Using Defects in Anion Excess Rhenium Trioxide Type Fluorides to Control Thermal Expansion; Ytterbium Zirconium Fluoride as a Case Study

Abstract

The average structure, thermal expansion, behavior on compression, high temperature stability, and moisture sensitivity of YbZrF₇, an anion-excess ReO₃-type material, was explored. Cubic YbZrF₇ (a = 4.10 Å) was synthesized by a fast quench from 1000 °C. Powder x-ray diffraction and density measurements showed reasonable agreement with prior work by Poulain, et. al. Elemental analysis confirmed stoichiometry. Attempts to synthesize monoclinic YbZrF₇ by a slow-cool from 675 °C resulted in a mixed-phase product, suggesting an incomplete reaction. Variable temperature powder x-ray diffraction performed at the Advanced Photon Source (17-BM) identified anomalous volumetric expansion behavior in cubic YbZrF₇ including a dependence on thermal history. When the material is initially heated above 310 K, negative thermal expansion persists to approximately 350 K. This is probably associated with an irreversible change in local structure due to the migration of fluoride. Zero thermal expansion close to 300 K was consistently evident on cooling from 500 K, while negative thermal expansion was observed below room temperature. Cubic YbZrF₇ underwent an abrupt amorphization on compression to ~ 0.9 GPa. This was preceded by a pressure-induced softening. The latter has been previously observed for similar ReO₃-type fluoride materials that display negative thermal expansion. Cubic YbZrF₇ begins to thermally decompose above 700 °C in dry nitrogen, where ZrF₄ is sublimated. Under ambient conditions, cubic YbZrF₇ appears to be air stable for at least 24 hours. For temperatures above 300 °C, it is susceptible to hydrolysis when exposed to moist air exceeding 43 percent relative humidity.