Raman characterization of Pb2Na1−xLaxNb5−xFexO15 and Pb0.5(5−x)LaxNb5−xFexO15 (0≤x≤1) solid solutions

The ferroelectric compounds Pb2Na1−xLaxNb5−xFexO15 and Pb0.5(5−x)LaxNb5−xFexO15 (0≤x≤1) with the tungsten bronze type structure have been investigated using Raman spectroscopy. The evolution of the spectra as a function of composition at room temperature is reported. In the frequency range 200-1000 cm−1 three main A1 phonons around 240 (υ1), 630 (υ2) and 816 (υ3) cm−1 were observed. The broadening of the Raman lines for high values of x originates from a significant structural disorder. This is in good agreement with the relaxor character of these compositions. The lowest-frequency part of the spectra, below 180 cm−1, reveals a structural change in the studied solid solutions. The behaviour of the Raman shift of the υ1 mode confirms that in Pb2Na1−xLaxNb5−xFexO15, a clear anomaly occurs in the vicinity of x=0.4.