In order to explain electron paramagnetic resonance data which had revealed a distortion at the H2- site, which lowered the defect symmetry from cubic to tetragonal, an investigation was made of the defect-induced lattice relaxation of an H2- ion in material with local Oh or D4h symmetry by using a first-principles embedded-cluster approach that was based upon the local density approximation of density functional theory. It was shown that the ground state in Oh symmetry was an orbitally degenerate tlu state whereas, in the case of D4h symmetry, it was an al state. This Jahn-Teller distortion of the H2--doped system was consistent with the experimental results. It was found that the position of the tlu root in the band-gap was sensitive to the inclusion of the 3d orbital at the nearest-neighbor Ca2+ sites; thus suggesting a bonding interaction. The Fermi contact spin density at the proton was in good agreement with the observed values, whereas a significant discrepancy was found at the Ca2+ sites. A Mulliken analysis indicated that the spin density at the proton was due mainly to core polarization of the H2- 1s orbital.

S.C.Ke, D.C.Patton, J.G.Harrison, H.T.Tohver: Journal of Physics - Condensed Matter, 1995, 7[49], 9625-33