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... oxide fuel cells (SOFC) are used in energy conversation devices, and have been of considerable interest in recent years due to their pollution-free electricity generation with higher effi- ciencies through electrochemical reactions, as well as due to their fuel adaptability and low emission of NOx and SOx gases [1][2][3]. In these devices; the electrolyte must be an oxide-ion conductor with negligible electronic contribution, and it must be dense to prevent gas mixing. The anode and cathode should have good electronic or mixed conductivity, in addition to some catalytic activity towards fuel oxidation and oxygen reduction, respectively. The use of SOFC at intermediate temperatures (from 600 to 800 °C) requires new oxide ion conductors with higher conductivity values than YSZ (yittria stabilized zirconia). Recently, the apatite silicates with general formula Ln 9.33À2x/3 RE x h 0.67 (SiO 4 ) 6 O 2 (Ln, rare earth elements) replaced YSZ, where the rare earth cations occupy two crystallographic sites (6 h and 4f) with seven and nine coordination polyhedrons [4][5][6] and were presented as a new class of oxide-ion conductor electrolytes due to their physical and chemical stability, and their excellent ionic conductivity and negligible electronic con- duction over a wide range of p(O2) [7][8][9][10]. The apatite structure ( Fig. 1) is made up of isolated [SiO 4 ] tetrahedrons with La 3+ cations located in the seven and nine coordinated sites and oxide-ion O(4) located in site 2a (0, 0, 0.25). The framework exhibits channels along the c-axis where oxide ions migrate [11][12][13]. In the literature, the apatite structure has usually been shown to have hexagonal sym- metry, and to crystallize in the P63/m space group [12,[14][15][16][17]. In this report, the crystal structure of lanthanum silicate has been studied by X-ray diffraction. The oxide ions conductivity of the apa- tites has also been measured and analyzed. The aim of the present work is to study the effect of La 3+ cations, which are partially substi- tuted by Mn 2+ or alkaline earth Ca 2+ , Sr 2+ , Ba 2+ cations on the ionic conductivity. The composition and structure of the electrolyte samples were studied by XRD and NMR. An accurate structural description of the oxygen network is thus very important to under- stand the relationship between the structure and the oxygen ion transportation in these ...
