STRUCTURE OF THE COMPLEX ARSENATES NACA(2)M(2)(2+) (ASO4)(3) (M(2+)=MG, NI, CO) - FIRST EXPERIMENTAL-EVIDENCE OF A GARNET-ALLUAUDITE REVERSIBLE POLYMORPHISM

It is shown by X-ray powder diffraction that the high-temperature polymorph of the garnet arsenates NaCa(2)M(2)(2+) (AsO4)(3) (M(2+) = Mg, CO, Ni) has an alluaudite-like structure. This is the first experimental evidence of a garnet-alluaudite reversible polymorphism. The cation distribution has been deduced from previous studies of similar compounds: the relatively small M(2+) cations are located on the octahedral M(2) sites of the alluaudite structure; the distribution of the Na+ + 2Ca(2+) cations over the X(2), X(1), and M(1) sites should be partially disordered. The vibrational spectra (Raman and infrared) of both polymorphs (garnet and alluaudite) of the Mg compound are presented and are briefly discussed on the basis of a factor group analysis and the occurrence of symmetry-allowed vibrational interactions. These spectra exhibit some diffuseness, in agreement with the partial disorder affecting the cation distribution. The possible occurrence of other cases of garnet-alluaudite polymorphism is discussed. (C) 1995 Academic Press, Inc.