Antiferrodistortive phase transition in (formula presented) A powder neutron diffraction study

Abstract

Results of a powder neutron diffraction study of the high-temperature monoclinic (formula presented) to a low-temperature monoclinic (formula presented) phase transition in (formula presented) discovered recently by Ragini et al. are presented for (formula presented) The powder neutron diffraction pattern of the (formula presented) phase contains superlattice reflections which have all odd hkl (formula presented) Miller indices with respect to a doubled elementary perovskite cell, characteristic of antiphase tilting of oxygen octahedŕa. In analogy to other perovskite systems, the appearance of these superlattice reflections is attributed to an instability at the R point of the cubic Brillouin zone. We show that the most plausible space group of the (formula presented) phase is Pc. Results of Rietveld refinement for this space group are presented to show that the oxygen octahedra indeed undergo antiphase rotations about [001] direction. © 2002 The American Physical Society.