Structural and Raman properties of the tetragonal tungsten bronze ferroelectricPb2.1/K1CxGdxNb5O15

摘要

Using X-ray diffraction and Raman spectroscopy we studied the Pb2.1/K1CxGdxNb5O15 (PKGN) ceramicsover the composition range 01. The results reveal two orthorhombic phases, with space groupCm2m and Pba2 for 00:2 and 0:30:6, respectively, and a tetragonal phase, with space groupP4bm, for 0:71. These suggest a rotation of polarization of 90, whenincreases, from the ab-plane(00:2) to the c-axis (0:31). The evolution of Raman spectra as a function of x confirmsthe structural changes in PKGN compounds and shows a strong dependence of the concentration of Gdmainly in the low-frequency part (<170 cm), which reflects the vibrations of cations in the tetragonaltungsten bronze (TTB) structure. The behavior of the ObO bending vibrations seems to be a signatureof the phase modifications as a function of x in PKGN. The Raman results provide strong evidence of thereduction of the disorder in the TTB structure when going from Pb to Gd. This disorder is connected to theorthorhombic distortion, which is reduced when going from orthorhombic to the tetragonal phase.