Antiferrodistortive phase transition in Pb(Ti_0.48Zr_0.52)O₃: Space group of the lowest temperature monoclinic phase

Ranjan et al. [Phys. Rev. B 65, 060102(R) (2002)] have recently presented results of a powder neutron-diffraction study of the high-temperature monoclinic (F_M^HT) to low-temperature monoclinic (F_M^LT) phase transition in Pb(Ti_1-xZr_x)O₃ discovered by Ragini et al. [Phys. Rev. B 64, 054101 (2001)]. They attribute the presence of superlattice reflections in the diffraction data to tilting of oxygen octahedra and propose a monoclinic space group Pc for the F_M^LT phase. It is shown that for the model proposed by Ranjan et al., the correct space group of the F_M^LT phase should be Cc. This has also been corroborated by a group-theoretical approach to the problem. A different set of refined structural parameters for the Cc space group obtained from the Rietveld analysis of the powder neutron-diffraction data of Ranjan et al. is also presented.