Acoustic Investigation of NaBi(MoO_4)_2 and NaBi(WO_4)_2 Crystals at High Temperatures

摘要

The results from investigations of the acoustic properties of NaBi(MoO_4_)2 and NaBi(WO_4)_2 crystals above room temperature are presented. Variations in ultrasound velocity that indicate the occurrence of phase transitions are observed. Phase transitions for NaBi(MoO_4)_2 and NaBi(WO_4)_2 crystals are observed near 309 and 319 K, respectively. It is assumed that these phase transitions are associated with the ordering of Na~+ and Bi~(3+) ions over cation sites. Observed anomalies in elastic properties and sound attenuation allow an interpretation based on the phenomenological Landau theory.