MODULAR CRYSTAL CHEMISTRY OF OXIDE SUPERCONDUCTORS REVISITED: POLYSOMATISM IN BISMUTH CUPRATES. M. O. Figueiredo, Crystallography and Mineralogy Center, IICT, Alameda D. Afonso Henriques, 41-4deg., P-1000 Lisbon, Portugal

Modular crystallchemistry stands on two fundamental concepts that have required successive refinements: i) polytypism, which is strictly concerned with layered arrangements, either imposing chemical identity to the planar modules in polytypes or allowing for slight chemical variations in polytypoids; ii) polysomatism, which releases the chemical constraint but restricts the structural affinity to the presence of identical modules, planar (layers) or columnar (beams). These concepts tie up crystallographic and chemographic relationships through a unified approach.

The basic square syngony of the atomic arrays in superconducting and allied oxide phases hinders true polytypism. Indeed, these layered arrays are based on coherent stackings of square-type anionic layers (labeled Q), either of a single metrics - in closest (Q_f) and simple (Q_s) stackings or in mixed sequences involving both stacking modes (Q_f/s) - or in composite arrays with layers of two distinct but coherent metrics (Q¹/Q²). Relationships with mineral structure types other than perovskite - namely, fluorite and diaboleite - have already been established (Figueiredo, 1993, Phase Trans. 43, 129).

An approach to polysomatic features parallel to those recognized in pyrolusite and afine oxide minerals is now considered. Polysomes in these minerals are generated by cutting out slabs of varied thickness from closepacked octahedral layers and interlinking these octahedral bands into columnar arrays with square or rectangular cross section.

A description is presented of identical structural processes relating the prototype structures of La₂CuO₄, Y₂BaCu₃O_7-( and Tl/HgBa₂CuO₅ with the series of bismuth strontium/barium cuprates, where examples of both commensurate (polysomatic interconnections) and incommensurable (modulated) adjustments may be found.