Raman spectroscopy and lattice-dynamics calculations of mixed layered copper-titanium oxides

We report micro-Raman spectra obtained from R 2 Ba 2 Ti 2 Cu 2 O 11 (R = Nd, Gd) and Gd 2 CaBa 2 Ti 2 O 12 ceramic samples. The analysis of the spectra was performed using the similarity between the investigated structures and related layered pure copper and titanium oxides. The assignment of the observed lines to definite atomic vibrations is supported by lattice-dynamics calculations, based on a shell model. The calculated frequencies for the IR-active modes are also presented. We stress that in contrast to Gd 2 CaBa 2 Ti 2 Cu 2 O 12 , where the ceramics consist of optically anisotropic plate-like microcrystals, in the case of the quadruple perovskites R 2 Ba 2 Ti 2 Cu 2 O 11 the microcrystals are isotropic, probably due to the fine twinning, rendering it impossible to obtain polarized Raman spectra along different crystal directions.