Phonons in Ca_2-xSr_xCuO₃(x=0, 0.2 and 0.4): Raman and infrared spectroscopy, and lattice dynamics calculation

Abstract

The polarized Raman spectra of Ca2CuO3, Ca1.8Sr0.2CuO3 and Ca1.6Sr0.4CuO3 single crystals and the IR transmission spectra of Ca2CuO3 and Ca1.6Sr0.4CuO3 have been measured. The frequencies of all Gamma -point phonons of Ca2CuO3 have been determined on the basis of assignment of the one-phonon Raman (Ag) and IR lines to definite normal modes and calculations of the lattice dynamics. The line assignment is consistent with earlier results for Ca1.8Sr0.2CuO3 as well as with the line shifts upon substitution of Sr for Ca. The intensive broad lines in the (bb)-polarized Raman spectra of both Sr-free and Sr-substituted samples (in addition to the two Ag lines already assigned) have been attributed to resonantly enhanced two-phonon scattering involving modes related to motions of Cu and/or O(2) atoms. A coincidence has been found between most of the line frequencies and half-widths in the (bb)-polarized Raman spectra and some of the maxima in the two-phonon density of states as obtained from lattice dynamics calculations.