Valences of Cu and Tl and chemical bond parameters in TlxBa2CuyO6+delta, Tl2Ba2CaCu2O8, and Tl2Ba2Ca2Cu3O10

The valences of Cu and Tl and chemical bond parameters, that is, bond coval ency and macroscopic linear susceptibility in TlxBa2CuyO6+delta, Tl2Ba2CaCu 2O8, and Tl2Ba2Ca2Cu3O10 have been calculated using a semi-empirical method . For TlxBa2CuyO6+delta, the results indicate that the average valences of Tl and Cu are between +2.0 and +3.0, indicating that part of Cu and TI is i n a state of +3.0. With the increase of oxygen content and Tl/Cu (x/y) rati os, the valence of T1 increases, while for the valence of Cu this trend is only observed for fixed Tl/Cu ratio samples. The valence of TI changes more rapidly than that of Cu. For samples where both oxygen content and TI/Cu r atio change, there exists an optimal valence of Cu and T1 for the highest T -c. On the other hand, for fixed TI/Cu ratio samples, the smaller the valen ces of Cu and T1, the higher the T-c. The bond covalencies of Ba-O, Cu-O (i n CuO2 layer), and Tl-O (the bond of T1 with the inserted oxygen between do uble TlO layers) are sensitive only to the Tl/Cu ratios, while those of the remaining Tl-O bonds and Cu-O (outside CuO2 layer) are influenced by both. The macroscopic linear susceptibility tends to decrease with the increase of oxygen content and Tl/Cu ratios. Finally, Tl2Ba2CaCu2O8 and Tl2Ba2Ca2Cu3 O10 have also been investigated. (C) 1999 Elsevier Science B.V. All rights reserved.