Evolution of Negative Thermal Expansion and Phase Transitions in Sc_1-xTi_xF₃

The cubic ReO 3 -type material ScF 3 exhibits strong isotropic negative thermal expansion (NTE) over a wide temperature range while remaining cubic. Control of its thermal expansion was investigated by forming Sc 1- x Ti x F 3 solid solutions, which were characterized by synchrotron powder diffraction at ambient pressure from 100 to 500 K. TiF 3 is fully soluble in ScF 3 at a synthesis temperature of 1338 K. The temperature for the cubic-to-rhombohedral phase transition in Sc 1- x Ti x F 3 varies linearly with composition (above 100 K), and, at large x, the transition is clearly first-order. The rhombohedral phase for each composition examined exhibits strongly positive thermal expansion, while the expansion of the cubic phase (between 420 and 500 K) is negative for all Sc 1- x Ti x F 3 .