The Pm3m-Fm3m transformation and phase equilibrium in solid solutions of CsC1 with KC1, RbC1, and CsBr

Thermal expansion and the Pm3m t Fm3m transition of cesium chloride have been studied with high temperature diffractometry, differential thermal analysis, and heating and quenching techniques. The onset temperature of the transition in pure CsCl, on heating, is 471ºC. There are no observable premonitory effects below the transition temperature and no apparent structural relationships between the polymorphs is apparent. Both phases coexist over a 20º to 50ºC temperature range and equilibrium thermal hysteresis occurs on heating and cooling. Hysteresis behavior is explained by addition of strain energy parameters to the free energy function. Initiation of the Pm3m t Fm3m transformation is probably related to generation of defects, primarily Schottky vacancies. The effects of addition of impurities on thermal hysteresis were studied. KCl and RbCl increase the width of thermal hysteresis loops and decrease the onset temperature of the transformation. CsBr increases the onset temperature of the transformation but has little effect on the width of the thermal hysteresis loop. Phase equilibria of the systems CsCl-KCl, CsCl-RbCl, and CsCl-CsBr were studied in detail. Addition of two mole percent KCl decreases the transformation temperature to 315 ± l0ºC; addition of 14% RbCl decreases the transformation temperature to 260º ± 10ºC. Miscibility gaps exist in both the CsCl-KCl and CsCl-RbCl systems with a consolute temperature of about 470ºC. Solid solubility is greater in the CsCl-RbCl system than in the CsCl-KCl system. The CsCl-CsBr system exhibits complete solid solution at all temperatures. Addition of 60% CsBr increases the transformation temperature to the solidus temperature of 610ºC. Liquidus curves in the systems CsCl with KCl, RbCl, and CsBr were found to agree reasonably well with reported values. The systems CsCl-NaCl, CsCl-KI, CsCl-Rbi, and CsCl-Csi were investigated partially. The CsCl-NaCl system has a eutectic with very little solid solubility; the transformation temperature was decreased by approximately 5ºC. The CsCl-KI and CsCl-Rbi systems are pseudo-binary with exchange reaction products, Csi and KCl or Csi and RbCl, the stable phases. The transformation temperature is decreased to approximately 300ºC in the presence of the reaction products. The CsCl-Csi system has a eutectic with maximum solid solubilitv of less than 10% in the CsCl end member and 12% in the Csi end member. The transformation temperature is decreased to approximately 404ºC in the presence of CsI --Abstract, pages ii-iii