Boron-Induced Hydrogen Localization in the Novel Metal Hydride LaNi<sub>3</sub>BH<i><sub>x</sub></i> (<i>x</i> = 2.5−3.0)
2005-06-13T00:00:00Z (GMT) by
The crystal structure and hydrogenation properties of the intermetallic boride LaNi<sub>3</sub>B were investigated. The hydrogen-free compound has a novel structure with orthorhombic symmetry, space group <i>Imma</i>, <i>a</i> = 4.9698(8) Å, <i>b</i> = 7.1337(8) Å, <i>c</i> = 8.3001(9) Å, and <i>V</i> = 294.26(7) Å<sup>3</sup>. Thermal gravimetrical analysis reveals a hydride phase that forms near ambient conditions within the compositional range LaNi<sub>3</sub>BH<sub>2.5</sub><sub>-</sub><sub>3.0</sub>. Single-crystal X-ray diffraction on both the alloy and the hydride, using the same crystal, shows an expansion in the <i>a</i><i>−</i><i>c</i> plane (by up to ∼8%) and a contraction along <i>b</i> (by ∼3%), while the symmetry changes from <i>Imma</i> to <i>Bmmb </i>(<i>Cmcm</i>) and the unit cell doubles along <i>a</i> and <i>b</i>. The cell parameters for the composition of LaNi<sub>3</sub>BD<sub>2.73(4)</sub> are <i>a </i>= 10.7709(7) Å, <i>b </i>= 16.0852(10) Å, <i>c </i>= 7.6365(5) Å, <i>V </i>= 1323.03(15) Å<sup>3</sup>, and space group <i>Cmcm</i>. Four nearly fully occupied interstitial hydrogen sites were located by neutron powder diffraction on deuterides and found to have tetrahedral, La<sub>2</sub>Ni<sub>2</sub> (D1,D2), trigonal-prismatic, La<sub>3</sub>Ni<sub>3</sub> (D3), and trigonal-bipyramidal, La<sub>2</sub>Ni<sub>3</sub> (D4), metal environments. The structure can also be described in terms of alternating quasi two-dimensional [NiD]<sup>-</sup> slabs (Ni−D = 1.62−1.97 Å) and La−B sheets for which bond-valence sums suggest the limiting formula La<sup>3+</sup>B<sup>0</sup>[Ni<sub>3</sub>D<sub>3</sub>]<sup>3-</sup>. 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.