Discriminating lattice structural effects from electronic contributions to the superconductivity of doped MgB₂ with nanotechnology

st partial altervalent/aliovalent substitutions for Mg or B in MgB ₂ studied to date depress the superconducting transition temperature (T_c) and, at higher replacements, completely suppress superconductivity of MgB₂. The diminution and loss of superconductivity in MgB₂ arise from the subtle interplay between the competing/cooperating effects of the electronic and lattice structural variations, which are induced by the different charge and atomic radii of the substituents. Here, we experimentally discriminate lattice structural effects from electronic contributions to superconductivity by exploiting the nanosize dependence of the lattice structure to modify structural parameters without resorting to chemical doping. It is found that the superconductivity of MgB ₂ is extremely sensitive to lattice parameter variation, such that contraction of Mg-Mg bond dramatically depresses T_c and eventually results in the loss of superconductivity as the average coordination of Mg to B falls from 12 to 8 due to the introduction of B vacancies for nanocrystalline MgB₂ of 2.5 nm diameter.