Phase Diagram and Compositional Studies of Sc2-x-yFexAlyW3O12: Discovery of New Zero and Low Thermal Expansion Materials.

This work reports on the discovery of new materials in the Sc2-x-yFexAlyW3O12 family. Members with compositions containing high concentrations of Fe are found to crystallize to a structure in the monoclinic P21/a symmetry, while compositions with low values of Fe and Al crystallize in the orthorhombic Pnca structure at room temperature. The orthorhombic samples in the compositional range x = 0.5, y ≤ 0.5 are found to transition to the monoclinic P21/a structure at 210-230 K. Specifically, Sc1.25Fe0.5Al0.25W3O12 is found to exhibit a volumetric coefficient of thermal expansion (CTE) of -0.48(7) × 10-6 K-1 over the range 300-1000 K, a near-zero thermal expansion material-this CTE value is lower than precision invar variants and it is persistent over seven times the temperature range. Increasing the Al concentration to y = 0.75 increases the CTE to 4.69(8) × 10-6 K-1 over the range 300-1000 K and decreasing it to y = 0 decreases the CTE to -4.22(6) × 10-6 K-1 over the range 300-1000 K. This work probes the phase-space with an intention to correlate doping regimes to the physical property of volume change.