Structural disorder in liquid and solid CuI at high temperature probed by x-ray absorption spectroscopy

A detailed investigation of local structure and phase transitions in copper iodide (CuI), at high temperatures, has been carried out performing x-ray absorption spectroscopy (XAS), x-ray absorption temperature scans, and energy scanning x-ray diffraction measurements. Low-noise XAS spectra have been recorded in a wide range of temperatures 298-903 K in the solid and liquid phases at Cu and I K-edges. Diffraction and temperature scans were used to monitor sample conditions and phase transitions. XAS data analysis has been performed simultaneously at both Cu and I K-edges in the framework of the ab initio GNXAS method. Average bond distance, variance, and skewness of the first-neighbor distribution function in the gamma-CuI phase are accurately measured and their statistical errors evaluated, taking into account thermal-expansion data. For the high-temperature superionic and liquid disordered phases, Cu-I and Cu-Cu short-range partial distribution functions are accurately reconstructed improving previous conflicting determinations. The inadequacy of the current structural models obtained using molecular-dynamics simulations is discussed. The first-neighbor Cu-I peak is found to be narrower and shifted to longer distances with respect to previous reverse Monte Carlo (RMC) models based on neutron diffraction or anomalous x-ray scattering data. Existence of short-range Cu-Cu correlations, resulting in a well-defined Cu-Cu short-distance peak in superionic and liquid CuI, is unambiguously determined as a function of temperature, exploiting the potential of the double edge XAS data analysis. Closest approach distance (about 2.5 Angstrom) and position of the Cu-Cu peak are compared with previous RMC estimates discussing the accuracy of the present results.