Solid solutions of multiferroic BiFeO3 doped with ferroelectric PbTiO3 (BFPT) can be prepared by conventional mixed oxide processing to produce a range of polycrystalline ceramics ranging throughout the xBiFeO3 - (1-x)PbTiO3 series. Sintered ceramics are shown to exhibit mixed tetragonal (P4mm) and rhombohedral (R3c) phase perovskite distortions from 0.4 ≤ x < 0.75, where at x ~
0.75 a morphotropic phase boundary exists and compositions x > 0.75 are entirely rhombohedral.
From extensive use of neutron diffraction experiments, the phase coexistence is attributed to compensation for the internal strain induced upon cooling through the ferroelectric Curie point from cubic, to the distorted tetragonal perovskite phase (ܿ/ܽ = 1.17). This drives a further partial transformation to the (~4 %) lower volume
rhombohedral phase as a relief mechanism.
Increasing the sinter temperature and fast cooling (> 900 °C/hr) sees the monolithic samples x ≤ 0.7 disintegrate to various levels of particulate size, when a critical
grain size is exceeded (7 μm), which in turn is inversely proportional to the grain boundary fracture energy. The magnetic properties studied using high resolution
powder diffractometry of these two states present G-type antiferromagnetism (AFM) in both the rhombohedral and tetragonal phases; but with Tn above ambient temperature for R3c, and below for P4mm for all compositions except x = 0.3. Compositions below this PbTiO3 rich solution are never observed to support antiferromagnetic order, as the dilution of magnetic iron ions exceeds the percolation threshold via substitution with titanium ions.
The rhombohedral phase is shown to exhibit an incommensurate, modulated magnetic order, with a propagation vector perpendicular to the magnetization
vector, which decreases in periodicity with increasing bismuth ferrite, from 840 Å for x = 0.75. At room temperature, transforming the paramagnetic tetragonally distorted powder to a G-type AFM rhombohedral phase, is observed with the application of hydrostatic pressure. Evident from neutron experiments, using the Pearl instrument
at ISIS, full transformation can be achieved with moderate pressures of 0.77 GPa, effectively ‘switching’ on the magnetic order.
The monolithic samples were used at 250 K to observe the changes in simultaneous structural and G-type antiferromagnetic properties as a function of applied electric field (0 to 10 MVm-1) for the most piezoelectrically active samples, around the MPB composition (x = 0.7), using neutron diffraction at the Berlin neutron scattering centre; instrument E2.
An observed increase in rhombohedral phase occurs with the application of electric field from peak analysis, which relates to a proportional increase in observed antiferromagnetic intensity (5 %).
These two behaviours are proposed to be linked by the internal strain developed within the system, from increased polarisation forcing a partial phase transformation
from the tetragonal to the rhombohedral phase which can support the antiferromagnetic order at room temperature.
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