X ray diffraction analysis of molten KCl and KBr under pressure: Pressure-induced structural transition in melt

The structures of molten KCl and KBr were studied by energy dispersive X ray diffraction experiments under high temperature and high pressure using synchrotron radiation. The radial distribution function g(r) for molten KCI and KBr was obtained at several points just above the melting temperatures up to 4 GPa. The second neighbor ionic distance decreases with pressure, although the nearest neighbor distance is almost constant. The coordination number of the nearest neighbor ions increases with pressure. Both melts transform from an open, simple-cubic-like structure into a more highly coordinated structure, probably a body-center-cubic-like structure. The structure of molten KCl changes before the crystalline phase but the molten KBr transforms into a dense phase about the same time as the solid. In both KCl and KBr melts the structural transition occurs over a narrow pressure range.