Electronic structure, phase stability, and chemical bonding in Th₂Al and Th₂AlH₄

We present the results of a theoretical investigation on the electronic structure, bonding nature, and ground-state properties of Th₂Al and Th₂AlH₄ using generalized-gradient-corrected first-principles full-potential density-functional calculations. Th₂AlH₄ has been reported to violate the “2-Å rule” for H-H separation in hydrides. From our total-energy as well as force-minimization calculations, we found a shortest H-H separation of 1.95 Å in accordance with recent high-resolution powder neutron-diffraction experiments. When the Th₂Al matrix is hydrogenated, the volume expansion is highly anisotropic, which is quite opposite to other hydrides having the same crystal structure. The bonding nature of these materials is analyzed in terms of density of states, crystal-orbital Hamiltonian population, and valence-charge-density analyses. Our calculation predicts a different nature of the bonding between the H atoms along a and c. The strongest bonding in Th₂AlH₄ is between Th and H along c which form dumbbell-shaped H-Th-H subunits. Due to this strong covalent interaction there is a very small amount of electrons present between the H atoms along c. This reduces the repulsive interaction between the H atoms along c and explains why Th₂AlH₄ has a shorter H-H separation than most other metal hydrides. The large difference in the interatomic distances between the interstitial regions where one can accommodate H in the ac and ab planes along with the strong covalent interaction between Th and H are the main reasons for highly anisotropic volume expansion on hydrogenation of Th₂Al.