2ND-ORDER ELASTIC-CONSTANTS OF AGCL FROM 20 TO 430 DEGREES-C

The three independent adiabatic second-order elastic constants of AgCl have been measured from 20 to 430 degrees 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 elasti c constant C-11'=(C-11+C-12+2C(44))/2 decreased by 37%, the shear cons tant C-44 decreased by 15%, and the shear constant C'=(C-11-C-12)/2 de creased by 65% over this temperature range. The longitudinal elastic c onstant C-11 decreased by 45%, the elastic constant C-12 decreased by 31% and the bulk modulus B-s=(C-11+2C(12))/3 decreased by 37%. The (00 1) 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 degrees C, whereupon C-11', 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 degr ees C. By contrast, the shear constant C' decreases linearly over the entire temperature range. The elastic constant behavior thus becomes a nomalous near the melting point, just like many of the other physical properties of the silver halides. This anomalous behavior may be attri buted to the unusually high defect concentration near the melting poin t. Similar changes in elastic constants are seen in superionic conduct ors 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 silv er halides are just starting the transition to the superionic state wh en the halide sublattice melts and the transition is frustrated.