Quasimolecular Jahn-Teller resonance states in the bcc metallic hydrides of vanadium, niobium, and tantalum

The unusual properties of the group Vb bcc hydrides are shown to derive from a particular d-band quasimolecular resonance state localized near interstitial hydrogen. This state is schematically shown to be grounded in fundamentals. The point of departure is an Anderson-Newns approach to the problem of a single hydrogen atom in a transition metal, with the metal states in a mixed nearly-free-electron-tight-binding representation. Orthogonalization of pseudo-plane-wave states to the hydrogen 1s ''core'' state leads to a Hamiltonian with site-diagonal disorder (Anderson Hamiltonian) for the d band. This gives the possibility of induced, localized d states. Assuming that localized electron states derived from the d band exist and are confined to the shell of metal atoms nearest the impurity, symmetrized linear combinations of atomic orbitals (-molecular orbitals) (LCAO-MO) are constructed for impurity polyhedra in the group Vb metals. A particularly simple <111> distortion proves to be key for understanding previously unexplained properties in the Vb metal-hydrogen systems, viz. cubic lattice distortion, hydrogen diffusion, and excess partial entropy