Extended and accurate determination of the melting curves of argon, helium, ice (H2O), and hydrogen (H2)

Abstract

The melting curves of argon, helium 4, ice $\left({\mathrm{H}}_2\mathrm{O}\right),$ and hydrogen $\left({\mathrm{H}}_2\right)$ have been measured from room temperature up to a maximum temperature of 750 K. This extends the previous determination of the melting lines of ${\mathrm{H}}_2$ and He by nearly a factor of 2 in pressure. The experiments were carried out with a resistively heated diamond anvil cell. Improved accuracy with respect to previous determinations, when existing, was achieved by the use of an optical metrology which gives an in situ measurement of both the pressure and temperature of the sample. The melting lines of argon and ${\mathrm{H}}_2\mathrm{O}$ are found to be well represented by the following Simon-Glatzel equations: $P=2.172\mathrm{}\times {10}^{-4}{T}^{1.556}-0.21$ (argon) and $P=2.17+1.253\mathrm{}\left[{\mathrm{}(T/354.8\mathrm{})\mathrm{}}^{3.0}-1\right]$ $\left({\mathrm{H}}_2\mathrm{O}\right).\mathrm{}$ But the Simon-Glatzel form was found inadequate to reproduce the melting data of ${}^4\mathrm{He}$ and ${\mathrm{H}}_2$ over the whole temperature range. In the case of ${}^4\mathrm{He},$ this deviation from a Simon law is explained by the softening of the pair interaction with density. A Kechin equation is proposed for ${\mathrm{H}}_2:$ ${T=14.025\mathrm{}(1+P/0.0286)\mathrm{}}^{0.589}\exp (-4.6\times {10}^{-3}P).\mathrm{}$ This form is in excellent agreement with all published experimental data for ${\mathrm{H}}_2$ and interestingly predicts a maximum on the melting curve at 128 GPa and 1100 K.