No abstract

A 2 M 3 O 12 materials (A = Al, Sc, Fe, In, Ga, Y; M = Mo, W) were synthesized using the non-hydrolytic sol-gel process. In contrast to previous studies, in which amorphous materials were recovered that required heating to 500-700°C to induce crystallization, several compounds could be crystallized in a temperature range compatible with low temperature process-

The high pressure behavior of negative thermal expansion materials continues to be of interest, as their potential use in controlled thermal expansion composites can be affected by irreversible pressure-induced phase transitions. To date, it is not possible to predict the high pressure behavior of these compounds, necessitating measurements on each composition. In this work, high pressure synchrotron powder X-ray diffraction studies of Cr2Mo3O12 and Y2Mo3O12 were conducted in a diamond anvil cell. Chromium molybdate, which adopts the monoclinic P21/a structure under ambient conditions, was found to not undergo any crystalline-crystalline transitions up to 8.9 GPa. The orthorhombic ambient pressure polymorph of yttrium molybdate was found to undergo a phase transition to the monoclinic P21/a scandium tungstate structure below 0.13 GPa. This structure is frequently observed for related materials at low temperatures, but has never been reported for Y2Mo3O12. No additional changes in this material were observed up to 4.9 GPa. The fact that the monoclinic polymorphs of these materials do not undergo phase transitions within the studied pressure range makes them unique among A2M3O12 materials, as most isostructural compositions undergo at least one phase transition to crystalline high pressure phases.

High pressure powder X-ray diffraction studies of several A 2 Mo 3 O 12 materials (A 2 = Al 2 , Fe 2 , FeAl, and AlGa) were conducted up to 6-7 GPa. All materials adopted a monoclinic structure under ambient conditions, and displayed similar phase transition behavior upon compression. The initial isotropic compressibility first became anisotropic, followed by a small but distinct drop in cell volume. These patterns could be described by a distorted variant of the ambient pressure polymorph. At higher pressures, a distinct high pressure phase formed. Indexing results confirmed that all materials adopted the same high pressure phase. All changes were reversible on decompression, although some hysteresis was observed. The similarity of the high pressure cells to previously reported Ga 2 Mo 3 O 12 suggested that this material undergoes the same sequence of transitions as all materials investigated in this paper. It was found that the transition pressures for all phase changes increased with decreasing radius of the A-site cations.