Local Structure Analysis of Na0.5Bi0.5TiO3 based Oxide Ion Conductor By DFT Calculations and RMC Method

Abstract

INTRODUCTION In recent decades, oxide-ion conducting ceramics are expected to be applied for various electrochemical devices, such as the solid oxide fuel cells (SOFC), and play an important role in the field of the solid-state electrochemistry. Among the devices, SOFC has attracted much attention because it realizes high power generation efficiency and low environmental load. The operating temperature of commercially available SOFC is too high, i.e. 1000 ˚C, at present and thus lowering the operating temperature to around 500 ˚C (intermediate temperature) is highly desired to put them into practical use. Recently, it has been found that Na0.5Bi0.5TiO3 based materials exhibit high oxide-ion conductivity and low activation energy, and the materials become potential candidates for electrolytes of SOFC at the intermediate temperature. In this work, in order to investigate effects of Bi and O deficits on atomic configurations of Na0.5Bi0.5TiO3-based materials and local environment of these deficits, we prepared Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ. The atomic configurations of the samples were investigated by analyzing neutron and synchrotron X-ray total scatterings with the density functional theory (DFT) and the reverse Monte Carlo (RMC) simulation. EXPERIMENTAL Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ were synthesized by solid-state method. In the solid–state method, mixture of Na2CO3, TiO2 and Bi2O3 was calcined at 850 oC for 3 h in air, and then sintered at 1100 oC for 2 h in air. The samples were identified by X-ray diffraction. The metal compositions were evaluated by inductively coupled plasma spectroscopy (ICP) and atomic absorption spectrometry(AA). Cross-section morphologies of the bulk samples were observed by scanning electron microscopy (SEM). The densities of the sintered pellets were evaluated by the Archimedes method. The conductivity measurements were carried out between 300-900 oC by the AC impedance method. Neutron powder diffraction experiments were performed at BL20B2 (J-PARC, Japan). The data were refined using Rietveld technique (Z-code). In order to discuss the effect of Bi and O deficits on the atomic configurations, Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ were also investigated by the RMC simulation (RMCProfile) using total scattering data measured by synchrotron X-ray (BL04B2, SPring-8) and neutron (BL21, J-PARC) sources in addition to the Bragg profile. RESULTS AND DISCUSSION It was found from XRD measurements that Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ could be attributed to the perovskite-type structure with a space grope of Cc. The analytical compositions of Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ evaluated by ICP and AA analysis were able to be controlled, and thus essentially equal to the nominal values. From the results of the Archimedes method, relative densities of the samples were more than 94 %. Figure 1 shows conductivities of the samples as a function of temperature. As seen in this figure, the conductivities of Na0.5Bi0.5-xTiO3-δ were increased with decreasing the Bi amount. In addition, by comparing the conductivity of Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ, it was demonstrated that better electrochemical property could be achieved in the case of Na0.5+yBi0.5-yTiO3-δ. Conductivities measured under conditions of oxygen and argon showed similar behavior against temperature, indicating dominant oxygen-ion conduction. In the Rietveld analysis, we assumed the space group as Cc, and then could refine the average structure successfully. In addition, in order to examine local structures and cation arrangements around the Bi vacancies around and O vacancies in detail, the synchrotron radiation X-ray and neutron total scatterings were measured for Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ. By using the data, the local structure analysis by the RMC method and the DFT was carried out. As a result, considerable differences in the bond distance between the each oxygen and bismuth was observed, and the effect on the conductive properties were suggested. REFERENCES 1)M.Li, M.J.Pietrowski et al., Nature Materials, 13, 31 (2014). Figure 1