Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite; I, Results from X-ray diffraction and selected-area electron diffraction

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 = 5.172 Å, b = 2.849 Å, c = 7.34 Å, β = 103.3° and a = 2.848 Å, c = 7.19 Å, γ = 120°, respectively. In addition to super-reflection networks, SAED patterns for NaBi and SrBi contain satellite reflections. On the basis of these experimental observations, 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 Mn3+ for Mn4+. The supercell A = 3a parameter arises from the ordered distribution of Mn3+-rich rows parallel to [010] and separated from each other along [100] by 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 interlayer 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 free of Mn4+. The idealized structural formula proposed for NaBi type II is Na0.333(Mn4+0.722Mn3+0.222Mn2+0.055)O2. NaBi type I has a lower amount of Mn3+ and its ideal composition would vary from Na0.167(Mn4+0.833Mn3+0.167)O2 to Na0.25(Mn4+0.75Mn3+0.25)O2. 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 structure of HBi consists of hexagonal octahedral layers containing predominantly Mn4+ with variable amounts of Mn3+ and layer vacancies. The distribution of layer vacancies is inherited from the former Mn3+ distribution in NaB. Interlayer Mn cations are located above or below vacant layer sites. The driving force of the NaBi to HBi transformation is probably the destabilization of Mn3+-rich rows at low pH.