Observation of phase transitions in shocked tin by molecular dynamics

We investigate the phase transition of a tin single crystal under shock loading by using large molecular dynamics simulations. The interatomic interactions are described by a Modified Embedded Atom Model (MEAM) potential. The comparison of the Hugoniot curves corresponding to four crystallographic orientations with the poly-crystal experimental Hugoniot curve ensures that MEAM is an acceptable choice. We show the necessity to use large samples (i.e., several hundred million atoms) in order to limit size effects. A precise analysis of the results shows that tin undergoes in simulations a solid/solid phase transition beyond 10 GPa, in good agreement with the static phases diagram. Moreover, the calculated shock melting point is close to the experimental melting curve. The final structure behind the shock is analyzed by using a combination of Steinhardt’s coefficients and the radial distribution function. We obtain a three phase mixture containing a compressed initial β phase, a γ phase predicted by the phase diagram, and an intermediate phase. The time transition is close to 100 ps. Finally, we calculate the integrated x-ray diffraction spectrum. The comparison with recent time-resolved experiments is satisfactory and validates our simulation method.