Second-order elastic constants of AgCl from 20 to 430 degreesC.

Abstract

The three independent adiabatic second-order elastic constants of AgCl have been measured from 20 to 430\ifmmode^\circ\else\textdegree\fi{}C using the McSkimin pulse-superposition technique. Two single crystals with (110) and (001) axes were used in the measurements. Measurements on the (110) crystal gave the complete set of constants and showed that the longitudinal elastic constant ${C}_{11}^{\ensuremath{'}}=\frac{({C}_{11}+{C}_{12}+2{C}_{44})}{2}$ decreased by 37%, the shear constant ${C}_{44}$ decreased by 15%, and the shear constant ${C}^{\ensuremath{'}}=\frac{({C}_{11}\ensuremath{-}{C}_{12})}{2}$ decreased by 65% over this temperature range. The longitudinal elastic constant ${C}_{11}$ decreased by 45%, the elastic constant ${C}_{12}$ decreased by 31% and the bulk modulus ${B}_{s}=\frac{({C}_{11}+2{C}_{12})}{3}$ decreased by 37%. The (001) crystal was used as a check on the consistency of the measurements. The decreases in the elastic constants are linear, as expected, until approximately 320\ifmmode^\circ\else\textdegree\fi{}C, whereupon ${C}_{11}^{\ensuremath{'}}$, ${C}_{44}$, ${C}_{11}$, ${C}_{12}$, and ${B}_{s}$ begin to decrease more rapidly than linearly and are 6.8, 0.8, 6.0, 9.2, and 8.0%, respectively, below the expected linearity at 430\ifmmode^\circ\else\textdegree\fi{}C. By contrast, the shear constant ${C}^{\ensuremath{'}}$ decreases linearly over the entire temperature range. The elastic constant behavior thus becomes anomalous near the melting point, just like many of the other physical properties of the silver halides. This anomalous behavior may be attributed to the unusually high defect concentration near the melting point. Similar changes in elastic constants are seen in superionic conductors near the transition into the superionic state: a large decrease in ${C}_{11}$, but only small changes in ${C}_{44}$. This may indicate that the silver halides are just starting the transition to the superionic state when the halide sublattice melts and the transition is frustrated.