Expansion of the superlattice of B layers, with AB2 structure, due to various intercalated A-atoms was studied in order to understand the emergence of high-TC superconductivity in diborides. The structure of the metal heterostructures at the atomic limit (with A = Al, Mg, Ti, Hf, Zr) was measured by using synchrotron X-ray diffraction. Increasing atomic radius of the intercalated A-ions produced an increase in the separation between the B layers, and of the tensile micro-strain of the B–B distance within the B layers. The results showed that the superconductivity appeared in a critical region in a phase diagram controlled by 2 variables: the micro-strain and the charge density.

High TC Superconductivity in a Critical Range of Micro-Strain and Charge Density in Diborides. S.Agrestini, D.Di Castro, M.Sansone, N.L.Saini, A.Saccone, S.De Negri, M.Giovannini, M.Colapietro, A.Bianconi: Journal of Physics - Condensed Matter, 2001, 13, 11689-95

Figure 10

Three-O Cluster Below B-Plane, Seen along [001]

Mg Atoms Not Shown