THERMAL DEFECTS AND THERMAL-EXPANSION OF IONIC-CRYSTALS AT HIGH-TEMPERATURES

Thermal expansion of ionic crystals at high temperature is treated as contributions from the perfect crystal and its lattice thermal defects. A semi-empirical quasi-harmonic model represents the thermal expansion of the perfect crystal, and contributions from lattice defects at high temperature are described by the theory of thermodynamics of point defects. The formation energies of thermal defects of those crystals considered are obtained from fitting thermal expansion differences between real and perfect crystals. Results show that there is a linear trend between the formation energies of thermal lattice defects and their Debye temperatures. The formation energies obtained are smaller than theoretical calculations and ionic conductivity measurements. This indicates that thermal defects are ordered at high temperatures.