Antiferrodistortive phase transition in Pb(Ti_0.48Zr_0.52)O₃ :A powder neutron diffraction study

Abstract

Results of a powder neutron diffraction study of the high-temperature monoclinic ( F^HT_M ) to a low-temperature monoclinic ( F^LT_M ) phase transition in Pb (Ti_1−xZr_x)O₃ discovered recently by Ragini et al. are presented for x = 0.520. The powder neutron diffraction pattern of the F^LT_M phase contains superlattice reflections which have all odd hkl ( h ≠ k ≠ l ) 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 F^LT_M 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.