Crystal Structure of and Chemical Bonding in MoNi₄

Single-phase MoNi4 was prepared by multi-step long-term thermal treatment. The crystal structure was re-evaluated by single-crystal and powder X-ray as well as powder neutron diffraction resulting in space group I4/m and lattice parameters of a=5.7300(6) angstrom and c=3.5649(6) angstrom. Despite the formation by complex solid-state reaction, the crystal structure of MoNi4 is fully ordered, whereby the single-crystalline micro-domains feature sizes below 20 mu m. The ordering is supported by the strong charge transfer from Mo to Ni hindering the formation of anti-sites. Two types of four-atomic bonds stabilize the crystal structure. The bonding is isotropic, allowing cleavage in different crystallographic directions with mostly nickel atoms on the surface. In combination with the negative effective charge of the nickel species, this enables to shed more light on the behaviour of MoNi4 in catalytic processes.