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 expansi on of the perfect crystal, and contributions from lattice defects at h igh 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 bet ween real and perfect crystals. Results show that there is a linear tr end between the formation energies of thermal lattice defects and thei r Debye temperatures. The formation energies obtained are smaller than theoretical calculations and ionic conductivity measurements. This in dicates that thermal defects are ordered at high temperatures.