Partial altervalent/aliovalent chemical substitutions for Mg or B were attempted to modify the Fermi-level density-of-states and to alter the lattice parameters, thus varying the TC of MgB2. However, in some oxide superconductors where replacement could increase TC from 35 to 93K, most of the substitutions in MgB2 depressed TC and, at higher replacements, completely destroyed the superconductivity. Such diminution and loss of superconductivity in MgB2 arose from a subtle interplay between the competing/cooperating effects of the electronic and lattice structural variations, which were induced by the differing charge and atomic radii of the substituents. Lattice structural effects were experimentally separated from electronic contributions to superconductivity by exploiting the nanosize dependence of the lattice structure in order to modify the structural parameters without resorting to chemical doping. It was found that the superconductivity of MgB2 was extremely sensitive to lattice parameter variations, such that contraction of the Mg–Mg bond markedly decreased TC and eventually resulted in the loss of superconductivity as the average coordination of Mg to B fell from 12 to 8 for nano-crystalline material which was 2.5nm in diameter.

Superconductivity of Nano-Crystalline MgB2. S.Li, T.White, J.Plevert, C.Q.Sun: Superconductor Science and Technology, 2004, 17, S589-94