High–Pressure Synthesis And Physical Property Study Of Novel Quadruple Perovskites

Quandruple perovskites AA′₃B₄O₁₂ and AA′₃B₂B′₂O₁₂ possess multiple novel interactions between magnetism and electricity.Compared to the simple ABO₃perovskite,quandruple perovskite accommodates transition metals in multiple atomic sites（A′–,B–,and B′–sites）,thus exhibits plenty of unique physical properties.With different combinations of the ions,diversified properties and functions can be realized in quandruple perovskite,which is also the frontier of condensed matter physics and material science.Considering the necessity of high pressure in the synthesizing procedure,many new quandruple perovskites are still waiting to be discovered and studied.In this dissertation,several novel quandruple perovskites with intriguring physical properties have been successfully prepared using the advanced high pressure and high temperature apparatus.The main contents are as follows.1.LaMn₃Cr₄O₁₂ was synthesized under high pressure and high temperature conditions.It maintains a cubic Im-3 space group from 2 to 300 K,and experences long–range antiferromagnetic transitions at T_Cr=150 K and T_Mn=50 K result from the G–type antiferromagnetic ordering along the[111]direction of B–site Cr³⁺and A′–site Mn³⁺,respectively.Magnetic point group analysis unveiled a polarized symmetry when considering both Cr³⁺and Mn³⁺spins,giving rise to electric polarization.Further measurements of the dielectric constant and electric polarization provided direct evidence of the spin induced polarization.As the first magnetoelectric multiferroics with cubic crystal lattice and unique collinear antiferromagnetic structure,LaMn₃Cr₄O₁₂ not only provides a prototype for multiferroics with new mechanisms,but also opens an avenue of exploring novel multiferroics.2.Under high pressure and high temperature conditions,the both A–and B–site ordered quadruple perovskite NaCu₃Fe₂Os₂O₁₂ was successfully synthesized.It crystallizes to Pn-3 space group,with rock salt ordering of Fe³⁺and Os^5.5+at the B–and B′–site,respectively.It experences a Cu²⁺（↑）Fe³⁺（↑）Os^5.5+（↓）ferrimagnetic transition at³80 K and possesses a relatively large magnetic moment of⁵.5μ_B/f.u..On the other hand,measurment of the magnetoresistance indicated half–metallic nature of NaCu₃Fe₂Os₂O₁₂.First–principle theoretical calculations further demonstrated that its spin–down energy band is conductive while the spin–up energy band is insulated with a wide gap of¹.6 eV.NaCu₃Fe₂Os₂O₁₂ opens a new route of exploring high–perfomance half–metal,and has potential applications for future advanced spintronic devices.3.The B–site doped quadruple perovskites LaCu₃Fe₄-_xOs_xO₁₂（x=1,2）were prepared under high pressure and high temperature conditions and systematically investigated.Compared with the undoped LaCu₃Fe₄O₁₂（T_N³93 K）,the charge transfer between A′–site Cu and B–site Fe is totally suppressed in the x=1 doped compound,in spite of the Im-3 space group is maintained.Besides,the disordered distribution of Fe and Os at B–site breaks the long–range antiferromagntic coupling between Cu and Fe,leading to a spin glass transition around 480 K.With further Os doping,the x=2 doped compound crystallizes to Pn-3 space group,indicating the rock salt ordering of B–site Fe and B′–site Os.Furthermore,it experiences a Cu²⁺（↑）Fe³⁺（↑）Os^4.5+（↓）ferrimagnetic transition around 520 K.Moreover,with the Os doping on the B–site,the interaction between the 3d transition metal Fe and 5d transition metal Os becomes stronger thus enhances the mangetic transition temperature.