The Effect Of Sr Doping At A-Site On Magnetism In Chromium-Based Oxides ReCrO₄（Re=La,Y）

On the basis of a large number of previous studies on the doping effect of 113 phase rare earth chromium oxide ReCrO₃（Re=rare earth）,this paper starts to study the doping effect of 114 phase rare earth chromium oxide ReCrO₄at position A to explore its influence on the crystal structure and magnetic properties.The valence state ofCrin 114 phase chromium oxide ReCrO₄is+5.Due to the instability of+5 valence,it is easy to be reduced to+3 valence state,so the quality of polycrystalline samples generally prepared is not high,single crystal samples are hardly reported.For the study of chromium based compounds of 114 phase,the structure of 114 phase LaCrO₄has changed compared with 113 phase,from perovskite structure to monazite structure,from orthorhombic system to monoclinic system,andCrO₆octahedral structure toCrO₄tetrahedral structure.In this paper,ReCrO₄（Re=La,Y）and its A-site strontium doped Re_1-xSr_xCrO₄samples are selected as the research object.The required samples are prepared by nitrate method.The crystal structure and magnetism of the samples are characterized by means of X-ray diffraction（XRD）and comprehensive physical property testing system（PPMS）.Aiming at the chromium based oxides of La and Y rare earth elements,the phase formation temperature and crystal structure of parent ReCrO₄（Re=La,Y）and Sr doped sample Re_1-xSr_xCrO₄were investigated,and the effect of sintering temperature on the phase formation of samples was analyzed.Subsequently,the effects of different Sr doping amount on the properties of samples were systematically explored.Re_1-xSr_xCrO₄（Re=La,Y,x=0,0.05,0.1,0.15,0.2）series samples were prepared by nitrate method.The phase analysis and magnetic test were carried out to explore the changes of micro crystal structure and magnetic properties of samples with the increasing doping amount.The first chapter is the introduction,which briefly introduces the crystal structure characteristics and magnetic properties of rare earth chromium oxides ReCrO₃and ReCrO₄,summarizes the research status and progress of rare earth chromium based oxides,and finally explains the research content and significance of this paper.The second chapter introduces the sample preparation method,experimental reagent,experimental equipment,sample characterization and analysis method and its principle.In Chapter 3,LaCrO₄and its strontium doped samples are studied.Firstly,the effects of samples prepared at two different temperatures on phase formation are explored,and 580℃was determined as the sintering temperature.The crystal structure of La_1-xSr_xCrO₄（x=0,0.05,0.1,0.15,0.2）system,according to XRD test and comparison standard PDF card analysis,it is concluded that the crystal structure and spatial group of all samples in the system have not changed,all belong to monazite structure,the spatial group is P2₁/n,and all the crystals in the system show good single-phase.Subsequently,the magnetic test results of the system show that in the range of x=0～0.1,the ferromagnetism of the sample increases slightly with the increase of doping amount.The magnetic ground states of all samples are inclined spin electronic states with the coexistence of various magnetism.The ferromagnetic transition temperature of the system samples does not change regularly.According to the calculated effective magnetic moment of the system,there is no significant change with the change of doping amount,Therefore,the system is likely to introduce oxygen vacancies by partial substitution.The fourth chapter studies YCrO₄and its strontium doped samples,and then studies the effects of different doping amounts on the crystal structure and magnetism of the parent.The crystal structure of Y_1-xSr_xCrO₄（x=0,0.05,0.1,0.15,0.2）system belongs to zircon type and the space group is I4₁/amd according to XRD test and comparison standard PDF card analysis.The magnetic test results of the system show that,similar to La_1-xSr_xCrO₄,doping substitution does not make the ferromagnetic transition temperature of the sample change regularly,and the effective magnetic moment of the system is very close to the theoretical value,indicating that the valence states ofCrin the system are positive pentavalent,the magnetic interaction does not change significantly,and some substitutions introduce oxygen vacancies.The antiferromagnetic transition similar to YCrO₃was found at～130K at high temperature at the doping ratio of 5%and 20%.