Raman phonons in RuSr2GdCu2O8

The Raman spectra of polycrystalline samples of RuSr2GdCu2O8, annealed under high pressure (3 Gpa, 1000 degreesC), 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.