Structure of liquid Sn over a wide temperature range from neutron scattering experiments and first-principles molecular dynamics simulation: A comparison to liquid Pb

The structure of liquid Sn was studied by neutron scattering experiments in the widest temperature range thatwas ever performed. Though, on increasing temperature, the existence of the shoulder in the structure factor,S(Q), becomes less clear in the change of the overall shape of the S(Q), the structure related to this shoulderseems to be present even at 1873 K. The first-principle molecular-dynamics ~FPMD! simulation was performedfor the first time for liquid Sn by using the cell size of 64 particles. The calculated results well reproducedS(Q) obtained by the neutron experiments. The angle distribution, g(3)(u ,rc), was evaluated for the anglebetween vectors from centered atom to other two atoms in spheres of cutoff radii rc’s. The g(3)(u ,rc) showsthat, with the decrease of rc from 0.4 to 0.3 nm, a rather sharp peak around 60 ° disappears and only a broadpeak around 100 ° remains; the former peak may be derived from the feature of the closely packed structuresand the latter one is close to the tetrahedral angle of 109 °. In addition, the coordination number, n, of liquidSn counted within the sphere of rc50.3 nm is found to be 2–3 and does not change with the increase oftemperature even up to 1873 K. These facts indicate that at least the fragment of the tetrahedral unit may beessentially kept even at 1873 K for liquid Sn. For comparison, the FPMD simulation was performed for thefirst time also for liquid Pb. No sign of the existence of the tetrahedral structure was observed for liquid Pb.Unfortunately, the self-diffusion coefficients, D’s, obtained from this FPMD for liquid Sn do not agree withthose obtained by the microgravity experiments though the structure factors, S(Q)’s, are well reproduced. Toremove the limitation of the small cell size of the FPMD, the classical molecular-dynamics simulations with acell size of 2197 particles were performed by incorporating the present experimental structural information ofliquid Sn. Obtained D’s are in good agreement with the microgravity data