Structure-Property Correlations in doped Bismuth Ferrite Ceramics

A detailed powder neutron diffraction study coupled with a complementary characterisation of the ferroelectricity and ferromagnetism, concerning two different compositions based on a parent BiFeO3 crystal lattice. The addition of Dy3+ into BiFeO3 ceramics shows how the parent polar R3c symmetry destabilises due to strain, linking to a large size variance between the two A-site cations (Dy3+ and Bi3+). This results in a loss of symmetry to polar Cc followed by a broad competitive phase system between strained Cc and non-polar Pnma symmetries. Increasingly strain on the lattice is seen until x=0.12 at which point the Pnma phase becomes dominant. However, phase co-existence persists until beyond x=0.25. Preliminary magnetisation measurements indicate a developing ferromagnetic character which increases in magnitude with Dy3+ content; electrical measurements suggest that whilst initial compositions of Bi0.95Dy0.05FeO3 show signs of polarity, more Dy3+ rich materials such as Bi0.70Dy0.30FeO3 shows relaxor like characteristics.

This study also examines the addition of Alkali Metal Niobates to BiFeO3, looking at the Bi1-xKxFe1-xNbxO3 solid solution. Despite much individual attention on both BiFeO3 and KNbO3 a solid solution between the two has received less interest. Reported in this work is a detailed neutron and synchrotron X-ray powder diffraction study which demonstrates a polar R3c ? polar P4mm ? polar Amm2 series of structural phase transitions similar to that exhibited by PbZrO3–PbTiO3. Early electrical measurements show a non-ohmic electrode effects, making it difficult to determine the polar nature of the P4mm phase. Preliminary magnetic data suggests that the G-type anti-ferromagnetic spin cycloid may be retained, but some canting may occur, however the exact nature of the magnetic structure is unconfirmed, and evidence may initially suggest paramagnetism, canted antiferromagnetism or a spin glass like structure. All materials seem to exhibit a similar magnetic transition consistent with the parent lattice.