THE ROLE OF CRYSTALLOGRAPHY FOR BETTER HIGH Tc SUPERCONDUCTORS. Catherine Chaillout, Laboratoire de Cristallographie, CNRS-UJF, BP 166, 38042 Grenoble Cedex 09, France

Since 1986, a large number of high Tc superconducting cuprates of general formula A_mM₂R_n-1Cu_nO_x have been discovered. Their structure consists in the stacking of (AO), (MO), (R), and (CuO₂) layers along the c-axis, where A= Bi Tl, Hg, Cu, or C, M= Sr, or Ba, R=Ca or a rare earth cation. The superconducting transition temperatures obtained for the more recently discovered phases are among the highest, namely 135K at ambient pressure and [[circleplus]]164K under [[circleplus]]35 GPa for the third member of the Hg-based phases.

This lecture will emphasize the role that crystallography can play in the study of these phases and in the search for new superconducting compounds with possibly higher T_c's.

Examples will be taken mainly in the Hg-based and oxycarbonates phases, to illustrate the influence of the oxygen stoichiometry, the cation substitution,.. on the physical properties. The role of pressure, either in the synthesis, or for the structure determination will be also discussed.

Some time will be devoted to the way the observations of structural defects can yield to the synthesis of new phases. The example of Hg₂Sr₂YCu₂O_x will be given.

All along this lecture, we will insist upon the necessity to use simultaneously different techniques such as diffraction, absorption, and microscopy in order to get the relationships between the local structure and the physical properties. This is due to either the complex structural arrangements, or the preparation method of the samples such as the pressure-induced synthesis.