The bond ionicity in diborides MB2 (M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al, Y) was studied by using a complex chemical bond theory based upon a generalization of the Phillips-Van Vechten-Levine scheme. The ionicity of M-B bonds decreased in the order: Mg, Al, Mn, Y, Cr, Zr, Hf, Nb, Ta, V, Ti. The Mg-B bond in MgB2 had the largest value of ionicity (96.8%) among the diborides examined. The calculations supported recent results of band structure calculations which suggested that Mg atoms were fully ionized so that they could donate valence electrons to the system. The observed superconductivity loss in the solid solution, Mg1-xAlxB2, with x > 0.1, could be understood in terms of the ionicity and the number of valence electrons.

The Bond Ionicity of MB2 (M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and Y). X.L.Chen, Q.Y.Tu, M.He, L.Dai, L.Wu: Journal of Physics - Condensed Matter, 2001, 13, L723-7