Calculated crystal and defect properties of ternary cuprates M₂CuO₃(M identical to Ca, Sr): relationship to La₂CuO₄ and high-T_c superconductivity

Attention is drawn to the requisite dimensionality of the superconducting networks in high-T_c ternary and quaternary cuprates. Calculations are reported of the lattice and defect properties of Ca₂CuO₃ and Sr₂CuO₃ compared with those of La₂CuO₄. The former consist of one-dimensional CuO₂ chains, and the latter of two-dimensional CuO₂ planes; both lead to calculated phonon densities of states which are similar in the 10 meV region. Different defect structures are calculated for the two types of material: La vacancies and holes in La₂CuO₄, and O interstitials and holes in Ca₂CuO₃ and Sr₂CuO₃. In both cases, however, the putative conducting networks are free of Cu and O vacancies. Li is predicted to substitute for both cation sublattices in Ca₂CuO₃ and Sr₂CuO₃, but Na solely for the alkaline-earth ions with the creation of holes. These are predicted to be largely O 2p band, as in Nd₂CuO₄, unlike La₂CuO₄, for which Cu 3d band holes were found to be lowest in energy.