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The correlation of phonon frequency co, and phonon damping yj to the ionic conductivity cr has been observed from the temperature dependence of infrared-reflectivity spectra of a Ag3SI single crystal. Two reststrahlen bands and a rapid increase of reflectivity in low-frequency region were observed above T, (=157 K), the P-y phase transition temperature, and described by the dispersion relation due to the phonons and the diffusing ions. The yj exhibits the drastic change at T, and has an anomalously large magnitude above T, . The shifts of y, and co, also showed appreciable changes at about 250 K. Furthermore, the toi of the low-frequency mode in the P phase increases with increasing temperature. We can qualitatively interpret these results from the disorder arrangement of the Ag ion above T, and the coupling of mobile ions above 250 K with phonons near the Brillouinzone boundary.

Resonant ultrasound spectroscopy has been used to measure the complete set of elastic constants of LiKSO 4 over the temperature range of 200 to 300 K including both the hexagonal room temperature phase and the lower temperature trigonal phase. Large step changes are observed in all the elastic constants, except C 13 , at 213 K on cooling and at 243 K on warming. These step changes are associated with the hexagonal/trigonal crystallographic phase transition. The bulk modulus is approximately 15% higher in the trigonal phase than in the hexagonal phase. The hexagonal-phase elastic constants exhibit very little temperature dependence while the trigonal-phase elastic constants show a stronger dependence on temperature. The transitions are noted to be quite sluggish, taking a few hours to equilibrate. The results are described in terms of a Landau-type free energy expansion using two Ising-like order parameters with strong coupling between the two parameters. Biquadratic coupling between the order parameters and strains is shown to account for the step changes in the elastic constants as well as the temperature dependence in the trigonal phase. Coupling linear in the strains is shown to be insufficient to explain the results.

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