A-Site Cation Disorder in Sr_x Ba_1-x Nb₂O₆ (x=0.25, 0.33, 0.50, 0.61) Studied by High-Resolution Resonant X-ray Powder Diffraction

The dielectric and ferroelectric properties of SrxBa1-xNb2O6 (SBN, 0.2 < x < 0.8) are known to be affected by the Sr fraction and can be further controlled by various quenching schemes. Changes in A-site cation configuration are believed to be linked to these changes in properties. In this work, we study the A-site cation disorder in SBN by the use of high-resolution resonant X-ray powder diffraction. The experimental results show that the larger Ba2+ is found exclusively on the larger A2 site, while Sr2+ is found on both the A1 and A2 sites, with an increasing amount on A2 with an increasing Sr fraction. At elevated temperatures, a small migration of Sr2+ from A1 to A2 is observed for SBN50 and SBN61. Linking this change in occupancies to changes in the average cation size on the A1 and A2 sites allows for rationalization of the property changes observed for quenched samples. Furthermore, SBN25 is shown to deviate from the tetragonal P4bm structure and is found to be orthorhombic with a Cmm2 structure.