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)BaxNb2O6, with x = 0.5 were investigated by using Raman spectroscopy over a wide temperature range from room temperature to 550 °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 ~350 °C. Especially, lowfrequency modes below ~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.