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. The orientation of atomic $d$ orbitals used for this construction is that of the pure metal. One of the MO's so obtained is shown to be particularly well suited, in terms of availability and closeness to the Fermi level, for screening purposes. This state is degenerate and, by analogy with other well-known states (e.g., impurity-vacancy pairs in Si), should have a strong Jahn-Teller interaction. 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. Following an argument that the Jahn-Teller distorted state should persist at higher concentrations because of its essentially molecular character, a blocking rule is formulated based on the assumed integrity of this state. The rule is shown to explain hydrogen partial entropy versus concentration, including a sharp decrease in entropy near [H]/[M]=1. It also predicts that hydrogen diffusion should become increasingly correlated at higher concentrations, agreeing with a recent experiment. Finally, exceptions to the blocking rule are discussed in terms of an additional Jahn-Teller state, and it is shown that observed superlattice structures can be understood in terms of one or the other of the two Jahn-Teller states.