STRUCTURE OF SYNTHETIC MONOCLINIC NA-RICH BIRNESSITE AND HEXAGONAL BIRNESSITE .1. RESULTS FROM X-RAY-DIFFRACTION AND SELECTED-AREA ELECTRON-DIFFRACTION

Synthetic Na-rich birnessite (NaBi) and its low pH form, hexagonal bir nessite (HBi), were studied by X-ray and selected-area electron diffra ction (XRD, SAED). SAED patterns were also obtained for synthetic Sr-e xchanged birnessite (SrBi) microcrystals in which Sr was substituted f or Na. XRD confirmed the one-layer monoclinic structure of NaBi and th e one-layer hexagonal structure of HBi with subcell parameters a = 5.1 72 Angstrom, b = 2.849 Angstrom, c = 7.34 Angstrom, beta = 103.3 degre es and a = 2.848 Angstrom, c = 7.19 Angstrom, gamma 120 degrees, respe ctively. In addition to super-reflection networks, SAED patterns for N aBi and SrBi contain satellite reflections. On the basis of these expe rimental obervations, structural models for NaBi and HBi are proposed. NaBi consists of almost vacancy-free Mn octahedral layers. The depart ure from the hexagonal symmetry of layers is caused by the Jahn-Teller distortion associated with the substitution of Mn3+ for Mn4+. The sup ercell A = 3a parameter arises from the ordered distribution of Mn3+-r ich rows parallel to [010] and separated from each other along [100] b y two Mn4+ rows. The superstructure in the b direction of NaBi type II (B = 3b) comes from the ordered distribution of Na cations in the int erlayer space. The maximum value of the layer negative charge is equal to 0.333 v.u. per Mn atom and is obtained when Mn3+-rich rows are fre e of Mn4+. The idealized structural formula proposed for NaBi type II is Na-0.333(Mn0.7224+Mn0.2223+Mn0.0552+)O-2. NaBi type I has a lower a mount of Mn3+ and its ideal composition would vary from Na-0.167(Mn0.8 334+Mn0.1673+)O-2 to Na-0.25(Mn0.754+Mn0.253+)O-2. Satellites in SAED patterns of NaBi crystals result from the ordered distribution of Mn4 and Mn2+ pairs in Mn3+-rich rows with a periodicity of 6b. The struct ure of HBi consists of hexagonal octahedral layers containing predomin antly Mn4+ with variable amounts of Mn3+ and layer vacancies. The dist ribution of layer vacancies is inherited from the former Mn3+ distribu tion in NaBi. Interlayer Mn cations are located above or below vacant layer sites. The driving force of the NaBi to HBi transformation is pr obably the destabilization of Mn3+-rich rows at low pH.