Structural and electronic properties of hexagonal yttrium trihydride

Abstract

The structural and electronic properties of yttrium trihydride with metal atoms in the hexagonal-close-packed (hcp) structure are studied by the pseudopotential method within the local-density-functional approximation (LDA). It is found that the hydrogen positions within the metal lattice have a major role in determining these properties. Calculations confirmed that hexagonal ${\mathrm{YH}}_3$ with unusual wavelike hydrogen displacements (space group ${\mathit{D}}_{3\mathit{d}}^4$ or P3¯c1) is energetically more stable than the cubic structure. This result is consistent with neutron-diffraction data for ${\mathrm{YD}}_3$ and for a heavy rare-earth trihydride ${\mathrm{HoD}}_3$. These hydrogen displacements are identified as Peierls-like distortions in this three-dimensional system. The calculated final LDA band structure for ${\mathrm{YH}}_3$ gives a semimetal rather than a semiconductor. With electron and hole pockets overlapping around the Γ point, the system is highly unstable. Other possible symmetry-breaking-distortion and many-body effects such as excitonic condensation are discussed.