Change in periodicity of the incommensurate magnetic order towards commensurate order in bismuth ferrite lead titanate

xBiFeO3–(1−x)PbTiO3 has been synthesised in bulk polycrystalline ceramic form and then self-disintegrated to form powder (P4mm) (x=0.7) or crushed (R3c) (x=0.75, 0.9) to relieve stress. High-resolution neutron powder diffraction has been employed to observe incommensurate antiferromagnetic ordering within the materials, and its dependence on phase at 7.5 K. It is shown that in the rhombohedral phase the period of the magnetic structure depends on PbTiO3 addition, and increases from 790 Å (x=0.9) to 840 Å (x=0.75). The tetragonal phase restores the collinear antiferromagnetic order.