The A-site deficient ordered perovskite Th 0.25□ 0.75NbO 3: a re-investigation

Abstract

The crystal structures of the non-stoichiometric perovskite Th 0.25□ 0.75NbO 3 prepared by quenching from a melt and by solid-state reaction are determined to be identical on the basis of single crystal X-ray diffractometry 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 symmetry of the structure [ P4/mmm; a=3.8956(1) and 3.8994(6) Å, c=7.8448(2) and 7.8502(11) Å, Z=2; where the smaller cell dimensions correspond to the powder data], is derived from the cubic perovskite aristotype by long range ordering of Th 4+ along the c-axis into 1 b sites. In response to this ordering, Nb atoms are displaced 0.08 Å along the c-axis towards the layers with vacant A-sites (1 a). The TEM data obtained in this study indicate that long-range ordering also resulted in the formation of antiphase domains oriented parallel to [100] p and [010] p , and having an average periodicity ( M) of 6 a p . In contrast to some previous studies of this compound, neither long-range ordering of Th 4+ within the layers, nor tilting of NbO 6 polyhedra was observed. This study presents a statistically superior refinement of the structure of Th 0.25□ 0.75NbO 3.