Boron-Induced Hydrogen Localization in the Novel Metal Hydride LaNi₃BH_x (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) Å, b = 7.1337(8) Å, c = 8.3001(9) Å, and V = 294.26(7) ų. Thermal gravimetrical analysis reveals a hydride phase that forms near ambient conditions within the compositional range LaNi₃BH_2.5-3.0. 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 LaNi₃BD_2.73(4) are a = 10.7709(7) Å, b = 16.0852(10) Å, c = 7.6365(5) Å, V = 1323.03(15) ų, 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 Å) 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) Å, respectively.