Defect chemistry of undoped La sub 2 CuO sub 4

La{sub 2}CuO{sub 4} is an oxygen-excess, p-type material over the range 700-900C and 10{sup {minus}2}-10{sup 5} Pa of oxygen. The major defects are interstitial oxygen and holes. The enthalpy of oxidation is near zero, and the hole mobility increases slightly with decreasing temperature, characteristic of band conduction. The preference for oxygen-excess nonstoichiometry is attributed to the presence of an interstitial site that is of adequate size for an oxide ion, to local charge and stress imbalances in the La{sub 2}CuO{sub 4} lattice that favor the presence of holes in the Cu-O planes, and to large, negatively charged oxygen interstitials in the NaCl layers of the intergrowth structure.