Large structural modulations in the relaxor ferroelectric and intermediate state of strontium rich members (x>0.6) of the SrxBa1−xNb2O6 (sbn) solid solution series

• Published in: Journal of solid state chemistry Vol. 246; pp. 167 - 175
• Main Author: Graetsch, H.A.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01-02-2017
• Subjects: Incommensurate modulation; Intermediate paraelectric state; Relaxor ferroelectric; Strontium barium niobate; Strontium barium niobate; Intermediate paraelectric state; Relaxor ferroelectric; Incommensurate modulation
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2016.11.013

The amplitudes of the positional, occupational and adp modulations of sbn mixed crystals are strongly enhanced for high strontium contents. The increase of structural modulations is accompanied by reduced spontaneous electric polarization largely due to smaller off-center shifts of the niobium atoms. Beyond the room temperature ferroelectric – intermediate transition near x=0.77, anomal large U33 atomic displacement parameters of the niobium atoms indicate static disorder caused by loss of orientational coupling between residual shifts of Nb atoms in neighboring NbO6 octahedra. Change of satellite intensities show a reduction from two-dimensional to one-dimensional modulation which is not consistent with tetragonal symmetry. The pseudo-tetragonally twinned crystal structure of sbn82 was refined in the orthorhombic super-space group A2mm(½0γ)000. The apparent tetragonal symmetry of the other investigated sbn samples also seems to be due to pseudo tetragonal twinning with equal twin volumes. The modulations mainly consist of cooperatively tilted NbO6 octahedra and wave-like ordered incomplete occupation of the largest cation sites (Me2a and b) by Ba2+ and Sr2+. Furthermore, the atomic displacement parameters of the Me2 sites are strongly modulated.
Satellite reflections and modulation coefficients in the solid solution series SrxBa1−xNb2O6 [Display omitted]
•The modulationed structures are refined for the whole composition range of sbn32–sbn82 in tetragonal and orthorhombic setting.•The amplitudes of positional, occupational and adp modulations increase strongly with the strontium content.•Evidence is presented that the sbn crystals are pseudo tetragonally twinned.•The ferroelectric–intermediate paraelectric transition is not accompanied by a change of symmetry.•Anomal adp of intermediate (non-ferroelectric) sbn82 indicate loss of coupling between off-center shifts of neighboring niobium.