STRESS AND CHARGE IMBALANCES IN SUPERCONDUCTING CUPRATES

La2CuO4 and Nd2CuO4 have layered structures, with alternating perovski te and NaCl layers in the former, and alternating perovskite and fluor ite layers in the latter. Because of bond length mismatches, the perov skite layers in La2CuO4 are in compression relative to the NaCl layers , while in Nd2CuO4 the perovskite layers are in tension. There are als o charge imbalances between the alternating layers that are in the opp osite sense in the two compounds. The compounds differ in their prefer red direction of nonstoichiometry, the solubility of aliovalent impuri ties, and the electrical properties of the doped materials in the norm al state, and these differences are correlated with the relief of the imbalances in stress and charge between the alternating layers. It is shown that as a result, the enthalpy of oxidation of acceptor-doped La 2CuO4 increases with increasing acceptor concentration, and this resul ts in a maximum in the hole concentration in the oxidized material as a function of acceptor concentration.