Defect-Structure Analysis on La2(Ni,Cu)O4+ d with Reverse Monte Carlo Simulation Using Neutron and Synchrotron X-Ray Total Scatterings

Abstract

Recently, it is highly expected to lower an operating temperature of the solid oxide fuel cell (SOFC). To satisfy this demand, a development of a novel air-electrode material can be considered as an important issue. Materials with high mixed oxide-ion and electronic conduction can be regarded as a promising candidate for the electrode since the materials enable to expand area for the oxygen-reduction reaction. As such materials, La2NiO4+δ with the layered perovskite structure has drawn much attention nowadays. In the oxides, interstitial oxide ions are introduced into the crystal due to a charge compensation of Ni oxidation from 2+ to 3+. This change of the defect structure leads to high mixed oxide-ion and electronic conduction and thus superior electrode property. It is also found that a partial substitution of Cu for Ni enhances electrochemical property of La2NiO4+δ [1]. Considering the mechanism for the mixed conduction mentioned above, the Cu substitution must affect the defect structure in the crystal: that is, an extra oxygen amount, a distribution and/or a local situation of the interstitial oxide ion. From such background, some previous works have carried out actively the crystal structure analysis using the Bragg profile and discussed the average position of the extra oxide ions. As is well known, however, the structure analysis (a so-called average-structure analysis) cannot give deep insight on the defect structure unfortunately. In this work, we focused on La2NiO4+δ and La2Ni0.6Cu0.4O4+δ with the layered perovskite structure, and investigated their defect structures by the reverse Monte Carlo (RMC) simulation using the Bragg profiles and the pair distribution functions (PDFs) simultaneously [2, 3]. Since PDF data include information on the atomic configurations without periodicity, we could clarify the defect structures of the samples on the basis of the atomic-configuration snapshots of the RMC simulations. La2NiO4+δ and La2Ni0.6Cu0.4O4+δ could be synthesized by a citric-acid method, according to literature [1]. The obtained products were identified by laboratorial X-ray diffraction measurements, and their metal compositions were evaluated by the inductively coupled plasma spectrometry. Average structures of the samples were determined by the Rietveld analysis (Z-Rietveld) using the neutron Bragg profiles which were recorded with iMATERIA installed at J-PARC. Neutron and synchrotron X-ray total scattering patterns were also measured with NOVA at J-PARC and BL04B2 at SPring-8, respectively. By using all the data, we performed the RMC simulations with RMCProfile for the specimens. In the analyses, we used super cells with 1152 atoms which were constructed based on the refined unit cells. The Rietveld analysis using neutron Bragg profiles confirmed that both the La2NiO4+δ and La2Ni0.6Cu0.4O4+δ had a single phase of the layered rock-salt structure with a space group of Fmmm. In order to determine distributions of the interstitial oxide ions, we analyzed atomic configurations of the samples by the RMC modeling using neutron PDF, neutron and synchrotron X-ray structure factors in addition to the neutron Bragg profile. As a result, it was found that a distance of Cu-O was longer than that of Ni-O, and the distribution of the distance became wider around Cu. Such a change in the local situation may be one of the reasons for higher oxide-ion diffusion in the Cu-substituted specimen. It was also indicated from the La-Ni and La-Cu distances that La2Ni0.6Cu0.4O4+δ had larger interstitial cavity volume in the rock-salt layer compared with La2NiO4+δ. Since the rock-salt layer forms the diffusion pathway of the interstitial oxide ion, the larger interstitial volumes in the Cu-substituted sample should contribute to better electrode property. From the analysis on cavities around interstitial oxide ions, it was suggested that an oxide-ion diffusion accompanied a significant change in the cavity volume around the mobile oxide ion in the case of La2NiO4+δ, whereas such a change was suppressed in La2Ni0.6Cu0.4O4+δ. This should be one of the reasons why the oxide-ion diffusion was enhanced by substituting Cu for Ni partially.