Optical Observations Concerning the Polymorphic Transition in the Potassium Halides

Abstract

At approximately 20 000 atm the potassium halides undergo a phase transition from the FCC (NaCl) structure to the SC (CsCl) structure. Using optical equipment developed in this laboratory various optical effects were observed including the rate of cutoff of the light and the wavelength dependence of the scattered light. The phase transition is apparently initiated by shear involving a large cutoff of light with little or no volume change. At the beginning and end of the transition the scattering is largely small particle scattering, with large particles dominating the middle region. Rb⁺ impurity has little effect on the transition. Na⁺ impurity apparently inhibits the nucleation strongly, and, to some extent, the growth. From the absorption spectra of Pb⁺⁺ ion impurity it is apparent the nuclei consist of hundreds of ions, rather than smaller units.