High-resolution powder neutron diffraction was used to study the crystal structure of the Mg11B2 superconductor (TC = 39.2K) prepared at high pressure at 2 to 293K. The experimental data provide clear evidence of temperature-independent phase inhomogeneity, which was modelled by coexisting Mg1+δB2 phases. Precise structural information for temperatures near TC also revealed extremely small volume and c-axis discontinuities; consistent with the existence of a specific heat anomaly at TC.

Phase Inhomogeneities and Lattice Expansion near TC in the Mg11B2 Superconductor. S.Margadonna, T.Muranaka, K.Prassides, I.Maurin, K.Brigatti, R.M.Ibberson, M.Arai, M.Takata, J.Akimitsu: Journal of Physics - Condensed Matter, 2001, 13, L795-802