Elastic constants of a superionicα-AgI single crystal determined by Brillouin scattering

For the first time Brillouin scattering experiments on single-crystal α-AgI have been performed. The spectra reveal all three acoustic modes of a cubic crystal, without any observable damping of the transverse modes in contrast to neutron scattering observations. The elastic constants were determined by fitting the observed Brillouin frequency shifts to the solutions of the dynamical equations for a cubic crystal of arbitrary but known orientation; the orientation relative to the light scattering configuration was established by transmission Laue x-ray diffraction. The obtained elastic constants, $C_{11}$=1.48, $C_{12}$=1.29, and $C_{44}$=0.63 in ${10}^{10}$ N ${\mathrm{m}}^{\mathrm{-}2}$, are noticeably small compared to those of low-conductivity β-AgI. The results reflect the disordered structure of α-AgI and are typical values of superionic conductors. The value of $C_{44}$ is similar to those of other ${\mathrm{Ag}}^{+}$-conducting systems, i.e., AgCl and AgBr, while $C_{11}$-$C_{12}$ is much smaller for α-AgI, in support of a suggested structure-related model to explain the high conductivity. Indications of a structural transition were found in a temperature range around 640 K, i.e., in the range of a proposed cationic order-disorder transition. This is demonstrated by the behavior of the longitudinal mode, which rapidly decreased in frequency, accompanied with increased intensity and increased linewidth in the proposed transition range.