High-pressure Synthesis And Physical Properties Of A-site-ordered Perovskites

The prototype of the perovskite material is calcium titanate（Ca Ti O₃）,with chemical formula of ABO₃ form,where A site were usually occupied by the alkali,alkaline earth and rare earth ions,and B site occupied by transition metal ions In the ABO₃ perovskite,both A-and B-site ions possess flexible configuration and charge configuration,therefore,perovskite materials not only show the various types of crystal structures,but also diaplay colorful features in ferroelectric,dielectric,giant magnetic resistance（GMR）,high-temperature superconductivity,multiferroic and other fields.In addition,perovskites are also the most widely used functional materials in the world.In this paper,on the basis of simple ABO₃ perovskite,by introducing transition metal ions into A-site,forming the A-site-ordered perovskite AA’₃B₄O12.We also carried out detailed structures and physical properties to this special kind of order perovskite.A-site-ordered perovskite La Cu₃Fe₄O12 shows a series of interesting physical phenomenons,e.g.rare temperature induced Cu-Fe intersite charge transfer,accompaning by negative thermal expansion and metal-insulator transition.In order to study the nature of the charge transfer and phase transition,we tried to introduce a second transition metal ions Cr into the B-site and study its infulence on the crystal structures and the of electronic states of Cu and Fe elements.Under the conditions of high pressure and high temperature,we successfully synthesised a series of La Cu₃Fe_4-x Cr_x O12（x = 1,2,3）solid solutions,and studied the details structures and physical properties.The results show that La Cu₃Fe_4-x Cr_x O12（x = 1,2,3）keep cubic A-site-ordered perovskite crystal structure,which belong to Im-3 space group.The magnetic susceptibility and resistivity of La Cu₃Fe₃ Cr O12 changes abnormally at 375 K,implying that there occurs similar Cu-Fe intersite charge transfer as La Cu₃Fe₄O12.When the temperature is further cooled to around175 K,the material shows signs of antiferromagnetic transition.With the increase of the content of Cr,no obvious magnetic phase transition was observed in La Cu₃Fe2Cr2O12.More interestingly,when the content of Cr increased further,two obvious magnetic phase transition appear,respectively,at 400 K and 200 K in La Cu₃ Fe Cr3O12,which associated with weak ferromagnetic properties.The research of the mechanism of these phase transition is underway.In conclusion,the results show that the introduction of Cr ions into the B-site of La Cu₃Fe₄O12 has remarkable effect on the electronic states,which can suppress the Cu-Fe intersite charge transfer gradually,and cause a series change ofmagnetic and electrical-transport properties.