STRUCTURAL CHANGE IN MOLTEN KCl AND KBr UNDER PRESSURE. S. Urakawa, (Department of Earth Sciences, Okayama University, 3-1-1 Tsushima-naka, Okayama 700, Japan) N. Igawa, H. Ohno, (Japan Atomic Energy Research Institute, Tokai 319-11, Japan) O. Shimomura, (Photon Factory, National Laboratory for High Energy Physics, Tsukuba 305, Japan)

Pressure-induced structure change in the molten KCl and KBr has been extensively studied by the X-ray diffraction analysis. Energy dispersive X-ray diffraction experiments were conducted up to 4 GPa by using the cubic type high pressure apparatus, MAX90, and the synchrotron radiation. The derived radial distribution functions show that the molten KCl and KBr transform from a NaCl-like structure into a CsCl-like structure. The structure change in both melts starts at the atmospheric pressure and continues with increasing pressure, accompanied by the gradual increase of the coordination number of the nearest neighbor ions. The phase transition in melt is completed at 1.5 GPa for KCl and at around 2 GPa for KBr. The dense CsCl-like high pressure melts are stable above these pressures. In the solid state, both alkali halides take the NaCl structure below 1.8 GPa, and they take the CsCl structure above this pressure. The phase transition pressure in molten KCl is clearly lower than that in solids. In these materials, the structural change in melt precedes the phase transition in solid.