Application of the embedded atom model to liquid metals: Liquid sodium

Abstract

The procedure for the calculation of the embedded atom model (EAM) potential for liquid metal, which involves the use of diffraction data on the structure of material in the vicinity of the melting point, is applied to sodium. In fitting the parameters of EAM potential, use is made of the data on the structure of sodium at 378, 473, and 723 K, as well as on the thermodynamic properties of sodium at pressures up to 96 GPa. The use of the method of molecular dynamics (MD) and of the EAM potential produces good agreement with experiment as regards the structure, density, and potential energy of liquid metal along the p ≅ 0 isobar at temperatures up to 2300 K, as well as along the shock adiabat up to pressures of ∼100 GPa and temperature of ∼30 000 K. The melting temperature of bcc model of sodium with EAM potential is equal to 358 ± 1 K and close to real. The predicted value of bulk modulus at 378 K is close to the actual value. The self-diffusion coefficients under isobaric heating increase with temperature by the power law with exponent of 1.6546. The values of pressure, energy, heat capacity, and the temperature coefficient of pressure are calculated in a wide range of densities. The compression to 45–50% of normal volume causes a variation of the structure of liquid; this results in the emergence of atoms with a small radius of the first coordination sphere (∼2.1 Å) and low coordination number, which form connected groups (clusters). Their concentration increases with decreasing volume and increasing temperature. The pre-peak of pair correlation functions, the height of which increases with heating, corresponds to these atoms. In the region of variation of the structure, the pressure decrease under isochoric heating follows the pattern of water anomaly.