A STUDY OF POSITIONAL DISORDER IN STRONTIUM BARIUM NIOBATE

The first systematic examination of trends in the site occupancies of strontium and barium ions in strontium barium niobate (SBN) at three c ompositions across the phase range is presented. X-ray powder diffract ion and Rietveld structure refinement were used to obtain refined stru ctures for SBN. Some problems were encountered in the structure refine ment due to the domain structure in the material. Barium was found onl y at the A2 site for each composition. Ba/Sr occupancy for the A2 site varied from 91% for SBN (72-28) to 99% for SBN (50-50) in the unannea led state, while strontium occupancy of the Al site varied from 70% to 54%. In annealed SBN (50-50), the A2 site occupancy decreased to 95%, while the Al site occupancy increased to 62%. It is proposed that a t wisting of the structure in the unannealed state decreases the sizes o f the cages at the A1, A2 and C sites. After annealing, the structure becomes more regular, enlarging the A1, A2 and C sites and allowing mo re strontium atoms to occupy the A1 site. Cation distribution is contr olled by the twisting of the octahedral framework. The overall propert ies of the material are modified by this distortion of the structure.