Mg{sub 8}Rh{sub 4}B - A new type of boron stabilized Ti{sub 2}Ni structure

The new magnesium rhodium boron compound Mg{sub 8}Rh{sub 4}B has been synthesized by reaction of the metal powders with crystalline or amorphous boron or the RhB precursor. The crystal structure of Mg{sub 8}Rh{sub 4}B was solved using single-crystal X-ray diffraction data (space group Fd3-bar m, a=12.1711(4)A, Z=8, 174 reflections, R{sub F}=0.016). The crystal structure can be described as a filled Ti{sub 2}Ni type where the interstitial sites 8b (12,12,12), located at the center of two nested Mg{sub 4}Rh{sub 4} tetrahedra, are occupied by boron atoms. Taking into account the absence of the Ti{sub 2}Ni-type phase in the binary Mg-Rh system, the boron atoms can be considered as stabilizing this structural motif. From the bonding analysis with the electron localization function the crystal structure is described as covalently bonded [Rh{sub 4}B]{sup 3-} anions, embedded in a cationic magnesium matrix.