Low Temperature Synthesis and Characterization of AlScMo3O12

Recent interest in low and negative thermal expansion materials has led to significant research on compounds that exhibit this property, much of which has targeted the A₂M₃O12 family (A = trivalent cation, M = Mo, W). The expansion and phase transition behavior in this family can be tuned through the choice of the metals incorporated into the structure. An undesired phase transition to a monoclinic structure with large positive expansion can be suppressed in some solid solutions by substituting the A-site by a mixture of two cations. One such material, AlScMo₃O12, was successfully synthesized using non-hydrolytic sol-gel chemistry. Depending on the reaction conditions, phase separation into Al₂Mo₃O12 and Sc₂Mo₃O12 or single-phase AlScMo₃O12 could be obtained. Optimized conditions for the reproducible synthesis of stoichiometric, homogeneous AlScMo₃O12 were established. High resolution synchrotron diffraction experiments were carried out to confirm whether samples were homogeneous and to estimate the Al:Sc ratio through Rietveld refinement and Vegard's law. Single-phase samples were found to adopt the orthorhombic Sc₂W₃O12 structure at 100 to 460 K. In contrast to all previously-reported A₂M₃O12 compositions, AlScMo₃O12 exhibited positive thermal expansion along all unit cell axes instead of contraction along one or two axes, with expansion coefficients (200-460 K) of αa = 1.7 × 10-6 K-1, αb = 6.2 × 10-6 K-1, αc = 2.9 × 10-6 K-1 and αV = 10.8 × 10-6 K-1, respectively.