Crystal Structure of High-Temperature Paraelectric Phase in Bi-Layered Perovskite Sr_0.85Bi_2.1Ta₂O₉

Abstract

The crystal structure of Bi-layered perovskite Sr 0.85 Bi 2.1 Ta 2 O 9 was determined at 750 K by X-ray Rietveld analysis. Sr 0.85 Bi 2.1 Ta 2 O 9 with space group A2 1 am at room temperature transforms to the paraelectric phase above 670 K, passing through a probable intermediate state at 510 K. Assuming two possible orthorhombic Fmmm and tetragonal I4/mmm space groups, analysis of the crystal structure of the paraelectric phase gave the final discrepancy factors R w p of 11.3% and 9.9%, respectively. Examination of possible reflections and peak positions at high-angles proved that the true space group is I4/mmm for the high temperature paraelectric phase. The lattice constants are a =3.927 (1), c =25.142 (1) Å at 750 K. The TaO 6 octahedron is not perfect and is slightly distorted along the c -axis, having the dipole moment of 0.478 debye.