ALLUAUDITE-LIKE STRUCTURE OF THE ARSENATE NA3IN2(ASO4)(3)

The arsenate Na3In2(AsO4)(3) has been synthesized and investigated by X-ray diffraction and vibrational spectroscopy. According to a Rietvel d analysis, it is monoclinic, space group C2/c, Z = 4, with a = 12.602 5 (1), b = 13.1699 (1), c = 6.8335 (1) Angstrom, beta = 113.7422 (5). It has an alluaudite-like structure, with the three Na+ cations locate d on the three large sites X(2), X(1), and M(1), and the two In3+ cati ons on the M(2) sites of this structure. This is a new type of chemica l formula (free from bivalent cations and with a full occupancy of the X(2), X(1), and M(1) sites by sodium ions) for an alluaudite-like str ucture. The Raman and infrared spectra exhibit a great number of sharp bands, as expected from the low symmetry and from the ordered distrib ution of the cations, but this complexity precludes a detailed assignm ent of the bands. Structural analogies or differences with other allua udite-like compounds are briefly discussed. (C) 1997 Academic Press.