The role of boron lattice expansion in superconducting diborides

Lattice structure of the diborides has been studied to explore a possible relation between high Tc superconductivity and expansion of the boron plane. High purity Mg1−xAxB2 diborides have been prepared by direct reaction of the elements and the lattice parameters are determined by X-ray diffraction, providing a measure to the strain ϵ of the B–B distance. The results show that the superconductivity in these intermetallics appears in a critical region of tensile strain and charge density in the boron plane.