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

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