Partial re-investigation of the ternary diagram La2O3 – Nb2O5 – CaO, synthesis and characterization of the Ca2La3Nb3O14 and Ca8La8Nb14.4□1.6O56 compounds
Résumé
This paper focuses on the synthesis and the structure determination of two previously reported compounds of unknown structure-Ca 0.96 La 1.56 Nb 1.48 O 7 and Ca 1.16 La 1.26 Nb 1.56 O 7-as well as the investigation of the center of the ternary phase diagram La 2 O 3-Nb 2 O 5-CaO. The samples are prepared by the conventional solid state ceramic route. Structure determination is performed by using microscopy electron diffraction, X-ray and neutron powder diffraction data. The formulas of the two compounds are found to be Ca 2 La 3 Nb 3 O 14 and Ca 8 La 8 Nb 14.4 □ 1.6 O 56 , respectively, where the symbol □ denotes cationic vacancies. Ca 2 La 3 Nb 3 O 14 crystallizes in the rhombohedral space group R 3 m (No.166) (Z ¼ 3) with the cell parameters a ¼ 7.6525(2) (Å) and c ¼ 18.2981(5) (Å). Ca 8 La 8 Nb 14.4 □ 1.6 O 56 crystallizes in the cubic space group Fd 3 m (No.227) (a ¼ 10.6111(1) (Å), Z ¼ 1). In both structures, Calcium and Lanthanum atoms are randomly distributed on the appropriate crystallographic sites. Both compounds belong to the Pyrochlore family A 2 B 2 X 7. Their conduction properties are studied with complex impedance spectroscopy. The central region of the ternary diagram La 2 O 3-Nb 2 O 5-CaO, investigated using Rietveld refinements on Xray diffraction data, is found to be composed of 8 zones which are in agreement with Gibbs rules from a thermodynamic point of view.
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