Preparation And Physical Properties Of Perovskite Type Cr-based Oxides Re_2/3AE_1/3CrO₃ ?RE=La,Y;AE=Mg,Ca,Sr,Ba?

In this paper,series of RE_2/3AE_1/3CrO₃?RE=La,Y;AE=Mg,Ca,Sr,Ba?polycrystalline oxides were prepared by sol-gel method.XRD,SEM,TEM,XPS,RAMAN,MPMS,and PPMS have been used to characterize their crystal structure,magnetic properties,and transport properties.For La_2/3Ca_1/3CrO₃,heterogeneous phase CaCrO₄ exists in the system after sintering at 800°C.The La_2/3Ca_1/3CrO₃ with no miscellaneous phase can be obtained by sintering at1000°C.The results of TEM show that Ca²⁺intercalation appears in the high-resolution lattice image of La_2/3Ca_1/3CrO₃ with the prolongation of the holding time,which shows that La_2/3Ca_1/3CrO₃ has a superlattice structure for holding time of 32h.The results of SEM show that the extension of the holding time and the addition of Ag can promote grain growth and densification of the sample.Ag exists between the La_2/3Ca_1/3CrO₃ grain boundaries.The?-T curves of La_2/3Ca_1/3CrO₃ and La_2/3Ca_1/3CrO₃+5%Ag under the magnetic field of 10Oe?ZFC and FC?and 50Oe?ZFC?show negative magnetization.The magnetic susceptibility of La_2/3Ca_1/3CrO₃ has the lowest value of-0.043?10Oe,ZFC?.When the magnetic susceptibility is zero,the corresponding temperatures of La_2/3Ca_1/3CrO₃+5%Ag are 257 K?10Oe?and 177K?50Oe?,respectively,and the magnetic susceptibility has a minimum value of-0.0205?10Oe,FC?.When the magnetic field is applied to 100 Oe,the system is antiferromagnetic,and it is presumed that due to the competition between between diamagnetic and ferromagnetism in the system,the?-T curve under different applied magnetic fields will change from diamagnetic to antiferromagnetic as the applied magnetic field increases.For La_1-xAE_xCrO₃?AE=Mg,Sr,Ba?,due to the difference in the radius of the alkaline earth and La ions,Mg doping is difficult to form single phase La_2/3Mg_1/3CrO₃,Sr can incorporate LaCrO₃,but SrCrO₄ heterophasic phase still exists in La_2/3Sr_1/3CrO₃.The results of XPS show that the valence states of Cr ions in nominal La_1-xBa_xCrO₃ are trivalent and hexavalent,and RAMAN shows that the components of nominal La_1-xBa_xCrO₃ are LaCrO₃and BaCrO₄.The?-T results of nominal La_2/3Ca_1/3CrO₃ and La_2/3Mg_1/3CrO₃ show that with temperature increasing,carrier concentration of La_2/3Ca_1/3CrO₃ and La_2/3Mg_1/3CrO₃ increases,thus the resistivity decreases.The?-T curve of nominal La_2/3Sr_1/3CrO₃ shows shows a sudden drop in resistivity at 133K,and the resistivity of the low temperature part?<77K?approaches2.18?10^-5?·cm.The?-T curve of the nominal La_2/3Ba_1/3CrO₃ has a resistivity drop of about10⁴ orders of magnitude starting from 290K,and the resistivity at 177K is 7.701?10^-8?·cm.The sudden drop in the resistivity of nominal La_2/3Sr_1/3CrO₃ and La_2/3Ba_1/3CrO₃ may be related to the electronic structure transformation and the heterogeneous interface.For Y_1-xAE_xCrO₃?AE=Mg,Ca?,Reitveld refinement of Ca²⁺doping with YCrO₃ shows that the lattice constant and cell volume of Y_0.9Ca_0.1CrO₃ are smaller than YCrO₃,which is because Cr³⁺is converted to Cr⁴⁺due to the charge compensation mechanism,resulting in an increase in CrO₆ octahedron distortion.The Mg²⁺and Ca²⁺doping have little effect on the antiferromagnetic transition temperature of YCrO₃,but the doping leads to a decrease in the effective magnetic moment of the paramagnetic state due to the decrease of the Y³⁺content of the system and the Cr³⁺with larger effective moments converts to Cr⁴⁺with a smaller effective magnetic moment.The ferromagnetic hysteresis loop that appears on the M-H curve is due to the weak ferromagnetism caused by the tilted G-type antiferromagnetic.The?-T curve of Y_0.9Ca_0.1CrO₃ shows a significant semiconductor transport behavior.As the temperature rises,the thermal activation causes the carrier concentration to increase and jump more violently,which leads to the increase of the electrical conductivity and the decrease of the resistivity.