XANES studies of thallium compounds and valence states and local environment of thallium in some of its superconducting cuprates

The position and shape of X-ray absorption discontinuities have been used to deduce structural and chemical bonding information on model compounds of thallium and its superconducting cuprates. The L X-ray absorption spectra were recorded on a Rigaku X-ray Spectrometer and monochromatized using a. at crystal of Si (1, 1, 1). The background subtraction was done using Victoreen function. The normalized absorption coefficient is plotted as a function of Xray photon energy. It is observed that the XANES of L-I edge has fewer features as compared to those of L-III edge. The L-II edge spectra of thallium are similar to L-III edge spectra. The L-III absorption discontinuity of thallium splits into three components whereas no splitting is observed in L-I edges. Thallium L-III edge spectra show a single absorption peak for monovalent compounds whereas in trivalent compounds a weak peak followed by a shoulder on rising absorption which culminates into a strong absorption peak. The important difference between the spectra of monovalent and trivalent compounds is the shift in the inflexion point energy of L-III edges and a shift of the main absorption maximum. The X-ray absorption spectra of superconducting cuprates show similarities with the spectra of Tl2O3 suggesting the presence of Tl3+ ions in octahedral configuration in these cuprates.