Controlling the Negative Thermal Expansion and Response to Pressure in ReO₃-type Fluorides by the Deliberate Introduction of Excess Fluoride: Mg_1–xZr_1+xF_6+2x, x = 0.15, 0.30, 0.40, and 0.50

Cubic ReO3-type fluorides often display negative or very low thermal expansion. However, they also typically undergo phase transitions upon cooling and/or modest compression, which are undesirable from the perspective of potential applications. Density measurements and total scattering data for Mg1–xZr1+xF6+2x, x = 0.15, 0.30, 0.40, and 0.50, indicate that the introduction of excess fluoride into cubic MgZrF6 is accompanied by the population of interstitial fluoride sites and the conversion of corner to edge shared coordination polyhedra. Unlike MgZrF6, no phase transitions are seen upon cooling these materials to 10 K, and the first high pressure phase transition in these compositions occurs at pressures much higher than that previously reported for MgZrF6 (0.37 GPa). The introduction of excess fluoride also provides for control of thermal expansion. For all of the compositions studied, negative thermal expansion is seen at the lowest temperature examined, and positive thermal expansion is observed at the highest temperature. The temperature at which zero thermal expansion occurs varies from ∼150 K for x = 0.50 to ∼500 K for x = 0.00. High pressure diffraction also indicates that increasing the amount of excess fluoride elastically stiffens the cubic ReO3 related structure and reduces the extent of pressure induced softening.