STRUCTURE OF SYNTHETIC MONOCLINIC NA-RICH BIRNESSITE AND HEXAGONAL BIRNESSITE: I. RESULTS FROM X-RAY DIFFRACTION AND SELECTED-AREA ELECTRON DIFFRACTION

Abstract

Synthetic Na-rich birnessite (NaBi) and its low pH form, hexagonal birnessite (HBi), were studied by X-ray and selected-area electron diffraction (XRD, SAED). SAED patterns were also obtained for synthetic Sr-exchanged birnessite (SrBi) microcrystals in which Sr was substituted for Na. XRD confirmed the one-layer monoclinic structure of NaBi and the one-layer hexagonal structure of HBi with subcell parameters a 5 5.172 A ˚, b 5 2.849 A ˚, c 5 7.34 A ˚, b 5 103.38 and a 5 2.848 A ˚, c 5 7.19 A ˚, g 5 1208, respectively. In addition to super-reflection networks, SAED patterns for NaBi and SrBi contain satellite reflections. On the basis of these experimental obervations, structural models for NaBi and HBi are proposed. NaBi consists of almost vacancy-free Mn octahedral layers. The departure from the hexagonal symmetry of layers is caused by the Jahn-Teller distortion associated with the substitution of Mn 31 for Mn 41 . The supercell A 5 3a parameter arises from the ordered distribution of Mn 31 -rich rows parallel to (010) and separated from each other along (100) by two Mn 41 rows. The superstructure in the b direction of NaBi type II (B 5 3b) comes from the ordered distribution of Na cations in the interlayer space. The maximum value of the layer negative charge is equal to 0.333 v.u. per Mn atom and is obtained when Mn 31 -rich rows are free of Mn