γ-ray diffraction in the vicinity of a ferroelastic transition: application to 'real crystal' of RbH2PO4and KH2PO4

The shape and the intensity of gamma -ray Bragg peaks in the vicinity of the ferroelastic-ferroelectric phase transition in RbH 2 PO 4 and KH 2 PO 4 are discussed. They reveal a distribution of the transition temperature and of the stress conjugate to the order parameter within the sample. The magnitudes of these distributions are evaluated. The temperature dependence of the diffracted intensity in the paraelectric phase is related to a severe extinction problem. The present study shows that this effect, which is often considered as an inconvenience in diffraction studies, can be favourable in the case of a phase transition. The quantitative estimates are consistent with a thermodynamic Landau-type model of the transition and how the ability of the extinction model over a large range of crystal perfection.