Our work on metastable oxides (Chapter 7) has culminated into an impactful research project despite being fairly secondary to my primary research focus (i.e. oxygen catalysts). As reported in Chapter 7, we were able to fully demonstrate a high-throughput methodology for form- ing and mapping metastable bismuth oxide (Bi2O3) over extremely rapid annealing conditions
using Robert Bell’s lateral gradient laser spike annealing (lgLSA) method.[13] While interests in Bi2O3 were driven by the application value of the delta-phase cubic polymorph (δ-Bi2O3) due
the structure’s incredibly high ionic conductivity, the broad utility of lgLSA studies expanded the scope to a broader systematic study on unknown or predicted metastable structures. Addi- tional solid oxide systems were studied using this high-throughput lgLSA method, which included metastable BaTiO3, MnFeTiO3 (varying composition), LiMnO2, Na6PbO5, Li4WO5, Li2MnWO6,
and MnTiO3. These oxide materials were selected from a computational study performed by
Jong et al., who computed the piezoelectric tensors of almost a thousand distinct inorganic com- pounds with various crystallographic symmetries, many of which were metastable and only com- putationally predicted, using a density functional perturbation theory model as a first-principle calculation.[44]
Each oxide system was synthesized using a simple ball-mill mixing method (this wasn’t done for all mixtures) and subsequently pressed into 1” diameter disks. Each target was placed in a tube furnace, undergoing solid-state sintering between 1000-1100 °C for 24 hours. Because there was a significant time constraint when synthesizing each of these targets prior to our CHESS run, there was not much time devoted to researching specific synthesis routes for each mixed oxides system, and consequently each oxide powder mixture underwent the same mixing and sintering
procedure. This was concerning because the chosen synthesis route may not be well-tailored to every oxide sample due to varying factors associated with powder morphology regarding particle size, wettability, thermal expansion, vapor pressure, etc. This was notably concerning for LiMnO2,
Li4WO5, and Li2MnWO6, which contain light-weight Li alkali metal, rendering these mixtures sus-
ceptible to Li-loss during sintering. It’s not known how much Li is lost during the high temperature sintering step, but I did compensate by adding an additional 5 wt% to the mixture. Overall, the tar- get compositions or compositional uniformities are not truly known. Thus, the quality of each film is predicated on the assumption that the oxide powders were well-mixed and sintered uniformly.
Each target was deposited onto 4” lgLSA calibrated silicon wafers using pulsed laser de- position (PLD). Each film then underwent a series of lgLSA scans, each corresponding to specific annealing condition based on peak temperature and anneal time. A series of XRD measurements were then taken laterally across the width of each laser scan using the fixed angle area XRD method at CHESS (See Chapter 4 and 7 for details on the XRD setup). We came across several interest- ing XRD patterns, some of which didn’t match any reported polymorphic structures. However, the difficulty of de-convoluting the measured diffraction peaks were compounded by systems with predicted low symmetry metastable structures, or systems comprised of multiple phases. This has prompted close collaborations with crystallographers and computer scientists to streamline XRD data processing based on computationally predicted structures.
MnTiO3
Among the previously listed piezoelectric materials that were measured, prelminary re- sults for MnTiO3were the most interesting. Over time, Dr. Robert Bell gained greater ownership of
this project.[12] Thus, his thesis provides a much more detailed summary on metastable MnTiO3.
To quickly summarize, MnTiO3has three reported phases (See Table 8.1). MnTiO3-I is the stable
atmospheric phase while the MnTiO3-II is a stable high pressure (∼6 GPa) phase.[114, 197] Both
Name Symmetry Point Group
Mineral Terms Space Group
Space Group Name MnTiO3-I Trigonal -3 Ilmenite
FeTiO3Type
148 R-3
MnTiO3-II Trigonal 3m LiNbO3Type 161 R3c
MnTiO3-III Orthorhombicmmm Perovskite
GdFeO3
62 Pbnm
Table 8.1: Polymorphs of MnTiO3.
MnTiO3-III is a metastable orthorhombic phase.
In the same manner in which we measured metastable bismuth oxide (Chapter 7), we obtained multiple XRD images across the lateral lgLSA profile of several laser scans. The XRD images were then integrated and stacked to generate a diffraction map versus Q-space (Figure S.4a). Each subfigure corresponds to the full XRD map across the width of the laser scane at a specific dwell time, all of which have peak temperatures of 1000 °C. These peaks were then de-convoluted in order to generate the phase map as a function of temperature and dwell time (Figure S.4b). At 5ms, there is a primary peak that matches the peaks corresponding to the trigonal MnTiO3-I and II, which are only differentiated by their relative peak intensities. For dwell times
below 5 ms, the pattern shows a primary peak near 2.3 Å-1and a secondary peak near 2.6 Å-1, none of which are matched with any known phase of MnTiO3. Moreover, we were able to confirm that
the 2.4 Å-1 peaks were not missing due to texturing as evidenced by the uniform diffraction ring intensities. Currently, we were not able to determine the base structure of this ostensibly MnTiO3
phase, which we labeled as MnTiO3-IV. Below 1000 °C at every measured dwell time, the film
appeared to remain amorphous. At higher temperatures of around 1250 °C with longer dwell times, the films formed a mixture of MnTiO3-I and II, which we distinguished by the measured peak
(a) (b)
Figure S.4: (a) Diffraction measures across multiple laser scans with peak temperatures of 1000 °C as a function of distance from each scans center, reciprocal vector, and logarithmic intensity (color). The dwell time for each laser scan is listed on the right. At the bottom are diffraction pat- terns from the ICSD database for MnTiO3-I (black)[114], MnTiO3-II (red)[114], and MnTiO3-III
(blue)[197]. All three previously reported phases have a large peak at ∼2.4 Å-1 which does not appear in our results until 5 ms, even for patterns that show an additional peak near 2.6 Å-1. The
peak near 1.8Å-1 is convoluted with a silicon peak known to appear at long dwells and high tem- peratures. (b) Temperature dwell transformation (TDT) diagram of predominant phase at different conditions. Each square is a discrete annealing condition measured. Filled backgrounds are used to denote regions with the same phases.[12]
This is currently an ongoing study, leaving open the prospect of a newly discovered MnTiO3-IV
phase.