The A-site deficient ordered perovskite Th-0.25 square 0.75NbO3: a re-investigation

The crystal structures of the non-stoichiometric perovskite Th(0.25)square( 0.75)NbO(3) prepared by quenching from a melt and by solid-state reaction a re determined to be identical on the basis of single crystal X-ray diffract ometry using a CCD area detector (final R=2.4%) and Rietveld refinement of the XRD powder pattern (R-F=7.1%), respectively. The overall tetragonal sym metry of the structure [P4/mmm; a = 3.8956(1) and 3.8994(6) Angstrom, c=7.8 448(2) and 7.8502(11) Angstrom, Z = 2; where the smaller cell dimensions co rrespond to the powder data], is derived from the cubic perovskite aristoty pe by long range ordering of Th4+ along the c-axis into 1b sires. In respon se to this ordering, Nb atoms are displaced 0.08 A along the c-axis towards the layers with vacant A-sites (1a). The TEM data obtained in this study i ndicate that long-range ordering also resulted in the formation of antiphas e domains oriented parallel to [100](p) and [010](p), and having an average periodicity (M) of 6a(p). In contrast to some previous studies of this com pound, neither long-range ordering of Th4+ within the layers, nor tilting o f NbO6 polyhedra was observed. This study presents a statistically superior refinement of the structure of Th(0.25)square(0.75)NbO(3) (C) 2000 Elsevie r Science S.A. All rights reserved.