A new crystalline phase of the boron-rich metal-boride family: the Mg2B25 species

A new complex magnesium boride, the Mg2B25 species, has been prepared and its crystal structure determined from laboratory X-ray powder diffraction data by the simulated annealing technique, followed by a conventional Rietveld refinement procedure. The title compound is R-centered trigonal (with a=11.0402(3) Åa=11.0402(3) Å and c=24.198(1) Åc=24.198(1) Å, hexagonal setting), space group R-3m, and is isomorphous with β-boron and with the rhombohedral compounds of the boron-rich metal-boride family. The magnesium atoms occupy interstitial sites with partial occupancies which can be interpreted with the aid of the analysis of “forbidden” interatomic MgMg contacts. The Mg atom in the F site is vicariant with the B(4) atom and falls into a pseudometallocenic environment with the 10 nearest-neighbour B atoms; all other magnesium atoms are placed in lattice cavities within corrugated slabs normal to c, with Mg(D) and Mg(E) occupying sites already known to be populated in other metal borides, and Mg(N) in a new one. These slabs present two equiprobable, but self excluding, configurations of ordered Mg atoms, which randomly stack along the c-axis. Mg2B25 shows a significant increase of the c-axis of the rhombohedral lattice with respect to the other members of the family, which we attribute both to the larger metallic radius of Mg, vs. those of 3d transition metal atoms and to the anisotropic stacking of the Mg atoms, and, possibly, to the presence of the rather stuffed Mg sublattices.

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