Isotropic Low Thermal Expansion over a Wide Temperature Range in Ti1–xZrxF3+x (0.1 ≤ x ≤ 0.5) Solid Solutions

TiF3 exhibits a rhombohedral to ReO3-type cubic phase transformation at ∼340 K. Here we report that, by introducing ZrF4 into TiF3, the cubic phase is stabilized at least down to 123 K in the Ti1–xZrxF3+x compounds. All compounds exhibit low thermal expansion (LTE) between 123 and 623 K, and a nearly zero thermal expansion (ZTE) was obtained in Ti0.7Zr0.3F3.3 (αL = 0.91 ppm/K). The analysis of pair distribution function reveals that the cation-centered octahedra are partially changed to pentagonal bipyramids in Ti1–xZrxF3+x due to the excess fluorines relative to the case of TiF3. Therefore, the cooperative rotation of the polyhedra tends to be restricted, and the cubic phase is thus stabilized. The restrained polyhedral rotations compete against the lattice softening caused by the introduction of Zr4+, giving rise to the LTE. Our present strategy is applicable to other rhombohedral metal trifluorides for the design of new isotropic ZTE materials.