STRUCTURAL AND ELECTRONIC-PROPERTIES OF HEXAGONAL YTTRIUM TRIHYDRIDE

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 YH3 with unusual wavelike hydrogen displacements (space group D3d4 or P3c1) is energetically more stable than the cubic structure. This result is consistent with neutron-diffraction data for YD3 and for a heavy rare-earth trihydride HoD3. These hydrogen displacements are identified as Peierls-like distortions in this three-dimensional system. The calculated final LDA band structure for YH3 gives a semimetal rather than a semiconductor. With electron and hole pockets overlapping around the point, the system is highly unstable. Other possible symmetry-breaking-distortion and many-body effects such as excitonic condensation are discussed. © 1995 The American Physical Society.