Valence states of thallium and copper in some substituted thallium high Tc superconductors by XAS and their influence on the critical temperature

✍ Scribed by F. Studer; N. Merrien; C. Martin; C. Michel; B. Raveau; A. Fontaine

• Publisher: Elsevier Science
• Year: 1991
• Tongue: English
• Weight: 997 KB
• Volume: 178
• Category: Article
• ISSN: 0921-4534
• DOI: 10.1016/0921-4534(91)90081-9

✦ Synopsis

An investigation of the valence states of thallium and copper in some high-To superconducting layered thallium cuprates has been undertaken by XAS at T1 Lm-and copper K-edges. At the thallium Lm-edge, a general shift of the edge transitions towards low energies has been observed on the superconductors spectra with respect to the Tl(III) reference compounds, indicating the presence of extra electrons on the [Tl-O ]oo layers of the structures. Furthermore, the problem of the increase of the 6s prepeak intensity on the superconductor edges with respect to the T1203 and Sr4T1207 compounds has been considered on the basis of the local oxygen environment of thallium ions.

The copper K-edge spectroscopy is very sensitive to the hole carrier density via the change in the admixture of configuration (al 3d9> +ill 3d°-L) with Og2-~j~2= 1 ) used to describe the essentially divalent Cu 1~ of the (CuO2)~o layer. In the square planar structure of the cuprate families hole doping increases ct 2 (whereas e-doping does the reverse as found in Nd2CuO4). Therefore the hole-doping is correlated to a shift of the copper K-threshold to higher energy value but also, at second order, to the crystal field symmetries, i.e. to the copper environment. As a general trend, the spectra at the copper K-edge of yttrium substituted cuprates ("1212" and "2212" types), in which copper ions are kept in a constant environment, show a decrease of the energy shift with increasing substitution. At the same time, Tc passes through a maximum for an energy shift of 6.2 eV suggesting the existence of an optimum hole density for superconductivity.