On the temperature and pressure dependence of the defect formation volume in ionic crystals

The Schottky formation volume in alkali halides not only exceeds the molecular volume but also depends on temperature and pressure. The coefficient β v for the thermal expansion of the defect-volume has been found in terms of the bulk expansivity β and the temperature dependence of the pressure derivative of the bulk elastic constants. The coefficient βv is much greater than β and increases slightly with temperature. The defect isothermal bulk modulus Bv has also been expressed in terms of the macroscopic bulk modulus B and its first and second pressure derivatives. The value of Bv is much smaller than that of B and decreases with temperature. Furthermore Bv decreases strongly with pressure, about 40 % up to 10 kbar. The temperature and the pressure dependences of Bv and βv have the same trend as required to explain the experimental diffusion and conductivity plots under hydrostatic pressure.