The bond ionicity of MB2(M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and Y)

Abstract

The bond ionicity in biborides MB₂ (M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and Y) has been studied by using the complex chemical bond theory based on a generalization of the Phillips-Van Vechten-Levine scheme. The ionicity of M-B bonds decreases in the following order: Mg, Al, Mn, Y, Cr, Zr, Hf, Nb, Ta, V and Ti. The Mg-B bond in MgB₂ has the largest value of ionicity of 96.8% among these examined diborides. Our calculations support the recent results of band structure calculations that Mg atoms are fully ionized so that they can donate valence electrons to the system. The observed superconductivity loss in the solid solution Mg_1-xAl_xB₂ with x>0.1 can be understood in terms of the ionicity and the number of valence electrons.