Crystal structure and compressibility of a high-pressure Ti-rich oxide, (Ti0.50Zr0.26Mg0.14Cr0.10)O1.81, isomorphous with cubic zirconia

A Ti-rich oxide, (Ti0.50Zr0.26Mg0.14Cr0.10)∑=1.0O1.81, was synthesized at 8.8 GPa and 1600 °C using a multi-anvil apparatus. Its crystal structure at ambient conditions and compressibility up to 10.58 GPa were determined with single-crystal X-ray diffraction. This high-pressure phase is isomorphous with cubic zirconia (fluorite-type) with space group Fm3¯m and unit-cell parameters a=4.8830(5) Å and V=116.43(4) Å3. Like stabilized cubic zirconia, the structure of (Ti0.50Zr0.26Mg0.14Cr0.10)O1.81 is also relaxed, with all O atoms displaced from the (14, 14, 14) position along 〈1 0 0〉 by 0.319 Å and all cations from the (0, 0, 0) position along 〈1 1 1〉 by 0.203 Å. No phase transformation was detected within the experimental pressure range. Fitting the high-pressure data (V vs. P) to a third-order Birch-Murnaghan EOS yields K0=164(4) GPa, K′=4.3(7), and V0=116.38(3) Å3. The bulk modulus of (Ti0.50Zr0.26Mg0.14Cr0.10)O1.81 is significantly lower than that (202 GPa) determined experimentally for cubic TiO2 or that (~210 GPa) estimated for cubic ZrO2. This study demonstrates that cubic TiO2 may also be obtained by introducing various dopants, similar to the way cubic zirconia is stabilized below 2370 °C. Furthermore, (Ti0.50Zr0.26Mg0.14Cr0.10)O1.81 has the greatest ratio of Ti4+ content vs. vacant O2− sites of all doped cubic zirconia samples reported thus far, making it a more promising candidate for the development of electrolytes in solid oxide fuel cells.