The commensurate crystal structure of the magnetic phase occurring below 7 K of the multiferroic Pb2MnWO6 perovskite has been solved by the introduction of the superspace approach. This lead based double perovskite is characterized by a complex ferrielectric noncentrosymmetric nuclear structure with orthorhombic symmetry stable in a wide temperature range. As indicated from the analysis of powder neutron diffraction data, the low temperature antiferromagnetic structure showing propagation vector κ= [1/4 0 0] is stabilized by a multistep process involving the evolution from incommensurate to commensurate spin ordering with a concomitant change of the magnetic symmetry. The determination of the Pb2MnWO6 magnetic structure offers a meaningful example of the superspace application and provides a detailed phase diagram of the involved magnetic states. Nowadays this ordered perovskite could be considered as a new type of multiferroic material encountering ferrielectric property and long period antiferromagnetic structure.
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