The temperature dependent structural phase transition from the tetragonal chalcopyrite like structure to the cubic sphalerite like structure in CuInSe₂ was investigated by in-situ high temperature synchrotron radiation X-ray diffraction. The data were collected in 1K steps during heating and cooling cycles (rate 38 K/h). The Rietveld analysis of the diffractograms led us to determine the temperature dependence of the lattice parameters, including the tetragonal deformation, and distortion. The thermal expansion coefficients of the tetragonal lattice constant which are related to the linear thermal expansion coefficient were obtained. The transition temperature is clearly identified via a strong anomaly. The temperature dependence of the anion position parameter was found to be rather weak. The cation occupancy values, derived from the Rietveld analysis, remain constant below the critical region. Close to the transition temperature, the number of electrons at the Cu site increases with a dercrease in the number of electrons at the In site with increasing temperature, indicating a Cu-In anti site occupancy, which is assumed to be the driving force of the phase transition.

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