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Structural, mechanical, thermodynamic, and electronic properties of thorium hydrides from first principles

We perform first-principles calculations of the structural, electronic, mechanical, and thermodynamic properties of thorium hydrides (ThH$_{2}$ and Th$_{4}$H$_{15}$) based on the density functional theory with generalized gradient approximation. The equilibrium geometries, the total and partial densities of states, charge density, elastic constants, elastic moduli, Poisson's ratio, and phonon dispersion curves for these materials are systematically investigated and analyzed in comparison with experiments and previous calculations. These results show that our calculated equilibrium structural parameters are well consistent with experiments. The Th$-$H bonds in all thorium hydrides exhibit weak covalent character, but the ionic properties for ThH$_{2}$ and Th$_{4}$H$_{15}$ are different due to their different hydrogen concentration. It is found that while in ThH$_{2}$ about 1.5 electrons transfer from each Th atom to H, in Th$_{4}$H$_{15}$ the charge transfer from each Th atom is around 2.1 electrons. Our calculated phonon spectrum for the stable body-centered tetragonal phase of ThH$_{2}$ accords well with experiments. In addition we show that ThH$_{2}$ in the fluorite phase is mechanically and dynamically unstable.