The Bulge in the Basal Plane Area of Cuprate Superconductors

Hole doping of cuprate superconductors removes electrons from the antibonding planar Cu3dx2−y2O2pxy states; thus increasing doping is expected to shorten the planar Cu–O bonds. All accessible crystallographic data, however, give evidence for a significant deviation from the Pauling-type contraction of covalent bonds on increasing covalency. Between the onset of superconductivity at nI−M = 0.07(2), and the closure of the large pseudogap at nPG = 0.21(2) the basal plane area (a 2 or ab) is always found concave away from the nh axis instead of convex toward it . This “bulge” exhibits a maximum around optimum doping nopt = 0.16(1), and collapses at nPG = 0.21(2), notably within the overdoped regime. Optimum doping may inflate the basal plane area by up to 30% of its overall variation.