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Brillouin scattering experiments from room temperature down to liquid helium temperature are reported. Two phase transitions occur in this temperature range at TI=40 K and TII=17 K, but elastic anomalies only happen at TI. Several attempts for the direct phason detection have been found unsuccessful. The elastic anomalies are qualitatively interpreted inside the frame of the Landau theory with simple group theory arguments.
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Écolivet et al. (1983) studied this question.
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