Preparation,Structure And Electrical/Magnetic Properties Of Layered Rare Earth Based (Oxy)Nitrides

Compared with oxygen ions,nitrogen ions can coordinate with more cations,which leads to（oxy）nitride with more degrees of freedom and diversity in structure,providing more development space for the exploration of new phases and new structures.In terms of performance,（oxy）nitride also show excellent performance in fluorescence,magnetism,photocatalysis,dielectric and other application fields.Therefore,the research on（oxy）nitride has attracted more and more people’s attention in recent years.However,the researches on（oxy）nitrides are mostly concentrated on Alkali metal nitrides and Alkaline earth metal nitrides.In contrast,there is still a big gap in the research on rare earth（oxy）nitrides.Based on the ion size effect,this paper explores the synthesis of new rare earth-based（oxy）nitride by unequal substitution of the cations of the reported nitrides and rare earth oxides,in order to find more rare earth（oxy）nitride with high-quality properties.The results of the study are as follows:（1）Based on the structure of melilite La_1.54Sr_0.46Ga₃O_7.27,which can provide 0.02-0.1 S cm^-1 conductivity at 600-900°C,the exploration of the preparation of La Sr Si₃O₄N₃ was carried out under high temperature and high pressure.Although the target oxynitride was not obtained,a new compound with chemical composition La₆Sr₃Si₆O₂₄ was found.The crystal structure of La₆Sr₃Si₆O₂₄ was confirmed as defective perovskite with space group R（?）m and cell parameters a=5.4326（8）?,c=19.6174（3）?by XRD,NPD and SAED.The（La/Sr）O₃ and（La/Sr）O₂ layers are stacked along the c direction in the order of（c’hh）₃.There are B-cation vacancies in the two h-（La/Sr）O₃ layers,and oxygen vacancies in the c’-（La/Sr）O₂ layers.La ions are enriched in the c’-（La/Sr）O₂ layers,and there are isolated silica tetrahedrons.La₆Sr₃Si₆O₂₄ also has high stabilities at room temperature and pressure,and its decomposition temperature is more than 800°C.AC impedance spectroscopy shows that La₆Sr₃Si₆O₂₄ is a semiconductor with a grain conductivity of 10^-6-10^-4 S cm^-1 in the range of 550-800°C,its electron transfer activation energy is 0.93 e V,and it has ionic conduction behavior that does not change with oxygen partial pressure.（2）Based on the structure of CaTiN₂,whose dielectric constant can reach 1300-2500 in the range of 10⁴-10⁶ H_Z,The composition design and exploration preparation of Ln MN₂（Ln=La,Sm,Gd,Pr;M=Ga,Al,Fe,Mn）series isomorphic compounds at different temperatures（800-1400°C）and pressures（0-10 GPa）were carried out.DFT calculation and comprehensive analysis of XRD,SEM and EDS indicate that the traditional high temperature solid phase method using binary nitride as raw material is not an ideal method for synthesis of Ln MN₂ series nitride.The main reasons are as follows:Ga N is easily volatile;Al N is too stable to participate in chemical reactions under high pressure.Iron nitride and manganese nitride raw materials decompose at500-600°C.But the layered oxynitride La₂O₂CN₂ found in the synthesis of La Ga N₂have diamagnetism and are potential superconducting materials.