Optical observations of the f.c.c. → s.c. transformation in single crystals of RbI at high pressures

The transformation of rubidium iodide from its low pressure sodium chloride structure to its high pressure cesium chloride structure takes place at a pressure of about 3500 b. This transformation has been studied optically in single crystals, using a high pressure vessel with sapphire windows, a modified Vickers metallographic microscope and time lapse photographic techniques. The transformation behavior of the crystal depends sensitively on the condition of its surface. Freshly cleaved crystals or crystals which have been etched in RbI solution transform in two stages. Firstly, the transformation proceeds from a few surface sites and spreads over the entire surface of the crystal, then the interior of the crystal transforms. We hypothesize that the first stage of surface transformation reflects a condition of relative compression of the surface layers of the NaCl structured phase of Rbl compared with the interior with the consequence that the surface layers transform at a lower value of applied pressure than does the interior. In cleavage crystals which have been stored under oil for a long time prior to observation, the surface transformation is absent and the pressure at which observable transformation occurs is much higher, in agreement with optical observations by other investigators. The equilibrium pressure between the two phases has been found to lie in the range 3430-3650 b at room temperature, somewhat lower than the range 3500-3900 b previously established by Bridgman's measurements.