Abstract: The phase diagram of zinc (Zn) has been explored up to 140?GPa and 6000?K, by combining optical observations, x-ray diffraction, and ab initio calculations. In the pressure range covered by this study, Zn is found to retain a hexagonal close-packed (hcp) crystal symmetry up to the melting temperature. The known decrease of the axial ratio (c/a) of the hcp phase of Zn under compression is observed in x-ray diffraction experiments from 300?K up to the melting temperature. The pressure at which c/a reaches (?10?GPa) is slightly affected by temperature. When this axial ratio is reached, we observed that single crystals of Zn, formed at high temperature, break into multiple poly-crystals. In addition, a noticeable change in the pressure dependence of c/a takes place at the same pressure. Both phenomena could be caused by an isomorphic second-order phase transition induced by pressure in Zn. The reported melt curve extends previous results from 24 to 135?GPa. The pressure dependence obtained for the melting temperature is accurately described up to 135?GPa by using a Simon?Glatzel equation: , where P is the pressure in GPa. The determined melt curve agrees with previous low-pressure studies and with shock-wave experiments, with a melting temperature of 5060(30) K at 135?GPa. Finally, a thermal equation of state is reported, which at room-temperature agrees with the literature.

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