Hydrothermal Synthesis and Characterization of Four Oxalatotitanates with Ti4O4(C2O4)8 Tetramers as Basic Building Blocks

The involvement of O- in oxide superconductivity is indicated in a variety of phenomenological rules such as a universal scaling of Tc with stoichiometric holes per O. Here, a crystal chemistry of O- creation and placement is developed, based on the competition of various types of planes (CuO2, E, B1). Generally, cuprate structural stability is dictated by slab matching, conventionally expressed in tolerance factors (t). Electronic liquefaction through subperoxide formation can ameliorate t, indicating a decisive influence of lattice pressure on the limits of O- formation. Comparison of calculation with experiment indicates trends to lattice commensurable hole patterns, of which the one responsible for optimal Tc is an alternate hole charge order (P2), for both simple and complex cuprates. For complex cuprates, tension on the layers containing M=Bi, Hg, T1 leads to additional reductive self-dopings. Calculations indicate that in optimally doped materials this leads universally to P2. The Tc decreases beyond this optimal doping, usually referred to as "overdoping," are here related to overfilling of this pattern. An extreme case of this occurs with nonsuperconducting Bi2Sr2CuO6, which appears to be P1. This overdoping is caused by severe lattice pressure causing the annihilation of superconducting pairs. A distinction has to be made with a second type of "early" Tc decreases. They occur before attainment of P2 and are due to an unfavorable hole placement crystal chemistry. An example is the disorder in [(LaSr)2CuO4]. Tc optimization involves navigating between these antagonistic principles.