Locating anionic hydrogen in Ba3(Yb,Lu)2O5H2: A combined approach of X-ray diffraction, crystal chemistry, and DFT calculations

By a combination of x-ray diffraction, structural chemistry, and DFT calculations, the presence and location of anionic hydrogen in the two new, layered lanthanide oxyhydrides, Ba3Ln2O5H2 (Ln = Yb, Lu) is inferred. Single crystals of the compounds have been synthesized from a molten barium flux with the addition of small amounts of BaH2. These phases crystallize in space group I4/mmm (#139, Z = 2) with lattice parameters a = 4.3336(2) angstrom and c = 22.7197(6) angstrom, and a = 4.3291(1) angstrom and c = 22.597(1) angstrom, respectively. The Ba3Ln2O5H2 phases comprise two different structural moieties: a perovskite double layer of stoichiometry Ba2Ln2O5H- formed by corner-connected LnO5 tetragonal bi-pyramids with a terminating hydrogen anion, and a puckered rocksalt-type (BaH)+ layer that is stretched along the c-axis. DFT calculations were used to arrive at hydrogen positions that minimize energy and are consistent with structural chemistry principles. Furthermore, the calculations show that the valence band edge is dominated by oxygen 2p orbitals with hydrogen 1s states admixed. The conduction band is formed by barium 5d-orbitals and Lu (Yb) 5d-orbitals. These are characteristics of materials with anionic H-. These new phases are isostructural with the Ba3Ln2O5Cl2 (Ln = Gd-Lu) family of compounds with the chlorine atom in the same apical position as the hydrogen atom. Steric effects limit the size of the lanthanide ion for Ba3Ln2O5H2.