al.30 bring theoretical and experimental unit cell parameters, organized in Table 1.2.

Table 1.2. Different approaches for unit-cell parameters of Li3AlH6.

Unit cell parameters

α-Li3AlH6

Experimental results (Kumar et

al.)33

Theoretical results (Kumar et

al.)33

Theoretical results (Vaajeston et

al.)39,40

Experimental results (Brinks and

Hauback)38

Theoretical results (Loovik et

al.)41

a (Ǻ) 8.12 8.01 8.04 8.07 8.07

c (Ǻ) 9.57 9.45 9.45 9.51 9.51

Unit cell parameters

β-Li3AlH6

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Vaajeston et al.39,40 define the successive transitions from α to β-Li3AlH6 in 18.64

GPa, β to γ at 28.85 GPa and γ to δ at 68.79 GPa. The γ-Li3AlH6 has lattice parameters of

a = 9.22 Å, b = 7.97 Å and c = 4.73 Å with a space group Pna21. Figure 1.6 illustrates an approach of lithium hexahydroaluminate, where Al is located at the center of each octahedra.

Figure 1.6. Structural representation of α-Li3AlH6.39

1.3.3 Lithium Amide (LiNH

2

)

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Li₂NH +H₂ +LiH ↔LiNH₂ +2LiH ↔ Li₃N +2H₂ (1.4) Chen et al.44 indicate that it is possible to store ~11.5 wt% of hydrogen following this relation. In neutron scattering experiments, Chien et al.45 confirmed the appearance of different quantities of LiD, LiND2 and Li2ND phases during isothermal heating of α-Li3N at different temperatures. A 7.26 wt% of hydrogen was determined during

deuteriding at 250°C. Ikeda et al.46 added magnesium to the LiNH2+LiH mixture, obtaining a dehydrogenation temperature in between 169 and 193°C with a confirmed reversibility in the process. Luo47 mention that by adding MgH2, partial substitution of Li

by Mg is achieved, creating a destabilization that favors the hydrogen absorption/desorption properties. An indicator of this fact is that the system LiNH2+LiH (see equation 1.4) has a decomposition enthalpy of 51 KJ/mol, while LiNH2+MgH2 have 34 KJ/mol.

Shani et al.48 found for the same mixture that increasing the duration of ball milling from 5 to 45 hours, an important development in hydrogen desorption kinetics is produced (up to 49%). This is caused mainly by the formation of Mg(NH2)2. The same

compound was detected by Liu et al.49 in a 1:1 mixture of lithium amide and MgH2.

Barison et al.50 agree with the results of extended ball milling time. The partial substitution of Li by Mg in LiNH2 was studied theoretically by Jin and Wu51, agreeing

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thermogravimetric and residual gas analyses Li2Mg2(NH)3 and a considerable amount of

ammonia in the products of the reaction.

The system 2LiNH2 +MgH2 was exposed to water saturated media by Luo et al.53, with a small increment in the final amount of hydrogen released. An interesting study for the proposes of this thesis is the performed by Lu et al.34, explaining the combination of Li3AlH6 with LiNH2. The next equation describes the behavior of that system. In a

separate study the authors comment that the reaction has approximately 7.0 wt% of hydrogen capacity under 300°C54.

₂ ₆ ₂ ₂ ₂

2 9 3

3LiNH Al Li NH H

AlH

Li + ↔ + + (1.5)

Noritake et al.55 made a crystal structure analysis of LiNH2+LiBH4. Tang et al.56 found that a CoCl2 doped mixture of LiNH2 with LiBH4 releases more than 8wt% of

hydrogen at 155 °C. Yang et al.57 made investigations about 2LiNH2+LiBH4+MgH4, having lower dehydrogenation temperatures and improved reversibility and kinetics response than 2LiNH2+MgH2. In this study, the formation of Li2Mg(NH)2 is responsible

for the acceleration of the reactions and the lower desorption temperatures mentioned. Yao et al.58 examined the characteristics of LiNH2 and LiNH2+LiH adding Mn, V, MnO2

and V2O5, which helps with thermal decomposition of lithium amide, but no relevant

effect was discovered on hydrogen desorption of LiNH2+LiH.

Xiong et al.59 studied the reaction between LiNH2 and LiAlH4, where 8wt% of H2

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using neutron diffraction affirms that the α phase of lithium amide is tetragonal (I4, z = 8) with unit cell parameters a = b = 5.03 Å and c = 10.25 Å. The bond angle between H– N–H is 99.97°. Chellappa et al.61 performed a vibrational mode assignment from ~50 to 1100 cm-1 and 2900 to 3600 cm-1. Figure 1.7 shows the crystalline structure of α-LiNH2.

Orimo et al.62 using first principles calculations approached lattice parameters of the α-LiNH2, obtaining values of a = b = 5.07 Å and c = 10.11 Å. High pressure studies

has been developed by Chellappa et al.61, who found a phase transition from orientational disordered α-LiNH2 to an ordered β phase that begins at ~12 GPa and completes at ~14

GPa. Also, that study indicates that during decompression the reverse transition occurs between ~9.4 and ~8.3 GPa. The system presents a considerable amount of hysteresis. In this thesis, some experiments on high pressure and temperature on lithium amide were made, agreeing with the behavior explained in the previously mentioned study. Preliminary results at high temperature are further presented.

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Chapter 2 High Pressure Experimental Techniques

2.1 Diamond Anvil Cells

A relatively simply device, the diamond anvil cell (DAC) has revolutionized the manner which high pressure studies experiments are conducted. Continued developments and better DAC designs are helping the scientific community around the world. The main function of the DAC is to exert a substantial amount of pressure on the solid particulate sample, while observing its molecular structural evolution under high pressure. Raman spectroscopy, x-ray diffraction, or other characterization methods like IR spectroscopy are used in conjunction with DAC device to observe changes in structures. In the following paragraphs, this high pressure apparatus will be explained.

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deformation when pressures are applied. This combination of factors produces an exceptional experimental system, which is extremely reliable device for laboratory research. Figure 2.1 illustrates the concept of the DAC.

Figure 2.1. DAC concept (a)65 and pictures (b, c).

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produced by the electroerosion phenomena. More details of the EDM machine applied to high pressure research are given in Lorenzana et al.66

Another critical function of the gasket in the DAC is related with the creation of a hydrostatic or quasi-hydrostatic medium in the sample, since it is important to have uniform pressure in the whole volume of the sample. This is necessary to obtain consistent results in an experiment, no matter which zone of the sample is chosen for analysis. Samples with gaskets demonstrated that they have lower possibilities of localized stress, as opposed to ungasketed systems67. Figure 2.2 shows an open DAC with a prepared gasket. With further development of the experiments and high pressure techniques based on the DAC, some ideas came to produce even more accurate measures. The introduction of a liquid media in the sample chamber also helps to maintain the hydrostatic pressure in the sample. A typical liquid medium used in DAC experiments is a 4:1 mixture of methanol:ethanol by volume, which is very nearly hydrostatic to about 10 GPa68.

Figure 2.2. Open DAC and gasket

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characterization method can suffer due the medium. Piermarini et al.69 describe a series of pressure media and their behaviors. As an alternative to liquid pressure media, the introduction of solid media and condensed gases is discussed in Eremets64 for special conditions and materials. However, the case of solid media doesn’t avoid the presence of shear stress, and the condensed gases are difficult to load.

For the research proposes previously listed, diamonds have their own classification. Depending on the nature of the experiments, and the characterization method used in sample analysis, the correct diamond must be selected. Diamonds are transparent to light (for energies ≤ 5 eV) and ideal for high photon flux sources like synchrotrons (energy 10 - 100 keV)70. In diamonds, some defects are caused by the presence of nitrogen. Type IIa diamonds (relatively pure diamonds) are used for specific applications like IR spectroscopy. Due to their pure nature, Type IIa diamonds are expensive and difficult to find. Owing to the presence of nitrogen impurities, standard diamonds (type Ia) are slightly yellow. However, they can still be used for Raman spectroscopy and x-ray diffraction70.

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alignment of the diamonds, and the optical geometry for the characterization method. Figure 2.3 shows in detail both culets in a DAC, and the manner in which they exert the high pressure in a sample. Historically, the first cells were provided with a piston – cylinder configuration. The Piermarini-Block cell (1975) and the Mao-Bell cell (1977) are examples of this. In the 1980s, the piston was embedded in the cell, reducing the size of the DAC, as in the case of the Mao-Bell cell. Eremets64 and Jayaraman63 explain this early development of cells.

Figure 2.3. DAC culets.

With the improvement of high pressure technology, the DAC started its miniaturization. The compact cylinder DAC, four-post DAC and Merril-Bassett cell are the most common used cells, due to their size and flexibility for experiments.

Figure 2.4. Merril and Bassett cell (left) and compact cylinder cell (right)

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The concepts of these three DAC models are similar, such as in guiding parts (metallic cylinders or posts) in order to open both halves of the cell by removing its screws and maintain the alignment of the diamonds while loading the sample. The Merril-Bassett cell has a triangular body with three Allen screws (instead of the cylindrical external shape of the four-post DAC). The posts can also fix the gasket in the correct position. Figure 2.4 illustrates two types of DAC. In the four posts and cylindrical DAC two of the screws are left handed and the other two right handed, in order to maintain the symmetry of the pressure applied in the system. The four-post cell can be observed in Figure 2.2.

The DAC has an independent set of screws for concentric and parallel alignment of the diamond anvils. The screws move the back of the cell on each half, which hold the diamonds. This fact is crucial in order to use the whole surface of the diamonds, and increase the pressure in the sample as symmetrically as possible. Additionally, the parallel alignment is crucial to avoid the creation of a transverse component during the application of force, since breaking the diamond can result from poor alignment64. The alignment of the diamonds has to be performed with a microscope, using transmitted and reflected light. It is necessary to view the diamonds from the side, and rotate the cell to check the concentric alignment.

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other. An excess of pressure of the diamonds in this step can break the diamonds. With the transmitted light of the microscope, and looking from the top of the DAC, it is possible to observe interference rings72. It is necessary to minimize the presence of these rings on the diamonds. Less than 3 to 4 rings over the whole surface of the diamonds indicates a good alignment. An accepted technique to establish a parallelism in the culets is to align the concentricity of the diamonds, and then loosen the alignment screws of the cell. The culets will be touching each other and the surfaces also will become parallel, due to the action of gravity. Then the screws can be tightened again.

2.2 Pressure Calibration by Ruby Fluorescence Method

In order to measure the pressure exerted on the sample by the DAC, the Ruby fluorescence method73 is used since it is accurated and especially convenient due to the extremely small size of the sample and culets of the diamonds. Once the sample is loaded in the DAC, the next step is to put some small particles of ruby in as well. When the DAC is closed and high pressure is applied, the pressure in the sample can be measured by the interaction of Laser with the ruby particles. This technique is based on the shift in wavelength of the R-line fluorescence of the ruby particles with changes of pressure in the DAC73. In this method, a Laser beam is focused on a ruby fragment. The fluorescence signal is collected, dispersed by a spectrometer, and interpreted.

An important property of Ruby is its chemically inert nature, which minimizes reactions with the sample. The composition of ruby is sapphire (Al2O3) with ~0.05% of

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sample and gasket appear. Figure 2.6 shows a typical ruby spectrum in the high pressure experiments. To collect the information just mentioned, a visible argon-ion Laser is used to illuminate the ruby particles, which fluoresce with a characteristic red color. The resonant fluorescence of the ruby is two peaks known as the R1 (694.2 nm) and R2 (692.8 nm) lines under ambient conditions. The variation of R2 is used to calculate the pressure by empirical relations. More details are given in Mao et al.74

Due the heating of the sample used in the experiments described later in this thesis, it is also necessary to take into account a temperature correction in the measurement of the pressure in the DAC by the ruby fluorescence technique. Vos and Schouten75 found an increase in the ruby wavelengths with increasing temperature at a rate of 0.0068 nm/K. In most experiments, two or three regions that contained ruby chips were selected to measure the pressure throughout the entire sample volume and to check that hydrostatic conditions prevailed in the material to be analyzed.

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Figure 2.6. Ruby fluorescence spectrum under high pressure. Obtained from the Renishaw InVia Raman micro-spectroscopy system with 514.5 nm argon-ion Laser (Spectra-Physics Model 163-C4210) used in this thesis.

The pressure measurements were performed in the same Raman system used for the characterization of the LiAlH4 samples. Details of this apparatus will be explained in

section 2.4.

2.3 Sample Preparation

It is vital to carry out meticulous sample preparation in order to obtain reproducible results in high pressure experiments. It is necessary to clean the DAC, its diamonds, and the gasket before each experiment to minimize the impurities in the sample. It is important to clean also all the devices involved with the sample manipulation (needles, tweezers, etc). This cleaning step can be performed with lint-free wipes and ethyl alcohol. The gasket is positioned in the DAC using the posts for alignment. Then, an indentation is made on the gasket by closing the DAC and applying

6 8 0 6 8 5 6 9 0 6 9 5 7 0 0 7 0 5

-1 0 0 0 0 0 1 0 0 0 0 2 0 0 0 0 3 0 0 0 0 4 0 0 0 0 5 0 0 0 0 6 0 0 0 0 7 0 0 0 0 8 0 0 0 0

In

te

n

s

it

y

(

a

rb

.

u

n

it

s

)

W a v e le n g h t (n m )

R2

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moderate pressure. The deformation produced in the gasket increases its hardness. The DAC is opened again and the gasket is removed in order to drill the hole in the center of the culet indentation. The holes drilled for this study were ~120 micron in diameter, and were made with the EDM machine.

Once the hole is ready, the gasket has to be cleaned again and returned to its position inside the DAC. To load the specimen in the DAC, it is necessary to use the microscope and a needle due to the small size of the parts involved. When there is a sufficient amount of the sample in the gasket, the material can be compressed with the needle to compact the particles and completely fill the hole. It is important to put a generous amount of the material in the hole, filling it as much as possible in order to avoid the presence of air pockets in the sample. When pressure is exerted in the DAC, if there is a considerable presence of air or another gas trapped in between the gasket and diamonds, the gasket can collapse and break at relatively low pressures. The ruby particles can be added by placing them on the other DAC culet, also with a microscope and a needle. When the DAC is closed, the diamonds will compact the sample and the ruby will be present. Then, the high pressure experiments can be initiated. Figure 2.7 shows the loading configurations.

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A controlled argon atmosphere was used to load the sample in the DAC and was achieved with a glove bag to avoid the reaction of LiAlH4 with moisture and oxygen in

the air. The strong reactivity of LiAlH4 with water makes this step necessary to maintain

the purity of the material and measure accurate results. For the same reason, a liquid pressure media was not used. A humidity gauge connected to a PC was used to monitor the humidity inside the glove bag. The sample container was open and the LiAlH4 was

loaded under approximately 0% to 4% relative humidity. The LiAlH4 sample in the DAC

was heated with an Omega temperature controller system attached to an adjustable resistive heater, presented in Figure 2.8.

Figure 2.8. DAC heater system.

2.4 Materials and Apparatus

The samples of LiAlH4 (95% purity) were purchased from Alfa Aesar, and were

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with a Renishaw InVia Raman micro-spectroscopy system with a 20x objective lens and 514.5 nm argon-ion Laser (Spectra-Physics Model 163-C4210) using 5 to 10 mW of Laser power incident upon the DAC. Further details of the four post DAC used and Raman spectrometer are available in Emmons et al76,77. The Raman scattering wavelength was calibrated using Ne lines, and a small offset correction was performed78. Several ruby chips were distributed within the sample for the pressure measurements.

The DAC was loaded and closed with minimal pressure. Because of the absence of a pressure medium, the effect of nonhydrostaticity needed to be considered. However, observation of the ruby R1 and R2 lines showed that they were well resolved, indicating a reasonably hydrostatic environment was maintained during all measurements. Inconel gaskets were used with no pressure media. The culets of the cell were of 600 µm of diameter and the hole in the gasket was in between 120 µm and 250 µm diameter with a thickness determined by a pre-indentation of about the maximum pressure of the experiment. The diamonds were type Ia (Raman low fluorescence). In this study, STYCAST 2850 FT Epoxy from Emerson and Cummings was used to glue the diamonds to the cell. The increments in the pressure were applied by tightening in a few degrees increments each screw and making measurements of the sample and the pressure.

2.5 Molecular Vibrations

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Born-Oppenheimer approximation considers that the electrons are less massive than the nuclei, having the capacity to move faster. That implies that in the calculations, the nuclei are considered stationary by the electrons. Also, the electrons are considered as a charged cloud by the nuclei. That is the manner which the electrons “adapt” to any position of the nuclei. It is important to mention that without the Born-Oppenheimer approximation, it is difficult to achieve a practical approach to computational chemistry, owing to the complexity of the many-body problem.

To proceed with the calculation of the vibrational frequencies, it is necessary to start with the solution of the Schrödinger equation for the electronic energy of the molecule or solid with constant values of bond angles and internuclear distances (also part of the Born-Oppenheimer approximation). Then, the bond lengths and the angles are changed, and a solution of Schrödinger equation for the new parameters is required. Repeating this procedure and using the electronic energy as an effective potential, a potential energy curve can be generated. The vibrational frequencies are obtained by the nature second derivative of this curve near the potential minimum.

A Schrödinger mathematical representation for the wavefunction of the nuclei (ψn) is presented in the next equation, where Hn is the nuclei Hamiltonian, Tn represents the kinetic energy of the nuclei, E(Rα) is the potential energy surface and Rα has all the

nuclear coordinates79:

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minimum in the potential energy. Rα reduces to a simple coordinate for a specific type of

vibrational mode. The entire problem can be reduced to a quantum mechanical simple harmonic oscillator, described in the next equation, with ħ = h/2π, being h the Planck’s constant, m the effective mass and k the spring constant80:

kx

( )

x E

( )

x dx

d

m n n

n ψ ψ

ψ = +       − 2 2 2 2 2 1 2 η (2.2)

The next expression provides the quantized energies of a simple harmonic oscillator:       + = 2 1 n hv

En (2.3)

Where v is the vibrational frequency and n a positive integral quantum number 80. Small deviations called anharmonicities are present in this formula because of the presence of cubic and higher order terms in the potential energy function. The normalized wavefunctions for the simple harmonic oscillator is shown in Equation (2.4), being

m k /

=

ω and Hn(x) the Hermite polynomial of order n.80

( )

                            −      

= m m x H m x

n x _n n n η η η ω ω π ω

ψ 4 2

1 2 exp ! 2 1 (2.4)

The intensity of a transition in the Raman spectrum will depend on the square of a matrix element with the form:

( )

(

)

, 1

( )

, 1

)

*

1

2 + −

∞ ∞ −  + +       =

=

_∫

m n mn mn

n

m n n

m h dx x x x

x δ δ

ω ψ

ψ ψ

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Where ψm(x) represents the final and ψn(x) the initial state wavefunction. δi,j is the Kronecker delta function80. From this mathematical approach a selection rule exist, involving the delta functions. Absorption or emission of a photon is only possible if n changes by ±1. This rule applies for Raman scattering, due the matrix elements of Equation (2.5) are also present in the equation for the Raman scattering intensity, further described in detail. Different types of motions can be observed in molecules and solids, with rotational, vibrational and electronic being three of the most important types. In the rotational motion, the nuclei maintain constant separations with each other. Vibrational motion involves the nuclei moving relative to each other.

Figure 2.9. Diagram of different types of vibrational modes. The + and − symbols indicate motion perpendicular to the plane of the paper65.

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twisting, wagging and rocking. Figure 2.9 offers a perspective of the vibrational modes. The Raman intensity (intensity of the scattered light) and the change in the frequency provide the Raman spectrum of the material, being source of information and analysis about the material and its characteristics. Historically, Gonikberg in 1959 performed the first Raman experiments with a piston – cylinder DAC65.

2.6 Raman Spectroscopy

Among the characterization approaches used in high pressure research, Raman spectroscopy is one of the most important. Its accuracy and precision allows experimenters to obtain valuable data, and its easy implementation in a laboratory, makes it a valuable characterization method in materials science. This technique is based on the Raman scattering effect, which is a physical effect related with the behavior of a light being scattered by a sample. Basically, when monochromatic light is focused and is incident in a sample, a fraction of it is going to be reflected.

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excitation above the ground vibrational state is required for Anti-Stokes transitions. The electric dipole approach can be used to describe mathematically the intensity of a transition in the Raman spectra81:

ψ1Mψ2 =

∫

ψ1Mψ2d3r (2.6)

The integral is taken all over the space, M is the transition moment, and ψ1 and ψ2

are the wavefunctions for the ground and excited vibrational states respectively. For a better comprehension of the scattering concept and its association with Raman spectroscopy, the transition moment P is the induced dipole moment due to an incident field, called polarization. As mentioned, in case that the initial and final vibrational states are equal, the effect is denominated Rayleigh scattering, while if they are different, the scattering it is termed Raman scattering. Here, P=αE, whereα is the polarizability tensor and E is the electric field vector, both in a matrix form. P and its components are presented in the next set of equations, where Ei is the electric field component and αij is a

component of the symmetric polarizability tensor82.

Px =αxxEx +αxyEy +αxzEz (2.7)

P_y =α_yxE_x +α_yyE_y +α_yzE_z (2.8) P_z =α_zxE_x +α_zyE_y +α_zzE_z (2.9) To determine the intensity in the infrared spectrum, the next matrix provides a useful expression:

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The matrix element for the polarizability tensor of Raman transitions can be written in terms of the Mij quantities in the form82:

∑

_

  

 

+ + − =

r rn

rm nr

rm rn mr n

m

v v

M M v v

M M

h ₀ ₀

1 ψ α

ψ (2.11)

Summarizing, to be effective in the Raman spectra, a vibrational mode needs matrix elements different to zero in at least one of the three components of the dipole moment (P) or six independent components of the polarizability tensor (α_{). For practical}

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Chapter 3 Development of Phase Diagram

3.1 Thermodynamic Basis

The primary objective in this thesis is to create a PT phase diagram of LiAlH4. As

mentioned in the experimental techniques section, this was executed by heating a DAC loaded with a LiAlH4 sample. The DAC provides high pressure and the heater makes

temperature increments. Using Raman spectroscopy, the structure of the compound was observed and the phase transitions defined. In order to proceed with the details about the results, it is important to clarify the scientific basis of phase diagrams and its relation with thermodynamics. This chapter includes an approach of thermodynamics and phase diagrams. Kou et al.83 cite some basic concepts that are important to define, to start a theoretical explanation of this topic:

System: a fraction of a material which is examined. The equilibrium state of the system is achieved if its states do not suffer transformations with time.

Phase: a part of the system which can be separated and identified from others by its molecular structure.

Component: is the number of independent constituents that conforms the composition of each phase related to the equilibrium of the phase.

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It is necessary to establish a relation with phase diagrams and thermodynamics, specifically with the Gibbs free energy conception. Gaskell84 explains several aspects of that concept. Gibbs free energy refers to a thermodynamic potential that rates the work obtainable from an isothermal and isobaric thermodynamic system. In other words, the Gibbs free energy is the highest quantity of non-expansion work that can be obtained from a closed system (a system that can exchange energy but no matter). This maximum can be extracted uniquely in a completely reversible process (a process which can be restored to its original thermodynamic state after occurs).

Because this is an ideal thermodynamic concept, Gibbs free energy can only decreases or remain constant for a process happening at constant pressure and temperature. Gibbs free energy can also be defined as a chemical potential, which is minimal when a system stabilizes its equilibrium at constant pressure and temperature. In a chemical reaction occurring at constant pressure and temperature, Gibbs free energy minimizes when the reaction stops. Gibbs free energy is defined by the next expression:

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diagrams, mentioning that at constant values of pressure and temperature, a stable phase will have the lowest Gibbs free energy in the system. That is completely understandable because when a material or a combination of materials interact with different pressures and/or temperatures, the molecular structure and its composition can change.

When a reasonable organization level is acquired in the system, Gibbs free energy will be the lowest and a phase will be stable. In other position, a high Gibbs free energy value will mean a phase with a considerable amount of molecular disorganization. That fact allows determining phase stabilities in a convenient manner. A common step to generate knowledge about a phase and its stability is by using graphs of Gibbs free energy versus composition. In those plots, the pressure is constant and there is one curve per each temperature. On each free energy curve, a tangent line is drawn at the lowest Gibbs free energy value, to define a stable phase in another related plot, corresponding to a phase diagram. Figure 3.1 shows those curves and how it matches with a phase diagram. The Gibbs phase rule determine a relation among the number of components (i), phases (j) and degrees of freedom (f). The next equation shows the relation:

f =i− j+2 (3.2)

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G =H(T)−T⋅S(T) (3.3)

Figure 3.1. Representation of Gibbs free energy versus pressure curves of H2O at

temperatures above, at and below the triple point temperature84.

The Enthalpy and Entropy terms can be re-expressed as follows:

= +

∫

T

p o

dT T C H

T H

298 298 ( ) )

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= +

∫

T p o dT T T C S T S 298 298 ) ( )

( (3.5)

Gibbs free energy can be obtained with Equation (3.6) as a function of heat capacities:

∫

_       + ⋅ − + = T T p o p o dT T T C S T dT T C H T G 298 298 298 298 ) ( ) ( )

( (3.6)

Gibbs-Helmholtz relation provides a useful manner to relate Cp,m and Gibbs free

energy. By partially differentiation on Equation (3.1) with respect to temperature at constant temperature:       ∂ ∂ = − T G

S (3.7)

Combining equation (3.7) with (3.1), multiplying by dT/T2 both sites of the equation: ₂ ₂ T GdT TdG T HdT − =

− (3.8)

That expression can be related with the identity d

(

x/y

) (

= ydx−xdy

)

/y2and Equation (3.8) can be written as presented next, being the Gibbs-Helmholtz equation.

( / ) ₂

T H dT T G d −

= (3.9)

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of Gibbs free energy and Enthalpy, being a useful expression in experimental thermodynamics: dT T G d T

H =− 2 (∆ / )

∆ (3.10)

Cp at constant pressure is defined by:

p p T H C       ∂ ∂

= (3.11) Making a substitution of Equation (3.10) in (3.11), defining the heat capacity in terms of Gibbs free energy84:

(

)

p p T T T G T C             ∂       ∂ ∆ ∂ − = / 2 (3.12)

3.2 Importance of Materials Characterization and Research

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characterization methods in a specimen of the material to analyze, towards a realistic and reliable model.

To construct a phase diagram there are experimental approaches involving characterization methods. X-ray diffraction, calorimetry, Raman spectroscopy and some other characterization options brings valuable data to organize in a diagram of this kind. Basically, the experiments measures composition and microstructure in a specimen under pressure and temperature conditions. The idea is to make observations in structural transitions or reorganizations of the atoms in a compound, in order to collect the data to plot in a phase diagram. Specifically, there are some methods to perform an accurate phase diagram. Kuo et al83. explain some strategies about data collection for phase diagram constructions. Among them, high temperature x-ray diffraction and high temperature optical microscopy exemplify the procedures commented in the previous paragraph.

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200 400 600 800 1000 In te n s it y ( a b s . u n it s )

Raman shift (cm-1)

7 8 1 9 3 7 8 7 8 8 1 9 6 8 9 5 0 8 4 3 7 3 1 7 2 1 4 1 6 8 1 4 9 2 7 0

1600 1800 2000 2200

1 8 3 9 1 7 5 9 1 7 2 5 Stretching Bending Libration Translation

200 400 600 800 1000

In te n s it y ( a b s . u n it s )

Raman shift (cm-1)

7 8 1 9 3 7 8 7 8 8 1 9 6 8 9 5 0 8 4 3 7 3 1 7 2 1 4 1 6 8 1 4 9 2 7 0

1600 1800 2000 2200

Chapter 4 Results and Discussion

4.1 Interpretation of the Ambient Raman Spectra of LiAlH

4

A strong indication of a phase transition due to a change in pressure or temperature is the discontinuous variation of the vibrational modes present in the material, observed by Raman spectroscopy. Bureau et al.85 studied the internal vibrational modes of LiAlH4, as well as the external lattice modes. The present study and that of

Chellappa et al.6 details LiAlH4 lattice vibrational, bending, and stretching modes. The

Raman spectrum of the material is classified into four zones at room temperature: translational, librational, bending, and stretching modes.

Figure 4.1. Mode assignments for LiAlH4 from this study (as-loaded sample, ~0 GPa),

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The vibrational modes of LiAlH4 at room temperature and ambient pressure

collected in the present study, and the band positions are labeled as shown in Figure 4.1. In order to compare the vibrational modes of LiAlH4 obtained in this project, Table 4.1

shows band positions and mode assignments reported by Chellappa et al.6 and Bureau et al.85 The data acquired in the present study are in agreement with Chellappa et al.6 A good reference for the librational motion in LiAlH4 is the study of Temme and

Waddington86.

Table 4.1. Comparisons of the vibrational mode assignment of LiAlH4 with experimental

Raman data. DAC as-loaded

(this work)

DAC as-loaded (Chellappa et al.)6

Powder Raman (Bureau et al.)85

Vibrational mode assignments

88 95

Translation

102 112

149 141 143

214 201 165

270 225 220

Librational

317 312 322

437 434 438

508 495 510

689 688 690 δ(AlH2)

781 778 780 sym-δ(AlH2)

819 816 830 δ(AlH2)

878 878 882 δ(AlH2)

937 933 950 δ(AlH2)

1725 1720 1722 ν (Al-H)

1759 1754 1762 asym-ν (Al-H)

1839 1829 1837 sym- ν (Al-H)

The relatively high frequency indicates that the AlH4- is a stable ion fixed in the

lattice. Gorbunov et al.87 also found a librational mode for LiAlH4 at 472 cm-1. The

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Al-H peak softens and shifts to lower frequencies as temperature or pressure is increased. The frequency shifts indicate that the temperature increase in the system at constant pressure is producing a phase transition in LiAlH4. Figure 4.2 presents the vibrational

mode assignment for LiAlH4 at high pressure (~6.6 GPa) and room temperature.

Figure 4.2. Mode assignments for LiAlH4 from this study (as-loaded sample, ~6.6 GPa),

showing the various vibrational modes.

An increase in the noise of the signal is visible in the Raman spectra at high pressure, being a normal phenomenon. A shifting in the band corresponding to the stretching mode is probably the most notorious difference in both Figures 4.1 and 4.2, moving from 1839 cm-1 to 1749 cm-1. The disappearance of the bands at 1725 cm-1 and 1759 cm-1 at high pressure in the Al-H bond is also important in order to considerate a new organization in the molecular structure of the sample due a phase transition. Other changes in the vibrational modes and peaks at high pressure are perceptible in the bending region, having a prominent peak at 1124 cm-1 and 1154 cm-1, with minor bands

200 400 600 800 1000 1200

Translation Bending 7 7 8 4 7 8 1 1 5 4 1 1 2 4 9 1 1 7 1 5 6 4 9 5 1 4 3 8 7 3 3 6 2 2 2 In te n s it y ( a rb u n it s )

Raman shift (cm-1)

2

5

2

Libration

1400 1600 1800 2000 2200

Stretching

1

7

4

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at 911 cm-1, 778 cm-1 and 715 cm-1. A shifting in the peaks of the librational mode is observed at ~6.6 GPa, with bands of moderated intensity at 336 cm-1, 387 cm-1, 478 cm-1, and 514 cm-1.

Finally, the translation mode has regular peaks also at higher frequencies than the data obtained at room temperature and pressure. In this case, two peaks at 222 cm-1 and 252 cm-1 are present in this zone of the spectra. There is no evidence or studies involving high pressure or temperature assignment of vibrational modes in LiAlH4.

4.2 Vibrational Studies

The vibrational spectra of alanates have been studied in the past by several authors. Table 4.2 summarizes the assignments for tetrahedral AlH4- ions in different

crystals both experimentally as well as theoretically using periodic DFT calculations. It is important to note that for the AlH4- group, the totally symmetrical stretching mode

(which is generally the strongest in the Raman spectra) is observed above 1700 cm-1, and the bending modes are observed between 750 and 900 cm-1.

Table 4.2. Raman spectra of the internal modes of AlH4- in crystals.

Compound υ1 υ2 υ3 υ4

KAlH422 1779 790 1711

NaAlH422 1762 772 1683 822

NaAlH4

DFT(GGA)88 1726-1731

744-748, 792-797

1649-

1655,1673-1681

813-

817,847-850 LiAlH422 1836 792 1769

LiAlH4