Multiple Softening of Low-Frequency Raman Modes of Tungsten-Bronze Lead Barium Niobate Relaxor Ferroelectric Single Crystals

The vibrational properties of tetragonal tungsten-bronze lead barium niobate, Pb((1-x))BaxNb(2)O(6), withx= 0.5 were investigated by using Raman spectroscopy over a wide temperature range from room temperature to 550 degrees C. The Raman modes in the measured spectrum were divided into three spectral ranges, internal symmetric and antisymmetric vibrational modes of the oxygen octahedra and low-frequency external lattice modes. The temperature dependences of the Raman shifts showed anomalous changes at the ferroelectric phase transition temperature of similar to 350 degrees C. Especially, lowfrequency modes below similar to 300 cm(-1)commonly exhibited multiple softening upon heating toward the transition temperature, indicating that the lattice becomes softer on approaching the ferroelectric phase transition temperature.