[en] Rietveld refinements using neutron powder profiles are reported for a series of PbZr_1-xTi_xO₃ samples (commonly known as PZT), with x ranging from ∼0.12 to ∼0.40. Cation shifts, octahedral distortion and tilts are determined with varying composition across the ferroelectric rhombohedral regions, F_R(LT) and F_R(HT), of the PZT phase diagram. These parameters are then used in conjunction with a simple Landau-Devonshire model to investigate the nature of the F_R(LT) - F_R(HT) phase transition. It is found that the cation shifts, octahedral distortion and tilt angles decrease with increasing Ti content, but, surprisingly, the octahedral strain, as indicated by the rhombohedral angle, increases. This is in contrast to the case for all other known rhombohedral perovskites. Furthermore, the refined anisotropic displacement parameters of the cations are anomalous and cannot be accounted for by the average crystal structure. A model is presented in which a domain-type 'local' structure is considered, containing 'ordered' additional cation displacements, consistently with the reports of extra reflections observed in electron microscopy studies by Viehland et al, Dai et al and Ricote et al. (author)