Isotropic negative thermal expansion in ZrW₂O₈ and HfW₂O₈ from 1100 to 1275°C

ZrW₂O₈ (ZrO₂•2WO₃) and HfW₂O₈ (HfO₂•2WO₃) have been the focus of thermal expansion studies due to their isotropic negative thermal expansion (NTE) measured previously at temperatures below 775°C. This work presents measurements of these materials at their thermodynamically stable temperature ranges of 1105 and 1257°C for ZrW₂O₈ and 1105–1276°C for HfW₂O₈, where they were characterized with in situ, powder X-ray diffraction. The linear coefficients of thermal expansion were measured to be −5.52 × 10⁻⁶ and −4.87 × 10⁻⁶°C⁻¹ for ZrW₂O₈ and HfW₂O₈, respectively. The mechanism leading to this NTE is discussed. Powder samples were synthesized by a solution-based process called the organic–inorganic steric entrapment method. In situ characterization in air was carried out at the National Synchrotron Light Source II using a hexapole lamp, optical furnace and the Advanced Photon Source using a quadrupole lamp, optical furnace to achieve elevated temperatures.