Negative thermal expansion and compressibility of Sc_{1- x}Y_xF₃ (x ≤ 0.25)

Scandium fluoride displays isotropic negative thermal expansion (NTE) from at least 10 to 1100 K and retains a cubic ReO₃-type structure over this range; the NTE is most pronounced at low temperatures. Control of thermal expansion was explored by forming Sc_1-xY_xF ₃ (x ≤ 0.25), which were characterized with synchrotron powder diffraction at ambient pressure from 100 to 800 K. The behavior of the solid solutions under pressure (≤0.276 GPa) was also examined while heating from 298 to 523 K. Insertion of the relatively large Y³⁺ ion into ScF ₃ results in a cubic-to-rhombohedral phase transition upon cooling from ambient temperature to 100 K, even at low substitution levels (5%). The coefficient of thermal expansion (CTE) of the solid solutions in the rhombohedral phase is strongly dependent on both composition and temperature; however, above 400 K, where all samples are cubic, the CTE appears to be largely independent of composition. The isothermal bulk modulus and CTE of ScF ₃, but not those of the solid solutions, are independent of temperature and pressure, respectively. Yttrium substitution lowers the bulk modulus, even at temperatures where the samples are cubic. Finally, the solid solutions stiffen upon heating.