Investigation of the mosaic structure of RbI-single crystals in the NaCl-phase near the transition point to the high pressure CsCl-phase

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Abstract
Pressure-induced changes of the mosaic structure in RbI crystals, with a thermodynamic equilibrium between the low-pressure NaCl-phase and the high-pressure CsCl-phase at 3.56 kbars, have been investigated by means of 412 keV-γ-rays at pressures up to 5.1 kbars. It has been shown that the actual transition in the four crystals under investigation took place between 4.7 kbars and 5.1 kbars, depending on the previous thermal treatment. The transition was accompanied by the disappearence of the Bragg-reflection of the low pressure phase and an audible click. No significant changes occurred at pressures up to 4 kbars but a few hundred bars below the transition pressure the mosaic structure started to decline in width (30 %) and in the integrated intensity (30 %). In agreement with our previous work [2] this narrowing of the mosaic width — i.e. the alignment of the mosaic blocks — and the decrease of the integrated intensity due to the formation of germs of the high pressure phase are supposed to prepare the transition