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

Abstract

Uniaxial ferroelectrics with tetragonal tungsten bronze structure are important functional materials with photorefractive, electrooptic, piezoelectric, and pyroelectric properties. SrxBa1−xNb2O6 (SBN100x) with x > 50 is known as a typical uniaxial relaxor ferroelectric, while CaxBa1−xNb2O6 (CBN100x) undergoes nearly normal ferroelectric phase transitions. Single crystals of CSBN100x = [x(CBN28) + (1 − x) (SBN61)] = xCa0.28Ba0.72Nb2O6 + (1 − x) Sr0.61Ba0.39Nb2O6 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, Tc. 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 Tc. The elastic anomaly and slowing down of relaxation time of CSBN100x crystals become diffusive in the vicinity of Tc 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.

Temperature dependence of incommensurate modulation in Ca0.28Ba0.72Nb2O6

“…2(d). Above T C , where the material is a relaxor, 3,19 the intensity of the superstructure spots tends to zero at a nominal temperature of 600 C. This corresponds well with ultrasound and Brillouin scattering measurements of the temperature T * at which polar nanoregions become completely transient, 3,5,6,19 taking into account the difference in the nominal temperature and the actual value at the position of measurement. The well-behaved changes in intensity and δ in the second heating/cooling cycle are, therefore, probably a result of the fine domains in the material after the initial heating/cooling cycle.…”

“…, with a partially filled tetragonal tungsten-bronze (TTB) structure, is a uniaxial ferroic material of interest for dielectric, 1 non-linear optics, 2 and ferroelectric 3 applications. Congruent melting occurs at x ¼ 0:28, producing a thermally stable compound (CBN28) with a high Curie temperature T C , typically reported to be between 250 3 and 275 C. 4 Brillouin scattering and ultrasound observations indicate that it is a normal ferroelectric below T C , transitioning to relaxor behavior above 3,5 with polar nanoregions detectable up to T * 530 C and a Burns temperature T B 800 C. 6 Structurally, 180 DWs are found at room temperature, which become fine needles elongated along the polar axis as T C is approached. 7 Like other TTBs, CBN28 only adopts the prototype P4/mbm (No.127) tetragonal structure (a ¼ b ¼ 12:95 Å, c ¼ 3:95 Å) at high temperature.…”