CRITICAL COVALENCE AND SUPERCONDUCTIVITY IN LN2-XCEXCUO4

The variations of superconductive properties with x of the n-type Ln2-xCexCuO4 (Ln = La0.5Nd0.5, Nd, or Gd) systems have been investigated. As the size of Ln3+ decreases, (i) the solubility limit x of Ce decreases, (ii) the value of x at which a transition from antiferromagnetic semiconductor to superconductor occurs increases, and (iii) the width Δx of the superconductive region decreases. The decreasing solubility of Ce with decreasing size of Ln3+ is due to decreasing tensile strain in the CuO2 sheets. The progressive shift of the semiconductor to superconductor transition to higher x values with decreasing size of Ln3+ is explained on the basis of increasing electrostatic Madelung energy EM caused by decreasing CuO bond length. A larger EM means a larger charge transfer gap Δ and a smaller covalent-mixing parameter λ and bandwidth W; so a decreasing size of Ln3+ necessitates a higher level of Ce-doping in order to achieve a critical covalence essential for superconductivity to occur. © 1992.