Polymorphic transitions of the alkali bromides and iodides at high pressures to 200°C

Abstract

LiBr and LiI have no transitions to 40 kbar in the temperature range 20–150°C. The temperature dependence of the transitions from the NaCl to the CsCl type structure was determined to 200°C for KBr, RbBr, KI and RbI. Easily measurable slopes were found for the RbBr and RbI transition lines in spite of the fact that Bridgman reported these transitions to be independent of temperature. The transition pressures of KBr and KI are almost independent of temperature and are thus exceptionally well suited for calibration of high-pressure equipment at elevated temperatures. These transitions are sluggish near room temperature, but above 50°C the hysteresis drops sharply to about ±30 bar. This is among the lowest hystereses encountered. Extrapolation of our high temperature data to room temperature probably yields data of higher precision on KBr and KI than those submitted by Kennedy and LaMori. NaBr and NaI appear to have very sluggish transitions near 10–14 kbars and 200°C. The evidence is not conclusive, however.