Antiferrodistortive phase transition in $\mathrm{Pb}\left({\mathrm{Ti}}_{0.48}{\mathrm{Zr}}_{0.52}\right){\mathrm{O}}_3:$ A powder neutron diffraction study

Results of a powder neutron diffraction study of the high-temperature monoclinic ${(F}_M^{\mathrm{HT}})$ to a low-temperature monoclinic ${(F}_M^{\mathrm{LT}})$ phase transition in $\mathrm{Pb}\left({\mathrm{Ti}}_{1-x}{\mathrm{Zr}}_x\right){\mathrm{O}}_3$ discovered recently by Ragini et al. are presented for $x=\mathrm{0.520.}\mathrm{}$ The powder neutron diffraction pattern of the $F_M^{\mathrm{LT}}$ phase contains superlattice reflections which have all odd hkl $\left(h\ne k\ne l\right)$ 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 $F_M^{\mathrm{LT}}$ 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.