Low Temperature Synthesis and Characterization of AlScMo₃O

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 A₂M₃O 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, AlScMo₃O, was successfully synthesized using non-hydrolytic sol-gel chemistry. Depending on the reaction conditions, phase separation into Al₂Mo₃O and Sc₂Mo₃O or single-phase AlScMo₃O could be obtained. Optimized conditions for the reproducible synthesis of stoichiometric, homogeneous AlScMo₃O 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 Sc₂W₃O structure at 100 to 460 K. In contrast to all previously-reported A₂M₃O compositions, AlScMo₃O exhibited positive thermal expansion along all unit cell axes instead of contraction along one or two axes, with expansion coefficients (200-460 K) of α = 1.7 × 10 K, α = 6.2 × 10 K, α = 2.9 × 10 K and α = 10.8 × 10 K, respectively.