Chemical bonding topology of superconductors. 5. The similarities between magnesium diboride and cuprate superconductors and the role of subvalent magnesium

The chemical bonding topology in the MgB2 superconductor is similar to that in the high T-c cuprates. Thus the conducting skeleton of MgB2 consists of the hexagonal boron layers, which are isoelectronic with graphite if the Mg is oxidized completely to Mg2+. However, comparison of the Mg-Mg distances in MgB2 with those in several ternary hydrides (Mg2RuH4, Mg3RuH3, and Mg4IrH5), assumed to contain subvalent magnesium as Mg-2(2+), suggests similar subvalent magnesium in MgB2. This incomplete oxidation of Mg to Mg2+ generates the holes in the valence band of the hexagonal boron layers leading to the experimentally observed hole superconductivity in MgB2. (C) 2002 Elsevier Science Ltd. All rights reserved.