Thermal expansion of LiF by X-ray diffraction and the temperature variation of its frequency spectrum

The coefficients of thermal expansion of LiF are determined at different temperatures using a diffractometer, Geiger counter, chart recorder and a specially designed furnace. Equations are obtained for the variation of lattice constants with temperature. The temperature dependence of the thermal expansion at high temperatures is shown to be related to the concentration of thermally generated Schottky defects. The energy of formation of Schottky pairs is found to be 2.42 eV. The Grüneisen constant γ is found to decrease with temperature. The mean frequency of the vibrational spectrum is found to decrease with temperature and the variation between 300 and 1000°K is about 13%.