DEFECT CHEMISTRY OF LA2-XSRXCUO4-X/2(O-LESS-THAN-X-LESS-THAN-OR-EQUAL-TO-1)

The hole concentration in oxygen-equilibrated La2-xSrxCuO4-x/2 increases linearly with x up to x ≈ 0.15, passes through a maximum value, and then decreases for further increases in x. This change in behavior is attributed to a shift in the charge-compensating defect from holes to oxygen vacancies. However, classical defect chemistry indicates that the hole concentration should only shift to a less rapid rate of increase with x, but not decrease. The observed decrease is caused by an increase in the enthalpy of the oxidation reaction that fills vacancies and replaces them with holes. This occurs because large concentrations of oxygen vacancies are effective in relieving imbalances in stress and charge that exist in the intergrowth structure of La2CuO4. As a result, the vacancies become increasingly favored over holes as the charge-compensating defect in the highly acceptor-doped material. The increase in the enthalpy of oxidation results in maximum values for both the hole concentration and Tc as functions of the acceptor content. © 1993 Academic Press, Inc.