CHARGE STATES OF ATOMS IN THE LATTICES OF THE HIGH-TEMPERATURE SUPERCONDUCTORS TL2BA2CAN-1CUNO2N-1CUNO2N+4(4 AND BI2SR2CAN)

To determine the charges of atoms in the lattices of the compounds Tl2 Ba2Can-1CunO2n+4 and Bi2Sr2Can-1CunO2n+4 (n = 1,2,3), the parameters o f the electric field gradient tensor at the copper sites of the indica ted lattices were found by emission Mossbauer spectroscopy on the isot opes Cu-61(Ni-61) and Cu-67(Zn-67), and a calculation of these paramet ers was performed in the point-charge approximation. A comparison of t he resulting values and the published data on Cu-63 nuclear quadrupole resonance showed that agreement between the experimental and computed values of the parameters obtains for models in which the holes result ing from a reduction in the valence of some of the thallium (bismuth) atoms are localized predominantly at oxygen sites located in the same plane as the copper atoms (for the compounds Tl2Ba2Ca2Cu3O10 and Bi1.6 Pb0.4Sr2Ca2Cu3O10-at oxygen sites in the same plane as the Cu(2) atoms ). (C) 1998 American Institute of Physics. [S1063-7761(98)02209-4].