STRUCTURAL AND CHEMICAL INVESTIGATIONS OF NA3(ABO4)3-CENTER-DOT-4H2O-TYPE SODALITE PHASES

The sodalite cage containing only an electron as a nonframework ''anion' is well adapted for the study of solvated ''electride'' Wigner lattices and quantitative mapping of the intra- and intercage electronic potential surfaces. In this paper we report the direct synthesis of anion free cage structures of the form Na3[](ABO4)3.nH2O (A; B = Al, Ga; Si, Ge) as possible precursors for sodalite electride synthesis. A novel, low-temperature sodalite dehydroxylation method is presented for the preparation of Na3[](AlSiO4)3.4H2O from Na4[OH](AlSiO4)3.1.72H2O. Results from a neutron diffraction study of Na3(ZnAsO4)3.4H2O give evidence of hydrogen bonding between the framework oxygen atoms and the beta-cage substructure and aid in explanation of analog unit cell trends. All of the investigated sodalites have an ordered array of their framework tetrahedral cations and crystallize in the cubic space group P43nBAR, Z = 2. Two of these new sodalites were characterized by single-crystal X-ray diffraction. Na3(AlGeO4)3.4H2O, with a = 8.965(1) angstrom, d(Al-O) = 1.746(4) angstrom, d(Ge-O) = 1.758(4) angstrom, and THETA(Al-O-Ge) = 129.6(2)degrees, has beta-cages which contain a defect-''cubane''-like Na3.4H2O array. Final disagreement factors: R = 1.93% and R(w) = 2.19% for 208 observed reflections [I > 3sigma(I)]. Na3(GaGeO4)3-4H2O, with a = 9.0033(7) angstrom, d(Ga-O) = 1.839(3) angstrom, d(Ge-O) = 1.745(3) angstrom, and THETA(Ga-O-Ge) = 125.3(2)degrees, has the same beta-cage contents. Final disagreement factors of R = 1.00% and R(w) = 1.29% were obtained for 183 observed reflections [I > 3sigma(I)]. The structure of Na3-[(GaSiO4)3].4H2O was determined by Rietveld refinement of room-temperature X-ray powder diffraction data, with a = 8.8614(2) angstrom, d(Ga-O) = 1.847(4) angstrom, d(Si-O) = 1.580(3) angstrom, THETA(Ga-O-Si) = 131.9(3)degrees, and the same beta-cage contents. Final disagreement factors of R(p) = 10.07% and R(wp) = 13.75% for 4099 data were obtained. The structure of Na3(ZnASO4)3.4H2O was determined from low-temperature continuous-wavelength neutron powder diffraction data, with a = 9.0276(7) angstrom, d(Zn-O) = 1.974(4) angstrom, d(As-O) = 1.689(4) angstrom, THETA(Zn-O-As) = 121.1(3)degrees, and the same beta-cage contents, with evidence of hydrogen bonding between cubane and framework atoms. Final disagreement factors of R(p) = 4.26% and R(wp) = 5.52% for 3268 data were obtained. These isomorphs are compared with other Na3[](ABO4)3.4H2O types of sodalite structures in terms of geometrical trends in 6-ring topologies and cation and water molecule sitings. Further structural information is obtained through presented spectral, thermogravimetric, and physical data.