THE ARSENATES NACA2M22- INFLUENCE OF CATIONIC SUBSTITUTIONS ON GARNET-ALLUAUDITE POLYMORPHISM((ASO4)(3) (M2+=MG, NI, CO) )

Cationic replacements in the arsenates NaCa2M22+(AsO4)(3) (M2+ = Mg, N i, CO) have a strong influence on their garnet-alluaudite polymorphism . The preferential formation of an alluaudite phase (at the expense of the garnet phase) is induced by the following cationic modifications: (i) The replacement of Na+ by larger cations such as Ag+, K+, or Tl+. The cation distribution is partly disordered in the Na and Ag compoun ds and ordered in the K and Tl compounds. (ii) The formation of vacanc ies through the (Na+ or Li+)-Ca2+ replacement, namely (Na or Li)(1-x)s quare(x/2)Ca(2+x/2)Mg(2)(AsO4)(3). The formation of a pure defect garn et phase is limited to small x values (x = 0.1 or 0.2) whereas a full series of defect alluaudite phases (terminating at Ca2.5Mg2(AsO4)(3), x = 1) can be obtained. (iii) The replacement of Ca2+ by Mg2+ or Co2+, leading to the new alluaudite-like compounds NaCaMg3(AsO4)(3), NaCaCo 3(AsO4)(3), and NaCo4 (AsO4)(3). Most of these results can be explaine d by the existence in the alluaudite structure, of three crystallograp hic sites X(2), X(1), and M(1), which are respectively larger, approxi mately equal to, or smaller than the dodecahedral sites of the garnet structure. A garnet-alluaudite transformation has not been observed in the corresponding vanadates, in which the garnet structure appears ve ry stable. (C) 1997 Academic Press.