Anomalous structural behavior and electronic structure in ZrBe2H2: density functional calculations

Density functional calculations are reported for ZrBe2, ZrBe2H2, and supercells representing ZrBe2Hx. We find that the H positions are soft in the sense that displacements along the hexagonal c axis are energetically favored for some orderings and displacement patterns. This is discussed in relation to the anomalous structural and nuclear magnetic resonance data reported for this compound. The mechanism for this softness is related to the electronic structure, specifically coupling to strongly hybridized bands occurring near the Fermi energy. In particular, certain patterns of H shifts along the c axis strongly mix H s and Zr d bands, leading to strong coupling between these shifts and bands at the Fermi energy and instabilities of phonons with these displacement patterns.