Brillouin Scattering Study of Ferroelectric Instability of Calcium-Strontium-Barium Niobate Single Crystals.

Uniaxial ferroelectrics with tetragonal tungsten bronze structure are important functional materials with photorefractive, electrooptic, piezoelectric, and pyroelectric properties. Sr x Ba 1- x Nb 2 O 6 (SBN100 x ) with x > 50 is known as a typical uniaxial relaxor ferroelectric, while Ca x Ba 1- x Nb 2 O 6 (CBN100 x ) undergoes nearly normal ferroelectric phase transitions. Single crystals of CSBN100 x = [ x (CBN28) + (1 - x ) (SBN61)] = x Ca 0.28 Ba 0.72 Nb 2 O 6 + (1 - x ) Sr 0.61 Ba 0.39 Nb 2 O 6 with nominal x = 0.00, 0.25, 0.50, 0.75, and 1.00 were studied to clarify the dynamical properties at the crossover from relaxor ( x = 0) to normal ( x = 1) ferroelectric behavior. The longitudinal acoustic (LA) and transverse acoustic (TA) modes and a central peak (CP) related to the relaxation process of polarization fluctuations along the polar c -axis were studied in uniaxial ferroelectric CSBN single crystals as a function of temperature via Brillouin scattering spectroscopy. A CBN28 ( x = 1.00) crystal shows the sharp elastic anomaly of the LA mode in the gigahertz range toward Curie temperature, T c . However, those of CSBN25 ( x = 0.25) and SBN61 ( x = 0.00) crystals show diffusive anomalies due to stronger random fields. The relaxation time determined from the width of a CP shows a critical slowing down in the vicinity of T c . The elastic anomaly and slowing down of relaxation time of CSBN100 x crystals become diffusive in the vicinity of T c as the CBN28 content decreases. The origin of the crossover from relaxor to normal ferroelectric phase transitions is discussed in terms of the difference in the A1 and A2 sites' occupancies.