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Two kinds of phase transitions are considered in the perovskite structure which involve instabilities located either at the Brillouin zone center (ferroelectric transition) or at the zone corners (antiferrodistortive transition). X-ray diffuse scattering and neutron inelastic scattering results are discussed. The steep narrowing of the energy spectrum recently observed in the cubic phase of NaNbO3 close to the transition temperature is related to the cross-over between the 2d and 3d fluctuation regimes.
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Comès et al. (1976) studied this question.
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