Doped Apatite Type Lanthanum Silicates: Structure and Property Characterization
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Doped Apatite Type Lanthanum Silicates: Structure and Property Characterization T. Kharlamova1, S. Pavlova1, V. Sadykov1, T. Krieger1, L. Batuev1, V. Muzykantov1, O. Lapina1, D. Khabibulin1, M. Chaikina2, N. Uvarov2, Y. Pavlukhin2, S. Petrov2, Chr. Argirusis3 1 Boreskov Institute of Catalysis SB RAS, 5 Lavrentieva Prospekt, Novosibirsk, 380090, Russia 2 Institute of Solid State Chemistry and Mechanochemistry, 18 Kutatetadze Street, Novosibirsk, 630128, Russia 3 Clausthal University of Technology, Clausthal-Zellerfeld, 38678, Germany ABSTRACT Structural features and transport properties of Fe- and Al-doped apatite-type lanthanum silicates were studied with relation to a role of the [SiO4] substructure. XRD, TEM, Mössbauer, UV-Vis electron, IR, 29Si and 27Al NMR spectroscopy were used for structural studies; impedance spectroscopy and oxygen isotope heteroexchange were used for transport properties characterization. INTRODUCTION Apatite type lanthanum silicates (ATLS) attract an interest as a new class of solid electrolytes having a high oxide-ion conductivity at intermediate temperatures [1, 2]. In comparison with perovskite- and fluorite-type systems, in which oxide-ion migration generally occurs through the jumps into vacancies, in ATLS materials an interstitial oxide-ion migration has been supposed [3]. The possibility of interstitial mechanism of ion transport is provided by peculiarities of apatite structure tolerant to different structural defects (cation vacancies and interstitial oxygen atoms) and extensive doping. In general, the ideal hexagonal apatite-type structure can be presented by crystallographic formula A10(RO4)6X2, where in the case of ATLS A = La, R = Si and X = O2-. It can be described as consisting of isolated tetrahedral SiO4 anions and La cations located in ninecoordinated 4f or seven-coordinated 6h sites. The seven-coordinated cations form channels along the c axis in which oxide-anions responsible for the high ion conductivity are located. The high ion conductivity of ATLS is caused by their defect/real structure, which can be controlled by doping. Thus, systems possessing cation vacancies, i.e. La9.33Si6O26, and/or oxygen excess, i.e. La9.67Si6O26.5 and La9SrSi6O26.5, show a high ion conductivity, while stoichiometric systems such as La8Sr2Si6O26 have a low conductivity. Numerous structural studies showed that some displacement of oxide ion in channels from the center into an interstitial position is observed for systems with cation vacancies and/or oxygen excess, which along with results of atomistic modelling suggest the interstitial mechanism of ion transport in apatite systems [2, 3]. However, recent 29Si NMR studies of silicon local structure in apatites and new results of atomistic modeling have shown that the defect apatite structure and the possible pathway of oxygen migration are more complex as it was supposed previously [2, 4, 5]. The present paper is devoted to the study of structural features and transport properties of Fe- and Al-doped ATLS with relation to a role
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