Hydrothermal synthesis and crystal structure of barium hewettite: BaV6O16·nH2O

Ba analogues of hewettite (CaV6O16·9H2O) were synthesized by the hydrothermal methods. The compounds exhibit two phases formulated by BaV6O16·nH2O and Ba1+xV6O16·nH2O (x ∼ 0.2, n ∼ 3), and the structure of BaV6O16·nH2O has been determined from a single crystal study. It crystallizes in the orthorhombic system Pnmm with a = 12.162(3) angstrom, b = 10.841(4) angstrom, c = 17.035(4) angstrom, and Z = 6 and the structure refinements led to R = 0.066 and Rw = 0.076 for 1,480 reflections with I > 3σ(I). The structure is basically analogous to that of γ-Li1+xV3O8 or CaV6O16·9H2O, consisting of V6O16 layers and interstitial hydrated Ba atoms. The V6O16 layers stack along the c axis with 8.518-angstrom spacing which is half of the c axis; adjacent layers are mirror images of each other. Ba atoms reside in three kinds of sites with totally different oxygen coordinations. Their interlayer distributions result in another long period along the b axis which is triple the ordinary 3.6-angstrom period of the hewettite compounds. This is the first single-crystal structural study of the synthetic hewettite compounds