VALENCE STATES OF POSTTRANSITION CATIONS (HG AND BI) AND OF COPPER IN SINGLE-LAYERED SUPERCONDUCTING MERCURY CUPRATES

In order to study the charge transfers in the mercury HTC superconductors, the valence states of heavy cations (Bi, Hg) and copper have been investigated by X-ray absorption spectroscopy Two new mercury single-layer cuprates have been chosen: one exhibiting the ''1201'' type structure, Bi0.5Hg0.5Sr1.5La0.5CuO5-delta and the second, Bi0.5Hg0.5Sr2Ca0.75Nd0.25Cu2O7-delta, of the ''1212'' type structure. By a quantitative analysis of the Cu K-edge, the copper formal charge has been evaluated between +2.2 and +2.25 in the [CuO2](infinity) planes in agreement with the superconducting properties of the compounds. At the Bi L(3) edge, the formal charge of bismuth has been found around +4 which is the result of a mixture of Bi3+ and Bi5+ in the [(Bi, Hg)O](infinity) planes. The Hg L(3) edges of some reference compounds, Hg2Cl2, HgO and CaHgO2, and of the superconductors show that mercury remains in the +2 formal charge in the latter compounds, the variation of the pre-peak (2p(3/2)--> 5d ) intensity being due to the density of oxygen vacancies in the [(Bi, Hg)O](infinity) planes perpendicular to the dumbbell axis of the mercury environment.