Computer simulation of Pu3+ and Pu4+ substitutions in gadolinium zirconate

Atomistic computer simulations have been used to determine the energetics of a variety of defect reactions related to the incorporation of Pu3+ and Pu4+ into the pyrochlore and the fluorite-type structures of Gd2Zr2O7. The lowest energy states were found for Pu3+ substitutions on Gd sites in the pyrochlore (−1.00 eV/Pu) and the fluorite-type (−1.55 eV/Pu) structures, so these defect reactions are the most likely configurations under reducing conditions that favor Pu3+ ions. Slightly higher, but still exothermic, energies (−0.26 to −0.45 eV) were calculated for Pu4+ substitutions on Zr sites for several fluorite-type cases, indicating that oxidizing conditions should favor Pu4+ incorporation on Zr sites in Gd2Zr2O7 hosts. Defect reactions involving cation vacancies or interstitials exhibited significantly higher energies, and are therefore not expected to occur. Mean field calculations indicated that the increases in crystal volume associated with Pu incorporation are minimized by the excess free volume associated with the Gd site in the pyrochlore structure. Volume changes upon thermal phase transformation from fluorite to pyrochlore are smaller for the material incorporating Pu by substitution than for the virgin material, with a slight advantage for the reducing conditions associated with Pu3+ substitutions on Gd sites.