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Doping and structural features of the apatite-type lanthanum silicate in the La2O3–SiO2–Al2O3 and La2O3–SiO2–Fe2O3 systems
|Tamara Kharlamova 1, Svetlana Pavlova 1, Vladislav A. Sadykov 1, Marina Chaikina 2, Tamara Krieger 1, Olga Lapina 1, Dzhalil Khabibulin 1, Arcady Ishchenco 1, Yurii Pavluchin 2, Christos Argirusis 3|
1. Boreskov Institute of Catalysis (BIC), pr. akad. Lavrentieva, 5, Novosibirsk 630090, Russian Federation
Apatite-type lanthanum silicates (ATLS) are of interest as a new class of solid electrolytes with a high oxygen ion conductivity at the intermediate temperature region . The flexibility of the apatite structure allows doping over a wide range of elements and compositions, providing modification of ATLS properties. However, doping possibilities for the lanthanum silicate can be limited depending on the element nature, while data on phase diagrams for such systems being scarce. This paper is devoted to the study of doping and structural features of the apatite in La2O3–SiO2–Al2O3 and La2O3–SiO2–Fe2O3 systems.
Samples prepared using mechanochemical activation were characterized by XRD, TEM, 29Si and 27Al MAS NMR, IR, Mössbauer and UV-Vis spectroscopy. The incorporation of dopants into the Si sites of the apatite structure was confirmed for all calcined samples. However, it was shown that there is a bi-phase domain in both La2O3–SiO2–Al2O3 and La2O3–SiO2–Fe2O3 phase diagrams at room temperature where apatite and LaAlO3 or LaFeO3 phases coexist, with a continuous, but limited range of solid solutions La9.33+x/3+ySi6-xMexO26+3y/2 (Me = Al, Fe; x = 0–1.5, y = 0–0.67) with the apatite structure being formed. The limit of the Si substitution is defined by the sample stoichiometry. Thus, the content of LaMeO3 for systems with the same Si/Me ratio decreases among the samples La9.67Si5MeO26>La9.83Si5MeO26.25>La10Si5MeO26.5, the latter being a single-phase apatite. In turn the apatite stoichiometry determines some of its structural features. Thus, a local structure of Si was shown to depend on the amount of the cation vacancies in the apatite. The formation of [Si2O7] fragments in this structure  appears to be typical for systems with cation vacancies. The results on the study of the dopant local structure will be presented as well.
This work is supported by EC 6 Framework Program within MATSILC Project.
Presentation: Oral at E-MRS Fall Meeting 2008, Symposium I, by Tamara Kharlamova
See On-line Journal of E-MRS Fall Meeting 2008
Submitted: 2008-05-08 09:36 Revised: 2009-06-07 00:48