Elastic Anomaly of Uniaxial Ferroelectric Strontium Barium Niobate with Very Weak Random Fields

Ferroelectric materials with tetragonal tungsten bronze (TTB) structure are technologically important for optical applications such as electro-optic, nonlinear optic, and photorefractive devices. Since the direction of spontaneous polarization is only along the c-axis of TTB structure, they are called uniaxial ferroelectrics. The structural formula of TTB ferroelectrics is expressed by (A1)2(A2)4(C)4-(B1)2(B2)8O30 with corner sharing distorted BO6 octahedra as shown in Fig. 1. In the case of ferroelectric Ba2NaNb5O15, the A1 and A2 sites are completely occupied by Ba and Na ions. Thus belongs to so called the filled TTB ferroelectrics, which undergo a normal ferroeletric sharp transition [1]. On the other hand, in SrxBa1-xNb2O6 (SBN100x), the A1 sites are occupied only by Sr ions and the A2 sites are occupied by both Ba and Sr ions. Since 1/6(A1+A2) sites remain unoccupied, it belongs to the unfilled TTB ferroelectrics. The randomly distributed vacancies at A1 and A2 sites are the main source of quenched random fields (RFs) due to the immanent charge disorder. They enhance the diffusivity of a broad phase transition. With the increase of the Sr content, the lattice entropy at A1 and A2 sites [2] increases as shown in Fig. 2, and the Curie temperature, TC decreases. Consequently, the diffusive nature increases due to the increase of the strength of RFs, which are believed to play a dominant role for the relaxorness. Thie is because quenched random fluctuations of RFs induce polar nanoregions (PNRs) which have a dominant contribution to the precursor phenomena of the ferroelectric phase transition [3,4]. In the present study, the effect of very weak RFs on the elastic properties of SBN26 (x = 0.26) single crystals was investigated using Brillouin scattering spectroscopy. The observed result of SBN26 is compared with that of SBN80 (x = 0.80) with strong RFs.