Heat capacity and structure of Rb_2KMeO_3F_3 (Me: Mo, W) elpasolites

摘要

X-ray and heat capacity measurements were performed on RD2KM0O3F3 and RD2KWO3F3 single crystals. A significant difference in the thermal parameters of atoms in 4a and 8c sites of Fm-3m structure was found, which depended on the central atom. Calorimetric measurements have revealed the heat capacity anomalies at 195 and 67 K, respectively, in RD2KM0O3F3 and Rb_2KWO_3F_3. The standard molar enthalpy and entropy were determined. Entropy changes associated with phase transitions in oxyfluorides under study and related Rb_2KTiOF_5 are discussed.