The A-site deficient ordered perovskite Th0.250.75NbO3: a re-investigation

The crystal structures of the non-stoichiometric perovskite Th0.250.75NbO3 prepared by quenching from a melt and by solidstate 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 (RF=7.1%), respectively. The overall tetragonal symmetry of the structure [P4/mmm; a=3.8956(1) and 3.8994(6) ?, c=7.8448(2) and 7.8502(11) ?, Z=2; where the smaller cell dimensions correspond to the powder data], is derived from the cubic perovskite aristotype by long range ordering of Th4+ along the c-axis into 1b sites. In response to this ordering, Nb atoms are displaced 0.08 ? along the c-axis towards the layers with vacant Asites (1a). 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 6ap. In contrast to some previous studies of this compound, neither long-range ordering of Th4+ within the layers, nor tilting of NbO6 polyhedra was observed. This study presents a statistically superior refinement of the structure of Th0.250.75NbO3.