Partial re-investigation of the ternary diagram La2O3 - Nb2O5 - CaO, synthesis and characterization of the Ca2La3Nb3O14 and Ca8La8Nb14.4□1.6O56 compounds

This paper focuses on the synthesis and the structure determination of two previously reported compounds of unknown structure -Ca0.96La1.56Nb1.48O7 and Ca1.16La1.26Nb1.56O7 - as well as the investigation of the center of the ternary phase diagram La2O3 - Nb2O5 - 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 Ca2La3Nb3O14 and Ca8L-a(8)Nb(14.4)square O-1.6(56), respectively, where the symbol square denotes cationic vacancies. Ca2La3Nb3O14 crystallizes in the rhombohedral space group R (3) over bar m (No.166) (Z = 3) with the cell parameters a = 7.6525(2) (angstrom) and c =18.2981(5) (angstrom). Ca8La8Nb14.4 square O-1.6(56) crystallizes in the cubic space group Fd (3) over barm (No.227) (a = 10.6111(1) (angstrom), Z = 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 La2O3 - Nb2O5 - CaO, investigated using Rietveld refinements on X-ray diffraction data, is found to be composed of 8 zones which are in agreement with Gibbs rules from a ther-modynamic point of view.