Pressure induced para-antiferromagnetic switching in BiFeO -PbTiO as determined using in-situ neutron diffraction

BiFeO-PbTiO exhibits both ferroelectric and antiferromagnetic order, depending on the composition. Moderate hydrostatic pressures have been used at room temperature to transform the crystallographic phase from P4mm to R3c for the compositions 0.7BiFeO-0.3PbTiO and 0.65BiFeO-0.35PbTiO, as determined using in-situ neutron diffraction. Using Rietveld refinements, the resultant data showed that, for both compositions, a transformation from para- to G-type antiferromagnetic order accompanied the structural transition. The transformation occurred over the range 0.4-0.77 and 0.67-0.88 GPa for 0.7BiFeO-0.3PbTiO and 0.65BiFeO-0. 35PbTiO, respectively; at intermediate pressures, a mixture of P4mm and R3c phases were evident. These pressures are far lower than required to induce a phase transition in either the BiFeO or PbTiO end members. The driving force for this pressure induced first order phase transition is a significant difference in volume between the two phases, P4mm > R3c of 4-5, at ambient pressure. Upon removal of the pressure, 0.65BiFeO-0.35PbTiO returned to the paramagnetic tetragonal state, whereas in 0.7BiFeO-0.3PbTiO antiferromagnetic ordering persisted, and the structural phase remained rhombohedral. Using conventional laboratory x-ray diffraction with a hot-stage, the phase readily reverted back to a tetragonal phase, at temperatures between 100 and 310 °C for 0.7BiFeO-0.3PbTiO, far lower than the ferroelectric Curie point for this composition of 632 °C. To our knowledge, the reported pressure induced para- to antiferromagnetic transition is unique in the literature.