THE RELATIONSHIP BETWEEN BOND-VALENCE SUMS AND TC IN CUPRATE SUPERCONDUCTORS

The recently demonstrated correlation of Tc with copper or oxygen bond valence sums for a wide range of superconducting cuprates is shown to contradict the behaviour observed for YBa2CU3Ox as x is varied from 6 to 7. This conflict is removed by introducing two independent variables, namely, the sum of and difference of the copper and oxygen bond valence sums. The latter is a measure of the hole density on the CuO2 plane and as this increases Tc passes through a maximum Tc (max). The former indicates the relative distribution of these holes on the oxygen or copper sites for a given compound. Tc(max) correlates closely with the preference for hole distribution on the oxygen atoms. A general phase-diagram using these bond sum parameters is presented. © 1990.