Sound velocities and dielectric losses ofAg2HgI4in the vicinity of the phase transition

We observed an anomaly in the velocity of longitudinal sound waves at the 51 °C order-disorder phase transition of ${\mathrm{Ag}}_2$Hg${\mathrm{I}}_4$, but not in the velocity of transverse waves. We found two relaxation peaks in the dielectric losses above the transition temperature; only one relaxation appears in the low-temperature phase. We interpret the anomaly in the sound velocity as due to a linear coupling between the order parameter and the volume change. We find a Debye temperature of 82 °K. The observation of the dielectric relaxations favors a model of electrical conduction with thermal-activated motion of ${\mathrm{Ag}}^{+}$ ions in contrast to the proposed polaron model for ${\mathrm{Ag}}_4$Rb${\mathrm{I}}_5$, in which the ${\mathrm{Ag}}^{+}$ ions are free to move in the lattice.