High-pressure Synthesis And Multiferroic Study Of CaFe₃Ti₄O₁₂ Perovskite And Its Doping System

A-site ordered quadruple perovskite oxide with the chemical formula AA?₃B₄O₁₂ can accommodate transition metal ions in both A?and B-sites,which can generate a wide variety of new magnetoelectric interactions,leading to a series of novel physical properties.In this paper,by means of the unique high-temperature and high-pressure conditions,a single-phase quadruple perovskite CaFe₃Ti₄O₁₂and its B-site doping system LaFe₃(Ti₃Fe)O₁₂ were prepared.The crystal structures and physical properties were systematically studied,and the related physical mechanisms were discussed in detail.The main innovative research results are shown as follows:(1)By using high-temperature and high-pressure experimental conditions,a single-phase CaFe₃Ti₄O₁₂ polycrystalline sample was synthesized,and the material was systematically studied on the basis of X-ray diffraction,neutron diffraction,magnetic susceptibility,magnetization,dielectric constant,electrical polarization,and ferroelectric hysteresis loop etc.The experimental results show that CaFe₃Ti₄O₁₂crystallizes to an A-site ordered quadruple perovskite structure with space group Im-3at room temperature.As the temperature decreases to 107 K,a paraelectric-ferroelectric phase transition is found to occur,and the space group changes to a polarized R3.When the temperature further reduced to 3.2 K,the A?-siteFe²⁺ions form a long-range antiferromagnetic ordering,which can break the space inverse symmetry and therefore induce electric polarization.Applying an external magnetic field can significantly tune the magnitude of electric polarization,giving rise to significant magnetoelectric effect(the magnetoelectric coupling coefficient is as high as 500 ps/m at 1 T).(2)The B-site doped LaFe₃(Ti₃Fe)O₁₂ polycrystalline samples were prepared under high-temperature and high-pressure conditions.X-ray diffraction experiments show that the compound also crystallizes to an A-site ordered quadruple perovskite structure with a cubic space group Im-3,in which theFe³⁺ions and Ti⁴⁺ions are randomly distributed at the B site.The dielectric measurement shows that the compound undergoes a relaxation ferroelectric phase transition near 12 K,indicating that the B-site doping can greatly reduce the ferroelectric phase transition temperature.In addition,the formation of B-site disorder can change the nature of the ferroelectric phase transition.These results indicate that Ti⁴⁺ions play an important role in the paraelectric-ferroelectric phase transition of CaFe₃Ti₄O₁₂.