Sistemas Críticos

2.7.1 Introduction

Atoms in solids vibrate at frequencies of 1 0 to 10*^ Hz [30]. These vibrations can be excited to higher energy states by the absorption of infrared radiation. In this work, infrared spectroscopy was used to determine the effect of temperature driven electronic transitions on the infrared absorption properties of the materials. It was been found that electronic transitions can be observed by changes in the infrared spectrum; they hence give a good indication as to the optical switching properties of the material as the temperature is increased.

2.7.2 Theory

The infrared region of the electromagnetic spectrum extends from the red end of the visible spectrum to the microwave region i.e. 14,000 to 20 cm '\ However, the most informative region for this study is the mid-IR region, 4000 to 50 cm '. Infrared vibrational spectroscopy involves the examination of the twisting, bending, rotating and vibrational motions of bonds within the crystalline lattice (Figure 2.13).

Figure 2.13 Stretching, deformation and rotational modes

The energy of a quantum harmonic oscillation is given by

E = V H---

h

2 7 1 \h

(32)

Where h is Planck’s constant, k is the bond strength, \i is the reduced mass of the molecule and v is the vibrational quantum number. At room temperature, most molecules are in the ground state (v=0) and promotion to the first excited state (v=l) requires the absorption of infrared energy.

The number and nature of absorption bands observed in the vibrational spectrum depends upon the number of atoms present in one unit of the material and its symmetry [31]. Group theory allows us to undertake a detailed analysis of allowed vibrational modes of the atoms within the unit cell (detailed descriptions can be found in many text books). However, in this work the infrared spectra were only used as a tool to investigate the manifestations of the electronic phenomenon and to determine how they can influence the infrared spectrum.

2.7.3 Experimental

Infrared vibrational measurements were carried out using a Nicolet Instruments 760 Magna FT-IR Spectrometer at a resolution of 4cm'^ and 32 scans were collected. A

DTGS KBr detector and a XT-KBr beam splitter was used for the Mid-IR measurements.

Samples of known mass (~lmg) were ground in a pestle and mortar; FTIR grade KBr (~200mg) was added and the materials ground together. They were then pressed in a Die (13mm in diameter) producing a disc at a pressure of 10 tonnes. This disc was then weighed, allowing the spectra to be normalised by weight to permit direct comparisons of peak intensities. To ensure a constant environment within the sample chamber and the spectrometer, a dry source of air flows through the spectrometer; CO2 was also removed from the air. The sample was placed in the spectrometer and

the spectrum obtained over the range 350 to 4000cm"\ For far infrared measurements, FTIR grade polyethylene was substituted for the KBr matrix. A spectrum was obtained over the range 50 to 700 cm"\ It is not possible to perform the far IR measurement quantitatively due to the nature of the polyethylene. A solid substrate beam splitter and DTGS polyethylene detector was used for the far infrared measurements.

Infrared spectra as a function of temperature were obtained using a modified Oxford Instruments cryostat with a temperature range of 12-800 K. The cryostat was sealed and evacuated to a pressure of ~10‘^ mbar using an Edwards Pico Dry Turbo pump. The temperature was controlled by an Oxford Instruments ITC4. The cryostat contains a cold head, which is cooled by a displex pump, a sample head, which contains a heating cartridge. The temperature controller balances the degree of heating and cooling of the sample to give a steady sample temperature. A bellows unit allows the same cryostat to be used for sub and post ambient temperatures. At sub-ambient

temperatures the bellows are filled with helium gas and drive the cold head onto the sample head. At higher temperatures the bellows are removed by vacuum. The cryostat was suspended above the infrared spectrometer and the background taken without the sample in the sample head after evacuation. The background was saved and used for each temperature measurement. The sample was then loaded into the cryostat evacuated and the sample head cooled to 12 K. Spectra were recorded at approximately 25 K intervals starting at 12 K up to a temperature of 800K.

In the next four chapters we now go onto to describe the structural and physical properties of the perovskites under study using the techniques detailed in this chapter.

2.8 References

[1] Moritmer R., Powell J., Vasantchacharya N.; J. Phys. Condens. Matter^ 9, 11209 (1997)

[2] Catlow C.R.A.; Applications o f Synchrotron Radiation, Chapter 2 (Eds CRA Catlow, GN Greaves) Blackie (1990)

[3] Rietveld H.M.; J. Appl Cryst. 2, 65 (1969)

[4] Werner P.E., Erikson L., Westdale M., J. Appl Cryst.^ 18, 367 (1985)

[5] The Rietveld Method (Ed R.A. Young) Oxford University Press (1993)

[6] McCusher L.B., Von Dreele R.B., Cox D.E., Louer D., Scardi P.; J. Appl Cryst,

32, 36 (1999)

[7] Larson A C., Von Dreele R.B.; GSAS Report No. LA-UR-86-740, Los alamos National Library N M (1987)

[8] Teo B.K.; EXAFS: Basic Principles & Data Analysis, Springer Verlag, Berlin (1986)

[9] Wong J.; Materials Science Engineering, 80,107 (1986)

[10] Gurman S.J.; Applications o f Synchrotron Radiation, (Eds C.R.A. Catlow G.N. Greaves) Blackie, (1990) Chapter 6

[11] Gurman S.J.; J. Synchrotron Rad., 2 56 (1995)

[12] Binstead N.; Excurv98 CCLRC Daresbury Lab Computer program (1998) [13] Gurman S.J., Binstead N., Ross I.; J. Phys. C: Solid State Phys., 17 143 (1984) [14] Rehr J.J., Albers R.C.; Physics Review B, 41, 8139 (1990)

[15] Rehr J.J., Albers R.C., Zabinsky S.I.; Physics Review Letters, 69, 3397 (1992) [16] Rao C.N.R., Gopalkrishnan J.; New Directions in Solid State Chemistry

Cambridge (1997)

[17] Cheetham A.K., Day P.; Solid State Chemistry - Techniques, Oxford Science Publications (1987)

[18] Cox P.A.; Transition Metal Oxides, Oxford (1995) [19] Stqewski E., Giordano N.; Adv Phys, 26,487 (1977)

[20] Gerloch M.; (1986) Orbitals, terms and states, Wiley, Chichester [21] Burdett J.K.; Molecular Shapes, Wiley, New York (1980)

[22] Goodenough J.B.; Prog. Solid State Chem., 5,145, (1971)

[23] Cox P. A.; The electronic structure and chemistry o f solids, Oxford University Press (1987)

[24] Rosenberg H.M.; The Solid State Oxford Science Publications (1997) [25] West A.; Basic Solid State Chemistry, Wiley (1996)

[27] Banwell C.N.; Fundamentals o f Molecular Spectroscopy, 3^^ Edn., McGraw-Hill (1983)

Chapter 3