CRYSTAL-STRUCTURE OF NA7FE4(ASO4)(6) AND ALPHA-NA3AL2(ASO4)(3), 2 SODIUM-ION CONDUCTORS STRUCTURALLY RELATED TO II-NA3FE2(ASO4)(3)

In the system Na2O-Fe2O3-FeO-As2O5, Na7Fe4(AsO4)(6) is one of the end members of a large domain of nonstoichiometry, including the sodium io n conductor II-Na3Fe2(AsO4)(3). It is rhombohedral, space group R(3) o ver barc$, with cell constants a = 13.807(1) and c = 18.354(3) Angstro m, and Z = 6. The structure is closely related to that of II-Na3Fe2(As O4)(3) and differs from it by a partial reduction of Fe-III into Fe-II , associated with an increase in sodium content for charge compensatio n; the reduction is localized on the Fe(1) site, and the Na(2) site is totally occupied, so that the structural formula can be written as Na (1)Na(2)(6)Fe-II(1)Fe-III(2)(3)(AsO4)(6). Na-3 Al-2(AsO4)(3) undergoes at 44 degrees C a reversible phase transition alpha reversible arrow beta. The beta form is rhombohedral and isotypic with II-Na-3 Fe-2(AsO 4)3. The room-temperature form, alpha, differs from beta by a slight m onoclinic distortion: a = 14.551(2), b = 13.303(1), c = 9.782(1) Angst rom, beta = 96.88(1)degrees, space group C2, and Z = 8. The sodium ion s exhibit a long-range ordering: among the six available sodium sites derived from the Na(2) site of the beta form, five are totally occupie d and one is vacant. A comparison with the structure of II-Na3Fe2(AsO4 )(3) reveals some important displacements of Na positions toward the v acancy square(26), which probably result in a minimization of electros tatic repulsions. A close examination of the framework distortions ind icates that the loss of symmetry (beta, rhombohedral --> alpha, monocl inic) could be associated with a loss of the three-dimensional charact er of the conduction. (C) 1995 Academic Press, Inc.