On the possibility of the enthalpy of a Schottky defect decreasing with increasing temperature

The upward curvature of the conductivity or diffusion plots of ionic crystals in the frame of a single mechanism, is attributed to an excessive fall of the Gibbs free energy g per defect. It is shown that, contrary to published suggestions, thermodynamics demands a simultaneous increase of the enthalpy h and the entropy s of the defect with temperature. The excessive fall of g is due to an increasing difference between h and T N .