Study On The Effect Of Ca And Cu Doping On The Structure,Magnetism And Electrical Properties Of LaCrO₃

The rare earth chromium-based oxide RECrO₃（RE=rare earth element）with perovskite structure has attracted much attention as the ideal candidate for solid fuel cell bonding materials.The research on it mainly focuses on improving the sintering performance,electrical transport and catalytic properties.Recently,the structure,magnetism and electrical properties of perovskite-type RECrO₃ have been reported.However,the research on the structure,magnetism and electrical property is not systematic and thorough.In order to study the influence of different positions of aliovalent doping on basic physical properties and explore the relationship between them,we chose LaCrO₃ as the study material.The effect of doping Ca²⁺at La position and Cu²⁺at Cr site on the structure,magnetism and electrical property of parent crystal were studied.Meanwhile,the effect of doping Ca²⁺at La position base on Cu²⁺doping at the Cr site were studied as well.The specific studies are as follows:In the first chapter,the structure,basic properties and applications of perovskite-type oxides are introduced.At the same time,the common basic concepts and magnetic structures,electrical theory and conductive model of perovskite-type oxides are briefly introduced.In the second chapter,the experimental equipment and reagents needed to prepare samples by sol-gel method are introduced,and the characterization methods and principles in the process of sample characterization are also introduced.In the third chapter,the optimum sintering temperature of LaCrO₃ prepared by sol-gel method is studied.The optimum sintering temperature is 1100℃,and the La_1-xCa_xCrO₃（x=0.1,0.2,0.3,0.4）replaced by Ca²⁺is sintered at this temperature.The results of physical properties analysis show that when the Ca with smaller ionic radius enters the parent lattice,the parent remains perovskite structure and Pbnm space group,but the lattice parameters reduced and the cell volume shrinks.The results of magnetic test show that valence state of Cr³⁺were increased because of the increase of the low-valent ions enter,the mixed state of Cr³⁺/Cr⁴⁺exists in the system,which leads to the competition of three exchange interactions among the different ions in the system.The magnetic state of the system is changed from G-type antiferromagnetic state to ferromagnetic and antiferromagnetic state co-existing state.When the doping amount is increased to 0.4,the hysteresis loop appears the contraction in low field and the weak exchange bias phenomenon.The results of electrical tests show that Ca²⁺doping improves the conductivity of the sample,and the electrical conductivity and temperature of the doped sample are fitted with the thermal activation model to show that the activation energy of the sample increases gradually with the increase of doping amount.In the fourth chapter,a series of LaCr_1-yCu_yO₃（y=0,0.1,0.2,0.3,0.4）samples replaced by Cu²⁺at La site were selected and sintered at 1100℃.The experimental results show that the effect of Cu²⁺on the system shows the coexistence of electron and size effect.The partially substituted samples still maintain the original structure,but there is a solid solution limit near y=0.2.The results of magnetic measurement show that the change of bond length bond angle induced by cell distortion induced by Cu²⁺entering the lattice leads to the transition from paramagnetic state to ferromagnetic state.At the same time,the negative magnetization caused by disordered freezing of Cr³⁺was observed in the samples doped with y=0.1,and the paramagnetic composition was more obvious at low temperature after further cooling.Electrical tests show that the electrical conductivity of the sample is greatly improved by Cu²⁺substitution,and the grain connectivity becomes better and the void decreases.By fitting the relationship between conductivity and temperature in the high temperature region,it is found that doping decreases the activation energy of the system.In chapter fifth,a series of co-doped La_1-xCa_xCr_0.8Cu_0.2O₃（x=0,0.1,0.2,0.3）samples have been synthesized at 1100℃.The results of phase analysis show that the solid solution limit of the doping amount is near x=0.3.Segregation and aggregation of elements were found in the samples above the solid solution limit.By full spectrum fitting,it is found that the crystal cell expands at x=0,and continues to contract as Ca²⁺enters the lattice cell.The twist and tilt of CrO₆ octahedron are found in the refinement of the structure.The results of magnetic test show that the antiferromagnetic transition temperature（T_N）and ferromagnetic transition temperature（T_C）appear with the decrease of temperature after the substitution of Ca²⁺,and the competition between antiferromagnetic interaction and ferromagnetic interaction appears with the increase of doping amount.It led to the same trend of T_C and dopant content,while T_N showed the opposite trend.The results of electrical test show that the electric conductivity of the sample is gradually improved by the double doping,but it is worse than that of the unit doping.The result is that the charge gravity of the conductive hole is gradually enhanced by the substitution of the low-cost elements for the"negative electric center".A higher energy is required to change the bound hole into a conductive hole,which is also illustrated by the gradual increase of activation energy obtained from the fitting of conductivity and temperature.