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 Ca2Cu03, C a l . s S r ~. ~C u O ~ and Ca1.6Sro.rCuO3 single oystals and the IR transmission spenra of Ca~Cu03 and Cal.aSro.rOuO~ have been measured. The frequencies of all I'point phonons of Ca2Cuo3 have been dclermined on the basis of assignment of the one-phonon Raman (4) and IR lines 10 definite normal modes and alculalions of the lattice dynamics. The line assignment is wnsislenl wilh earlier mults for Cal.gSr0.2ChO3 as well BS with the line shifts upon substitution of Sr for Ca. I h e inlensive bmad lines in the (bb)polarized Raman speclra of both Sr-free and Sr-subtiluted ?ample (in addition l o the two As Lines already assigned) have been attributed to resonanlly enhanced two-phonon scattering involving modes related 10 motions of Cu andlor O(2) atoms. A coincidence has k e n found between most of the line frequencies and half-widlhs in the (bb)polarized Raman spectra and some of the maxima in the two-phonon density of stales as oblained fmm lattice dynamirs calculations.

Origin of the forbidden phonons in Raman scattering spectra of uranium‐doped Ca2CuO3, a spin 1/2 chain system

“…Hence, the A g mode phonons could be observed in the c(a, a)c and a(c, c)a scattering configurations. Indeed, they were the only observed features here, and therefore could unambiguously be associated with the Ca movement (307 cm −1 ) and O(1) movement (530 cm −1 ) [3,10] along the c-axis. For reasons unknown, no structure due to the B 1g and B 2g modes was observed.…”

“…2. Two studies have been previously reported for Sr-doped Ca 2 CuO 3 by Yoshida et al [3] and by Zlateva et al [10] As they used single crystals, the Raman-allowed phonons could be identified. According to group theory, the six Raman active modes ( -point phonons, k = 0) in the space group Immm (D 25 2h ) are 2A g +2B 1g +2B 2g (the other nine modes, 3B 1u +3B 2u +3B 3u , are the IR active).…”

“…This assignment well interprets the observed fact that the wavenumbers of the forbidden modes showed only a small variation among the doped and undoped samples, the crystalline and polycrystalline samples, despite differences in chemical content and structure. [3,5,10,11] The coincidence in wavenumbers is apparent especially for a group of higher resonances at 890, 943, 1140, 1215, and 1337 cm −1 (Fig. 2), which correspond to 880, 940, 1140, 1200, and 1330 cm −1 in Sr-doped Ca 2 CuO 3 (the inset, Fig.…”

“…The bands marked by arrows, which are better visible in the doped samples, correspond to 235, 435, 1003, and 1390 cm −1 . These four bands may be practically considered as absent for x = 0 (the first two of them were also not seen for x = 0 in the Sr-doped Ca 2 CuO 3 [10] ). For the purpose of comparison, the Raman scattering spectra for the Sr-doped Ca 2 CuO 3 , as obtained from Refs 3 and 10, are given in the inset in Fig.…”

“…Among them, only nine bands were visible for the undoped samples. [3,5,10,11] Considerable effort has been made to identify the origin of these forbidden modes in the undoped Ca 2 CuO 3 . There were two theoretical studies from Drechsler et al [4] and Hoang et al: [9] the first one based on the tight-binding approach and the second on the ab initio cluster modeling.…”