Charge states of atoms in the lattices of the high-temperature superconductors Tl2Ba2Ca n−1CunO2n+4 and Bi2Sr2Ca n−1CunO2n+4

Abstract

To determine the charges of atoms in the lattices of the compounds Tl2Ba2Can−1CunO2n+4 and Bi2Sr2Can−1CunO2n+4 (n=1,2,3), the parameters of the electric field gradient tensor at the copper sites of the indicated lattices were found by emission Mossbauer spectroscopy on the isotopes 61Cu(61Ni) and 67Cu(67Zn), and a calculation of these parameters was performed in the point-charge approximation. A comparison of the resulting values and the published data on 63Cu nuclear quadrupole resonance showed that agreement between the experimental and computed values of the parameters obtains for models in which the holes resulting 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 Bi1.6Pb0.4Sr2Ca2Cu3O10—at oxygen sites in the same plane as the Cu(2) atoms).