Phase Stability of Beta-Gallia Rutile Intergrowths

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Alfred University. Faculty of Materials Science and Engineering. Kazuo Inamori School of Engineering
Beta-gallia rutile intergrowths are crystal structures consisting of a β-gallia component and a rutile-structured oxide component in the stoichiometry Ga_{4}Mn_{n-4}O_{2n-2} where M is Ge, Sn or Ti, and n ≥ 5. This structure is of interest as an ion-storage material due to the 2.5Å hexagonal tunnels suitable for the intercalation of small to medium sized cations such as Li+. The purpose of this study was to attain low n-value intergrowths due to their higher tunnel density, using a reducible rutile-structured component. Two systems were studied. The Ga_{2}O_{3}-Al_{2}O_{3}-TiO_{2} system was studied because Ti4+ is reducible. Previous research showed that only higher n-value intergrowths were stable in the Ga_{2}O_{3}-TiO_{2} system; therefore Al^{3+} was added in an attempt to stabilize lower n-value phases. Solid state synthesis at 1400ºC was used to prepare samples with the stoichiometry Ga_{4-4x}Mn_{n-4}O_{2n-2} combinations of n ranging from 4 to 29 and x ranging from 0 to 1 and x-ray diffraction was used to characterize samples. Results show that while the addition of Al3+ decreases the cell volume of the β-gallia, pseudobrookite and corundum structures, it does not stabilize intergrowth structures with n ≤ 17. For those samples prepared as n = 5, x = 0.1, 0.3, 0.5, 0.7, and 0.75, β-gallia and pseudobrookite were formed, while samples prepared as n = 5, x = 0.85, 0.90 and 1.0 corundum and pseudobrookite were formed and the sample prepared as n = 5, x = 0.80, was triphasic with corundum, pseudobrookite and β-gallia. Samples prepared as n = 7, 9, 15 and 17 with x = 0.1 and n = 9, x = 0 were biphasic with n = 17 intergrowth and pseudobrookite, whereas samples prepared with the same n-value and x = 0.5 contained pseudobrookite and n = 33 intergrowth. Samples prepared as n = 7, 9, 15, 17, 21 and 25, x = 0.3 were triphasic with pseudobrookite and n = 17 and n = 21 intergrowth. The sample prepared as n = 9, x = 0.9 contained rutile and pseudobrookite. Samples prepared as n = 15, 17, 19, 21, 23, 25 and 27, x = 0, n = 17, 25 and 27, x = 0.1, n = 25 and 29, x = 0.2 all contained phase pure intergrowths whereas n = 19, 21, x = 0 contained n = 21 intergrowth and the rest contained n = 33 intergrowth. The sample prepared as n = 25, x = 0.5 was triphasic with n = 33 intergrowth, rutile and pseudobrookite. The sample xi prepared as n = 25, x = 0.7 was biphasic with pseudobrookite and rutile. An isothermal phase diagram is presented. The Ga_{2}O_{3}-MnO_{2} system was chosen because manganese, like Ti4+ is well-known as being reducible and is widely used in the battery industry. Hydrothermal synthesis was used to prepare samples with n = 5, 7 and 9, pH ranging from 0.7 to 9.8 with temperatures of 150º-235ºC and autogenic pressures. Samples were characterized with xray diffraction, thermal gravimetric analysis, differential scanning calorimetry and transmission electron microscopy. Results showed a strong correlation between pH, temperature and phases attained. Samples prepared with a pH < 7 contained diaspore as a major phase, and samples prepared as pH > 7 generally contained spinel as a major phase. Four samples prepared as n = 9 with pH > 9 and processing temperature less than 170ºC did not contain spinel but unknown major phases. One sample prepared as n = 9, pH = 6.6 and processing temperature of 225ºC contained rutile in addition to diaspore and one sample prepared as n = 9, pH = 8.7 and processing temperature of 230ºC contained bixbyite in addition to spinel. Unknown minor phases were a function of processing temperature. Analysis of thermal analysis data and x-ray diffraction after thermal events was able to yield possible transitions occurring in known major phases and was not able to determine the composition of the unknown major phases or transitions occurring.
Rutile, Gallia, Phase stability, Titanium oxide ore, Ceramics