Melting Curve of Molecular Crystal GeI4

In situ synchrotron x-ray diffraction measurements were carried out to determine the melting curve of the molecular crystal GeI4. We found that the melting line rapidly increases with a pressure up to about 3 GPa, at which it abruptly breaks. Such a strong nonlinear shape of the melting curve can be approximately captured by the Kumari-Dass-Kechin equation. The parameters involved in the equation could be determined from the equation of state for the crystalline phase, which was also established in the present study. The melting curve predicted from the equation approaches the actual melting curve as the degree of approximation involved in obtaining the equation is improved. However, the treatment is justifiable only if the slope of the melting curve is everywhere continuous. We believe that this is not the case for GeI4's melting line at the breakpoint, as inferred from the nature of breakdown of the Kraut-Kennedy and the Magalinskii-Zubov relationships. The breakpoint may then be a triple point among the crystalline phase and two possible liquid phases.