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 [1]. 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 La_9.33+x/3+ySi_6-xMe_xO_26+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 La_9.67Si₅MeO₂₆>La_9.83Si₅MeO_26.25>La₁₀Si₅MeO_26.5, 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 [1] 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.

[1] E. Kendrick, et al. J. Mater. Chem. 17 (2007) 3104.

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