Boron-Induced Hydrogen Localization in the Novel Metal Hydride LaNi3BHx (x = 2.5-3.0)

The crystal structure and hydrogenation properties of the intermetallic boride LaNi3B were investigated. The hydrogen-free compound has a novel structure with orthorhombic symmetry, space group Imma, a = 4.9698(8) (angstrom), b = 7.1337(8) (angstrom), c = 8.3001(9) (angstrom), and V = 294.26(7) (angstrom)^3. Thermal gravimetrical analysis reveals a hydride phase that forms near ambient conditions within the compositional range LaNi3BH2.5-3.0. Single-crystal X-ray diffraction on both the alloy and the hydride, using the same crystal, shows an expansion in the a-c plane (by up to ~8%) and a contraction along b (by ~3%), while the symmetry changes from Imma to Bmmb (Cmcm) and the unit cell doubles along a and b. The cell parameters for the composition of LaNi3BD2.73(4) are a = 10.7709(7) (angstrom), b = 16.0852(10) (angstrom), c = 7.6365(5) (angstrom), V = 1323.03(15) (angstrom)^3, and space group Cmcm. Four nearly fully occupied interstitial hydrogen sites were located by neutron powder diffraction on deuterides and found to have tetrahedral, La2Ni2 (D1,D2), trigonal-prismatic, La3Ni3 (D3), and trigonal-bipyramidal, La2Ni3 (D4), metal environments. The structure can also be described in terms of alternating quasi two-dimensional [NiD]slabs (Ni-D = 1.62-1.97 (angstrom)) and La-B sheets for which bond-valence sums suggest the limiting formula La3+B0[Ni3D3]^3-. The La-B planes do not accommodate deuterium; the B-D and D-D interactions appear to be repulsive. The shortest B-D and D-D contacts are 2.52(2) and 2.33(2) (angstrom), respectively.