Ferroelectric Tungsten Bronze‐Type Crystal Structures. I. Barium Strontium Niobate Ba0.27Sr0.75Nb2O5.78

TLDR: The Ferroelectric Ba0.27Sr0.75Nb2O5.78 as discussed by the authors is a tungsten bronze-type structure crystallizing in the tetragonal system, with lattice constants a = 12.43024

Abstract: Ferroelectric Ba0.27Sr0.75Nb2O5.78, with Tc = 348° ± 15°K, is a tungsten bronze‐type structure crystallizing in the tetragonal system, with lattice constants a = 12.43024 ± 0.00002 and c = 3.91341 ± 0.00001 A at 298°K, space group P4bm, and five formulas in the unit cell. The integrated intensities of 6781 structure factors were measured with PEXRAD, 875 symmetry‐independent structure factors being significantly above background. The metal‐atom positions were determined from the three‐dimensional Patterson function and the oxygen atoms from subsequent Fourier series. The final agreement factor between measured and calculated structure factors is 0.0508. The structure consists of close‐packed slightly puckered layers of oxygen atoms separated by nearly c / 2. The Nb atoms are slightly displaced from one layer, the Ba and Sr atoms from the other and in the same sense. The oxygen atoms in the Ba and Sr layer are disordered. Neither of the two independent sites occupied by the Ba and Sr atoms is fully filled....