高温还原La1-xSrxCr0.5Mn0.5O3钙钛矿的晶体结构研究OA北大核心CSTPCD

Introduction La1-xSrxCr0.5Mn0.5O3(LSCM)perovskite as a SOC fuel electrode material has attracted much attention due to its excellent comprehensive performance.However,there is still a lack of systematic and in-depth research on the its structural stability that is affected by some factors such as Sr content,temperature,and atmosphere under operating conditions of SOC.In particular,previous work indicate that this material exhibits different electrode performances and crystal structures under Sr-free and Sr-containing conditions,having the Sr content impact on its structural stability and electrochemical performance.Therefore,this paper was to analyze the crystal structures of La1-xSrxCr0.5Mn0.5O3 heat-treated in a reducing atmosphere at elevated temperatures from 500℃to 1 000℃.In addition,the mechanism of crystal structural transformation of LSCM in a high-temperature reducing atmosphere was also discussed. Methods The LSCM precursors with different Sr contents(i.e.,x of 0,0.1,0.2,and 0.3)were prepared by a citric acid sol-gel method.The precursors were calcined into LSCM powders in a muffle furnace at 1100℃.The LSCM powders were then thermally reduced in a reducing atmosphere at 500℃-1 000℃.After reduction,the phase transformations of each sample were characterized by X-ray diffraction(XRD).The lattice constants of the samples were calculated based on the Rietveld method using a software named Rietica.In addition,the first-principles calculations,Fourier transform infrared spectroscopy(FTIR),and X-ray photoelectron spectroscopy(XPS)were also performed to reveal the effects of Sr content and reduction heat treatment on the crystal transformation of LSCM. Results and discussion The samples with different Sr contents were calcined in air at 1 100℃.A single trigonal phase appears when the Sr contents x≤0.2,while a small amount of SrCrO4 phase occurs besides the perovskite phase when x=0.3.After reduction at elevated temperatures,the sample with x of 0 transforms from a trigonal phase to an orthorhombic phase,and its crystal structure is relatively stable according to the fact that there are no other phases formed with increasing the reduction temperature.The XRD patterns of LCM samples reduced at different temperatures are analyzed using the Rietveld method.The results show that the unit cell volume of the reduced perovskite samples increases with the increase of heat treatment temperature from 700℃to 1 000℃.However,the unit cell volume of the sample reduced at 600℃is smaller than that at 500℃.This is because the sample begin to lose a considerable amount of oxygen at 500-600℃during the reduction heat treatment,leading to a significant change in the shape of the crystal unit cell.For the samples with x of 0.1 and 0.2,trigonal and orthorhombic phases co-exist,and the proportion of trigonal phase increases with increasing x value but decreases with increasing reduction temperature.A layered perovskite oxide phase of(La,Sr)2MnO4 appears in the sample with x of 0.2 when the reduction temperature increases to 900℃,indicating that this sample has a poorer structural stability,compared with the sample with x of 0.1.According to the XPS results,compared with the LCM without Sr,the Mn3+concentration in the reduced LSCM is relatively low as the Sr content increases,which can alleviate the Jahn-Teller distortion.Therefore,LSCM tends to form a trigonal phase,while LCM undergoes a more significant Jahn-Teller distortion,making it tends to transform into a more stable structure of orthorhombic phase than trigonal phase.The FTIR spectra reveal that the Mn/Cr—O bond energy increases in the LSCM perovskite with the increase of Sr content.The lattice parameters of each sample are calculated using the Rietveld method based on the XRD data,and subsequently the lengths of Mn/Cr-O bonds of each crystal are calculated.The results demonstrate that the length of Mn/Cr—O bond in the crystals decreases with the increase of Sr content,which is consistent with the conclusion derived from the FTIR spectra.However,the first-principles calculations confirm that the binding energy of the crystal of LSCM gradually increases with the increase of Sr content.Therefore,in summary,the existence of trigonal phase and the increase of binding energy caused by Sr doping jointly lead to the fact that the structural stability of Sr-containing LSCM perovskite is lower than that of Sr-free LCM under high-temperature reduction atmosphere conditions. Conclusions The structural transformation process and regularity of the LSCM perovskites under high-temperature reducing atmosphere were mainly controlled by the Jahn-Teller distortion,which depended on the concentration of Mn3+.The LCM without Sr underwent a more significant Jahn-Teller distortion,making it form an orthorhombic phase.The increase of Sr content mitigated the Jahn-Teller distortion,making the Sr-containing LSCM perovskites more likely to form a trigonal phase as well as possess an increased binding energy,thus reducing the structural stability of LSCM.