Dynamical properties of crystals of MgF2, ZnF2and FeF2. II. Lattice dynamics and infrared spectra

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Abstract
For pt.I see ibid., vol.21, p.2749 (1988). Very careful studies of infrared spectra of crystals of magnesium, zinc and iron fluoride are reported. Using several models and methods, the authors discussed the experimental infrared spectra temperatures from 10 to 300 K in a previous paper. Using a full shell model with both ions polarisable, the optical frequencies and elastic constants are calculated and compared with existing data. Dielectric properties such as effective charges, ionic polarisabilities, dielectric constants and infrared mode strengths are determined. These models are applied to the study of the behavior of B1g, A2u and Eu modes, and C44 and Cs elastic constants with temperature in ZnF. They show that, above 100 K, this behaviour can be explained by the change of some short-range force constants, while, below 100 K, it is necessary to take into account the increase of the effective charge ZF. Lastly the authors have calculated the contribution of the second-order processes to the infrared spectra. Comparison with experimental data allows a precise interpretation to be made of supplementary IR bands. These results explain the difficulties encountered in trying to represent the IR data by a classical oscillator model particularly for the Eu spectrum in ZnF2 and FeF2.