Density Functional Theory Study of the Structural Properties of PuH_x, 2<=x<=3
Plutonium dihydride and trihydride show strikingly similar crystal structures when viewed as close-packed Pu planes with ABC and AB stacking, respectively. The similarity serves as a framework for density functional theory (DFT) calculations of PuH2, PuH3, and intermediate compositions. Agreement between structures observed in experiment and in the DFT description of the Pu-H system requires accounting for the strong electronic correlation in the f orbitals, achieved here with the addition of a Hubbard parameter U. The hysteresis measured between hydriding and dehydriding can be attributed to the effect of stacking of the close-packed Pu planes on the energy as a function of stoichiometry, calculated using the GGA+U approach. Changes in the interstitial positions occupied by the H atoms affect the energy by amounts that are negligible compared to room temperature; changes in the magnetic structure lead to equally small energy differences.