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Abstract:
Two ternary borides MNi9 B8 (M=Al, Ga) were synthesized by thermal
treatment of mixtures of the elements. Single-crystal X-ray diffraction
data reveal AlNi9 B8 and GaNi9 B8 crystallizing in a new type of
structure within the space group Cmcm and the lattice parameters
a=7.0896(3) A, b=8.1181(3) A, c=10.6497(4) A and a=7.0897(5) A,
b=8.1579(4) A, c=10.6648(7) A, respectively. The boron atoms build up
two-dimensional layers, which consist of puckered [B16 ] rings with two
tailing B atoms, whereas the M atoms reside in distorted
vertices-condensed [Ni12 ] icosahedra, which form a three-dimensional
framework interpenetrated by boron porphyrin-reminiscent layers. An
unusual local arrangement resembling a giant metallo-porphyrin entity is
formed by the [B16 ] rings, which, due to their large annular size of
approximately 8 A, chelate four of the twelve icosahedral Ni atoms. An
analysis of the chemical bonding by means of the electron localizability
approach reveals strong covalent BB interactions and weak NiNi
interactions. Multi-center dative BNi interaction occurs between the
Al-Ni framework and the boron layers. In agreement with the chemical
bonding analysis and band structure calculations, AlNi9 B8 is a
Pauli-paramagnetic metal.
