Pressure dependence of the structure of liquid copper halides

X-ray diffraction of liquid CuI, CuBr and CuCl has been measured up to 19 GPa using synchrotron radiations. Static structure factor S(Q) and pair distribution function g(r) were obtained. For liquid CuI, CuBr and CuCl, S(Q) and g(r) change their shapes continuously with increasing pressure, indicating anisotropic compression of the local structures. The pressure dependence of the peak position ratio r₂/r₁ of g(r) shows that Cu atoms are located in a tetrahedral site at low pressures and then in an octahedral site at high pressures in these liquid copper halides. At higher pressures, the local structure of liquid CuCl is similar to the intermediate structure between NaCl and CsCl structures. These results are compared with the pressure-induced structural transformation in the crystalline phase.