Phase transition studes in selected selenate crystals

Abstract

The study of phase transitions in crystals is an interesting area of research in solid-state physics. Even though a large number of experimental and theoretical works have been done in this area a large number of questions still remain unanswered regarding various crystals. The aim of the present study is to investigate phase transitions in certain Selenate crystals. A variety of experimental techniques can be used for the investigation, like the study of its electrical properties, elastic properties, thermal properties etc. Appropriate technique is to be selected depending on the chosen sample. Depending on the type of phase transition in the crystal various physical properties of the crystal undergo anomalous changes near the transition temperature. We have chosen electrical conductivity and dielectric studies to investigate the phase transition. Phase transitions occur when a material undergoes a physical change from one state to another. Phase transition from one solid state to another occurs when there is a rearrangement of molecules that results in a change in the material's lattice structure. The phase transitions in solids are accompanied by interesting changes in many of material properties. Measurement of any sensitive property across the phase transition provides a mean of investigating the transition. Changes in properties at the phase transition are often of technological interest. Frequency dependant measurements of the ac conductivity have been recognized as an important tool for the study of ionic transport properties of the solids. For this, the conductivity data of solid electrolytes are generally analyzed in terms of equivalent circuits composed of frequency independent resistances and capacitances. In most of the cases the ac response cannot be simulated correctly by simple R-C circuits, e.g. the cases of depressed semicircles in the impedance formalism. To explain such deviation from the ideal behaviour various empirical functions have been formulated which are helpful to fit the experimental data. In recent years Johnscher's universal law of dielectric response has been used to model the observed properties of the solid electrolytes. The dielectric behaviour of materials under an external field has been the focus of numerous investigations, in view of its high scientific and technological importance. Measurements are made in a very wide range of frequencies and temperatures, and for different materials. In this thesis we have measured the ac conductivity, dc conductivity and dielectric properties of some selected selenate crystals-Zinc Selenate Hexahydrate (ZnSe04.6H~0), Zinc Selenate Monohydrate (ZnSe04.HzO), Magnesium Selenate Hexahydrate (MgSe04.6HzO). Measurements of electrical conductivity and dielectric properties are powerful tool to investigate the phase transition present in the crystal. These studies are reporting for the first time in the above said selenate crystals. The variation of these properties with temperature gives an insight in to the physical properties of these crystals. Measurements are carried out along [I 001, [OlO] and directions.