Isostructural phase transition of fcc Ce: Molecular dynamics simulations

MJ Diwu and XM Hu, ACTA PHYSICA SINICA, 68, 203401 (2019).

DOI: 10.7498/aps.68.20190884

Ce is a rare earth element in the periodic table. In the range of low temperature and low pressure, there are two face-centered-cubic (FCC) phases (alpha-Ce and gamma-Ce) and a double-hexagonal-close-packed phase (beta-Ce) for metallic Ce. At ambient temperature and about 0.7 GPa pressure, Ce undergoes gamma ->alpha phase transition with a volume shrink of 14%-17% discontinuously. In this paper, an embedded-atom method (EAM) potential compatible for alpha-Ce and gamma-Ce was developed. This EAM potential has been employed to study several basic properties of cerium in these two FCC phases, such as equilibrium lattice constants, cohesive energies, and elastic constants. These results showed good accordance with experiments and first principle calculations. The lattice defects have been studied with the formation energy calculations of vacancies, interstitials, surfaces, stacking faults, and twinning defects in alpha-Ce and gamma-Ce lattice. The lattice dynamics of alpha-Ce and gamma-Ce have been analyzed using our EAM potential. The lattice vibrational entropy was calculated and plotted as functions of temperature for each phases. The vibrational entropy change across the alpha-gamma phase transition showed to be similar to 0.67 k(B) per atom at ambient temperature. Using molecular dynamics simulation with our EAM potential, several isotherms and radial distribution functions were calculated. These isotherms and radial distribution functions demonstrate a first order phase transition between two FCC structures, corresponding to alpha-Ce and gamma-Ce, with a critical point sets at T-c approximate to 550 K and P-c approximate to 1.21 GPa. Thus the newly developed EAM potential could provide a reasonable description of FCC Ce and its alpha-gamma phase transition within the scale of classical molecular dynamics simulation.

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