Temperature-dependent Raman scattering study of the defect pyrochlores RbNbWO6 and CsTaWO6

Lattice dynamics calculations and temperature-dependent Raman scattering experiments were performed on RbNbWO6 and CsTaWO6 pyrochlore oxides. The observed bands were assigned to the respective motions of atoms in the unit cell. The spectra showed the presence of additional Raman bands not allowed for by the Fd (3) over barm cubic structure. We have shown that these bands appear due to both substitutional disorder in the 16c sites and displacive disorder of the A ions. Raman studies also revealed the presence of an additional 80 cm(-1) band at room temperature for RbNbWO6, not observed for CsTaWO6. The presence of this band has been attributed to off-center displacement of the Nb and W ions due to structural phase transition into a tetragonal ferroelectric phase. The temperature evolution of the 80 cm(-1) band intensity revealed that it disappeared at a much higher temperature (about 650 K) than the reported phase transition temperature (about 360 K). This behavior is reminiscent of chemically disordered perovskite ferroelectrics, including relaxor ferroelectrics, and was attributed to the presence of small polar regions with local tetragonal distortion embedded in the paraelectric matrix of the Fd (3) over barm structure.