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From an experimental determination of the temperature and pressure dependence of the R 25 soft-mode frequency in RbCaF 3 , KZnF 3 and CsCaF 3 , it is shown that the overall temperature-dependent shift of the soft mode is mainly due to the self-energy (the purely anharmonic effect) whereas the quasi-harmonic contribution (the thermal expansion effect) is one order of magnitude lower.
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Ridou et al. (1984) studied this question.
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