Zero Thermal Expansion and Abrupt Amorphization on Compression in Anion Excess ReO₃‑Type Cubic YbZrF₇

Heat treatment of cubic YbZrF₇, after quenching from 1000 °C, leads to a material displaying precisely zero thermal expansion at ∼300 K and negative thermal expansion at lower temperatures. The zero thermal expansion is associated with a minimum in the lattice constant at ∼300 K. X-ray total scattering measurements are consistent with a previously proposed model in which the incorporation of interstitial fluoride into the ReO₃-related structure leads to both edge and corner sharing coordination polyhedra. The temperature dependence of the experimental pair correlation functions suggests that the expansions of edge and corner sharing links partly compensate for one another, supporting the hypothesis that the deliberate incorporation of excess fluoride into ReO₃ structure materials can be used as a design strategy for controlling thermal expansion. Cubic YbZrF₇ has a bulk modulus, <i>K</i>₀, of 55.4(7) GPa and displays pronounced pressure-induced softening [<i>K</i>₀′ = −27.7(6)] prior to an abrupt amorphization on compression above 0.95 GPa. The resulting glass shows a single sharp scattering maximum at <i>Q</i> ∼ 1.6 Å^–1