Ultrasonic Investigation of Elastic Anomalies in Lithium Sodium Sulphate Hexahydrate Single Crystal

Abstract

An extensive study of the thermal properties of Lithium Sodium Sulphate Hexa hydrate (LSSW) single crystal, with Trigonal structure, has been carried out using ultrasonic Pulse Echo Overlap (PEO) technique, Differential Thermal Analysis (DTA) and Thermo Gravimetric Analysis (TGA). The temperature variation of elastic constants of LiNa 3 (SO 4 ) 2 · 6H 2 O single crystal have been reported for the first time. The second order elastic stiffness constants C 11 , <i>C</i>₃₃, <i>C</i>₄₄, along the various directions in the crystal have been determined in the temperature range 300 - 330 K. The change in velocity with temperature with respect to the room temperature value has been measured using PEO technique. Significant anomalies were observed in <i>C</i>₁₁ and <i>C</i>₃₃ at 316 K. The elastic constant <i>C</i>₁₂ has shown no variation in the temperature range 300 - 319 K. A minor deviation for <i>C</i>₄₄ at 305 K following a parabolic change has been observed. The minor anomalies observed in the elastic constants of LSSW may be due to its dehydration of water of crystallization in the range 304 - 319 K. DTA studies showed an appreciable endothermic change in the range 309 K-369.79 K. TGA curve exhibited a decrease in weight of 1.687 mg in the temperature range 304 K-360 K. The minor anomalies observed in the elastic constants of LSSW may be due to loosing of its water of crystallization in the range 309 - 319 K. On loosing water there will not be any change in chemical structure but there will be physical change associated with loosing of water molecule.