Raman phonons in RuSr₂GdCu₂O₈

The Raman spectra of polycrystalline samples of RuSr₂GdCu₂O₈, annealed under high pressure (3 Gpa, 1000°C), were studied between 10 and 420 K. The ferromagnetic ordering has a little (if any) effect on the variations with temperature of the position and width of the Raman lines at 158, 318, 440 and 659 cm^-1. In contrast, for the line near 270 cm^-1 one observes a local maximum of the halfwidth near the magnetic transition and strong hardening upon cooling below T_c. The observation of the latter line, which has no analog in the YBCO-type compounds, supports either I4/mcm or P4/mbm crystal structure, where, due to the tilting of the RuO₆ octahedra, the unit cell is doubled and new Raman modes are activated. The lattice dynamical calculations (LDC) predict that the Raman mode with the closest wavenumber to that of the 270 cm^-1 line is the A_1g mode involving rotations of CuO₅ pyramids. Such an assignment alone cannot explain the anomalous behavior near T_c. A reasonable explanation could be given considering the 270 cm^-1 line as a superposition of two A_1g lines corresponding to rotations around c-axis of CuO₅ pyramids (265 cm^-1) and RuO₆ octahedra (275 cm^-1), respectively.