FREQUENCY-SHIFTS OF OPTICAL PHONONS DUE TO SIZE-EFFECT INDUCED CHARGE-TRANSFER IN TL(BA1-XSRXLA)CUO5

Abstract

Raman-scattering and infrared-absorption spectra of optical phonons in the ceramic Tl(Ba1-xSrxLa)CuO5 oxide superconductor that contains a single Tl-O and Cu-O layers are reported as functions of Sr content. Replacing Ba-ions by isovalent Sr-ions leads to remarkable changes in the electronic and superconducting properties of these materials. As a consequence of the charge transfer from the TlO to the CuO2 plane (by a redox mechanism), the Raman-active apical-oxygen phonon at 526 cm-1 and the 623 cm-1 IR-active phonon (associated with the TlO-plane oxygen ions) show respectively an approximately 18 cm-1 and a approximately 20 cm-1 frequency increase with increasing x. The changes in the Raman-active phonon linewidth with increasing x are attributed to a decreasing disorder due to the smaller ionic radii mismatch in the Ba site.