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We have investigated the magnetic ground state of the multiferroic relaxor ferroelectric Pb͑Fe 1/2 Nb 1/2 ͒O 3 with SR spectroscopy and neutron scattering. We find that a transition to a partially disordered phase occurs below T = 20 K that coexists with long-range antiferromagnetic ordering. The disordered phase resembles a spin glass. No clustering of magnetic ions could be evidenced by SR so that the coexistence appears homogeneous in the sample.

The relaxor ferroelectric PbMg1/3Ta2/3O3 was studied by single‐crystal neutron and synchrotron X‐ray diffraction, and its detailed atomic structure modelled in terms of static Pb displacements that lead to the formation of polar nanoregions. Similar to the other members of the Pb‐based relaxor family like PbMg1/3Nb2/3O3 or PbZn1/3Nb2/3O3 the diffuse scattering in the [H00]/[0K0] scattering plane has a butterfly shape around the h00 Bragg reflections and is orthogonal to the scattering vector for hh0 peaks. In the [HH0]/[00L] plane the diffuse scattering is elongated along the 〈112〉 directions and is orthogonal to the scattering vector for hhh reflections. It is found that a model consisting of correlated Pb displacements along the 〈111〉 directions reproduces adequately the main features of the diffuse scattering in PbMg1/3Ta2/3O3 when the correlation lengths between the Pb‐ion displacement vectors are longest along the 〈111〉 and shortest along the 〈11〉 and 〈10〉 directions.

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