First-principles thermodynamic calculations for δ-Pu and ε-Pu

With the aim of investigating several long-standing issues, e.g. the equilibrium atomic volumes of the high-temperature δ-phase (fcc) and ε-phase (bcc) of plutonium metal, a first-principles thermodynamic calculation has been carried out on the basis of (i) accurate calculations of 0 K total energies with the full-potential linearized augmented-plane-wave method within the generalized gradient approximation to the exchange-correlational functional and (ii) the newly developed classical mean-field statistics where both the cold and thermal parts of the Helmholtz free energy are derived entirely from the 0 K total energy. The calculated results show that at 0 K the antiferromagnetic states are energy preferable, both for the fcc and for the bcc phases. For the first time, an ab initio thermodynamic calculation gives calculated atomic volumes for δ-Pu and ε-Pu: these are, respectively, 24.97 angstroms³ and 23.82 angstroms³, which are very comparable with the corresponding experimental values of 24.89 angstroms³ and 24.29 angstroms³.