Lattice dynamics and the phase diagram of sodium at high pressures and temperatures from ab initio calculations
The melting of sodium was investigated by two ab initio methods: (i) based on the calculations of the phonon spectra in the quasi-harmonic approximation with the use of the Lindemann criterion and (ii) by molecular-dynamics simulation. It is shown that the anomalous behavior of the melting curve Tm(p) for Na is explained well by the change in the phonon spectrum under compression; in particular, the decrease in Tm(p) at p > 30 GPa is due to the strong softening of transverse-phonon frequencies. The results obtained within both approaches are in good agreement and yield a reasonable quantitative description of the experimental melting curve of sodium at pressures up to ∼1 Mbar and temperatures from 300 to 1000 K. The good agreement between the two approaches indicates the smallness of anharmonicity effects in Na.
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