The curvature in conductivity plots of silver halides as a consequence of anharmonicity

At high temperatures the conductivity plot of silver halides shows a pronounced upward curvature to higher conductivity values. This curvature has been attributed to an excessive decrease of the Gibbs formation energy g^f per Frenkel defect. Such a decrease is known to arise from Lidiard-Debye-Huckel interactions and expansivity. However a quantitative evaluation for AgCl and AgBr has shown that the said effect is not sufficient to explain the experimental data. The present paper proposes a further reason for the excessive decrease of g^f; it is due to an excessive fall of the bulk modulus B at temperatures near the melting point. For AgCl a fall by approx. 10% would give the needed decrease of g^f.