The cubic to rhombohedral phase transition is considered within the Girshberg‐Tamarchenko approach for the vibronic anharmonicity of the soft TO modes and a Landau free energy expansion for the strictive coupling of optic and acoustic modes. It turns out, that one needs a not too simple model for the lattice repulsion forces for a comprehensive description of the phase transition. The spontaneous rhombohedral shear below T c appears as secondary (induced) order parameter. The relation to the work of Littlewood is discussed.
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P. Enders (1984) studied this question.
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