Systematic Raman study of Tl2Ca/sub n//sub -1/Ba2 Cu/sub n/O/sub 4+2//sub n/ (n = 1,2,3) high-temperature superconductors

We report the results of Raman measurements performed on polycrystalline samples of superconducting Tl2Ca/sub n//sub -1/Ba2Cu/sub n/O/sub 4+2//sub n/ (n = 1,2,3) with well-characterized transport, magnetic, and structural properties. The room-temperature first-order Raman spectra of polycrystalline samples of predominantly Tl2Ba2CuO6, Tl2CaBa2Cu2O8, and Tl2Ca2Ba2Cu3O/sub 10/ phases, with respective superconducting transition temperatures of 10, 110, and 125 K display a systematic variation with the number of CuO2 layers and great similarities with the Raman spectrum of YBa2Cu3O7. Comparison with Raman spectra of the impurity phase candidates BaCO3, BaO2, BaCuO2, CaCuO2, Tl2O3, and TlCa5Cu6O/sub x/ allows identification of intrinsic Raman modes of the superconducting phases. Factor group analysis of the three body-centered tetragonal structures space group 14/mmm, yields the mode classifications and the atoms involved in the modes of A1/sub g/ and B1/sub g/ symmetry corresponding to c-axis motion that produce the strongest modulation of the polarizability. The systematic variation between the spectra of the three phases and comparison with the well-characterized YBa2Cu5O/sub x/ Raman spectrum suggests mode assignments for the high-frequency modes of A1/sub g/ and B1/sub g/ symmetry involving oxygen motion