Raman and infrared studies of Sr2TiO4: A material isomorphic to (La,Sr)2CuO4 superconductors

Abstract

The high-temperature superconductor (La2-xSrx)CuO4 has the K2NiF4 structure, as does Sr2TiO4. In order to understand better the phonon spectra obtained from ceramic (La2-xSrx)CuO4, we have performed Raman and infrared measurements on ceramic, insulating Sr2TiO4. All four Raman-allowed phonons are observed and found to be quite sharp, because in this centrosymmetric structure the Raman-mode frequencies are isotropic. The infrared reflectivity spectra are more difficult to interpret because reststrahlen bands are measured, which extend from the transverse-optic (TO) frequency to the longitudinal-optic (LO) frequency. Symmetry forbids overlap of the 3A2u bands as well as the 4Eu bands. The interpretation, nevertheless, is difficult because the A2u and Eu bands can overlap one another. By comparison with single-crystal results in fluorides and chlorides having the same structure, we have been able to identify many of the infrared modes in our spectra. This knowledge permits a determination of most of the infrared active modes in (La2-xSrx)CuO4. In the superconductor, two other effects aid our understanding of the spectra. (a) The effective charges are smaller than in Sr2TiO4, reducing the TO-LO splitting, and thus the overlap of the reflectivity bands. (b) The electron plasma is confined to the xy plane, and consequently it interacts strongly only with the Eu phonons, and leaves the A2u phonons largely unaffected. © 1988 The American Physical Society.