INFLUENCE OF CATIONIC SUBSTITUTIONS IN NA3FE2(ASO4)(3) - TRANSITION FROM THE GARNET TO THE ALLUAUDITE STRUCTURE

The substitution in Na3Fe2(AsO4)(3) of Na+ by either Li+ or Ca2+ and a vacancy, is able to stabilize an alluaudite structure at the expense of either the garnet or the rhombohedral structure of the parent compo und. In the system (Na1-xLix)(3)Fe-2(AsO4)(3), pure garnet phases are obtained at 700 degrees C in the 0 less than or equal to x less than o r equal to 0.333 composition range, whereas pure (or nearly so) alluau dite phases are obtained at 870 degrees C in the 0.333 less than or eq ual to x less than or equal to 0.666 composition range (namely, from N a2LiFe2(AsO4)(3) to NaLi2Fe2(AsO4)(3)). In contrast, vacancies (as in (Na(1-x)square(x/2)Ca(x/2))(3)Fe-2(AsO4)(3)) are hardly accepted in th e garnet structure, whereas the range of formation of a pure alluaudit e (at 870 degrees C) is more extended (0.166 less than or equal to x l ess than or equal to 0.666), the last composition corresponding to Na square CaFe2(AsO4)(3). The stabilization of an alluaudite structure at the expense of a garnet structure has already been observed in previo us work and is discussed in relation with the structural role of lithi um and vacancies. (C) 1998 Academic Press.