  The phase diagram of 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 crystal symmetry up to the melting temperature. The known decrease
of the axial ratio 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 the axial ratio reaches the square root of 3 value, around 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 polycrystals. In addition, a noticeable change in the pressure
dependence of the axial ratio 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. The determined melt
curve agrees with previous low-pressure studies and with shock-wave
experiments, with a melting temperature of 5060 K at 135 GPa. Finally, a
thermal equation of state is reported, which at room-temperature agrees with
the literature.
