Phonons in Ca<sub>2-x</sub>Sr<sub>x</sub>CuO<sub>3</sub> (x=0, 0.2 and 0.4): Raman and infrared spectroscopy, and lattice dynamics calculation
G A Zlateva; V N Popov; M Gyulmezov; L N Bozukov; M N Iliev; G A Zlateva; Fac. of Phys., Sofia Univ., Bulgaria; V N Popov; Fac. of Phys., Sofia Univ., Bulgaria; M Gyulmezov; Fac. of Phys., Sofia Univ., Bulgaria; L N Bozukov; Fac. of Phys., Sofia Univ., Bulgaria; M N Iliev; Fac. of Phys., Sofia Univ., Bulgaria
Журнал:
Journal of Physics: Condensed Matter
Дата:
1992-11-02
Аннотация:
The polarized Raman spectra of Ca<sub>2</sub>CuO<sub>3</sub>, Ca<sub>1.8</sub>Sr<sub>0.2</sub>CuO<sub>3</sub> and Ca<sub>1.6</sub>Sr<sub>0.4</sub>CuO<sub>3</sub> single crystals and the IR transmission spectra of Ca<sub>2</sub>CuO<sub>3</sub> and Ca<sub>1.6</sub>Sr<sub>0.4</sub>CuO<sub>3</sub> have been measured. The frequencies of all Gamma -point phonons of Ca<sub>2</sub>CuO<sub>3</sub> have been determined on the basis of assignment of the one-phonon Raman (A<sub>g</sub>) and IR lines to definite normal modes and calculations of the lattice dynamics. The line assignment is consistent with earlier results for Ca<sub>1.8</sub>Sr<sub>0.2</sub>CuO<sub>3</sub> 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 A<sub>g</sub> 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.
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