Grangeon Sylvain, Lanson Bruno, Lanson Martine
University of Grenoble Alpes, ISTerre, F-38041 Grenoble, France.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2014 Oct;70(Pt 5):828-38. doi: 10.1107/S2052520614013687. Epub 2014 Sep 18.
In surficial environments, the fate of many elements is influenced by their interactions with the phyllomanganate vernadite, a nano-sized and turbostratic variety of birnessite. To advance our understanding of the surface reactivity of vernadite as a function of pH, synthetic vernadite (δ-MnO2) was equilibrated at pH ranging from 3 to 10 and characterized structurally using chemical methods, thermogravimetry and modelling of powder X-ray diffraction (XRD) patterns. With decreasing pH, the number of vacant layer sites increases in the octahedral layers of δ-MnO2 (from 0.14 per layer octahedron at pH 10 to 0.17 at pH 3), whereas the number of layer Mn(3+) is, within errors, equal to 0.12 per layer octahedron over the whole pH range. Vacant layer sites are capped by interlayer Mn(3+) sorbed as triple corner-sharing surface complexes (TC sites). The increasing number of interlayer Mn(3+) with decreasing pH (from 0.075 per layer octahedron at pH 10 to 0.175 at pH 3) results in the decrease of the average Mn oxidation degree (from 3.80 ± 0.01 at pH 10 to 3.70 ± 0.01 at pH 3) and in the lowering of the Na/Mn ratio (from 27.66 ± 0.20 at pH 10 to 6.99 ± 0.16 at pH 3). In addition, in-plane unit-cell parameters are negatively correlated to the number of interlayer Mn at TC sites and decrease with decreasing pH (from b = 2.842 Å at pH 10 to b = 2.834 Å at pH 3), layer symmetry being systematically hexagonal with a = b × 3(1/2). Finally, modelling of X-ray diffraction (XRD) patterns indicates that crystallite size in the ab plane and along the c* axis decreases with decreasing pH, ranging respectively from 7 nm to 6 nm, and from 1.2 nm to 1.0 nm (pH 10 and 3, respectively). Following their characterization, dry samples were sealed in polystyrene vials, kept in the dark, and re-analysed 4 and 8 years later. With ageing time and despite the dry state, layer Mn(3+) extensively migrates to the interlayer most likely to minimize steric strains resulting from the Jahn-Teller distortion of Mn(3+) octahedra. When the number of interlayer Mn(3+) at TC sites resulting from this migration reaches the maximum value of ∼ 1/3 per layer octahedron, interlayer species from adjacent layers share their coordination sphere, resulting in cryptomelane-like tunnel structure fragments (with a 2 × 2 tunnel size) with a significantly improved layer stacking order.
在地表环境中，许多元素的归宿受到它们与叶状锰酸盐水钠锰矿（钠水锰矿的一种纳米级且具有 turbostratic 结构的变体）相互作用的影响。为了加深我们对水钠锰矿表面反应性随 pH 值变化的理解，合成水钠锰矿（δ - MnO₂）在 pH 值为 3 至 10 的范围内进行平衡，并使用化学方法、热重分析法以及粉末 X 射线衍射（XRD）图谱建模对其结构进行表征。随着 pH 值降低，δ - MnO₂八面体层中的空层位点数量增加（从 pH 值为 10 时每层八面体 0.14 个增加到 pH 值为 3 时的 0.17 个），而在整个 pH 范围内，层状 Mn(Ⅲ) 的数量在误差范围内等于每层八面体 0.12 个。空层位点由作为三重角共享表面络合物（TC 位点）吸附的层间 Mn(Ⅲ) 封端。随着 pH 值降低，层间 Mn(Ⅲ) 的数量增加（从 pH 值为 10 时每层八面体 0.075 个增加到 pH 值为 3 时的 0.175 个），这导致平均 Mn 氧化度降低（从 pH 值为 10 时的 3.80 ± 0.01 降低到 pH 值为 3 时的 3.70 ± 0.01）以及 Na/Mn 比降低（从 pH 值为 10 时的 27.66 ± 0.20 降低到 pH 值为 3 时 的 6.99 ± 0.16）。此外，面内晶胞参数与 TC 位点处的层间 Mn 数量呈负相关，并随着 pH 值降低而减小（从 pH 值为 10 时的 b = 2.842 Å 降低到 pH 值为 3 时的 b = 2.834 Å），层对称性系统地为六方晶系，a = b × 3(1/2)。最后，X 射线衍射（XRD）图谱建模表明，在 ab 平面和沿 c*轴的微晶尺寸随着 pH 值降低而减小，分别从 7 nm 减小到 6 nm，以及从 1.2 nm 减小到 1.0 nm（分别对应 pH 值为 10 和 3）。在对其进行表征之后，干燥样品被密封在聚苯乙烯小瓶中，保存在黑暗中，并在 4 年和 8 年后重新进行分析。随着老化时间的推移，尽管处于干燥状态，层状 Mn(Ⅲ) 仍大量迁移到层间，最有可能是为了最小化由 Mn(Ⅲ) 八面体的 Jahn - Teller 畸变引起的空间应变。当这种迁移导致的 TC 位点处层间 Mn(Ⅲ) 的数量达到每层八面体约 1/3 的最大值时，相邻层的层间物种共享它们 的配位球，形成具有显著改善的层堆积顺序的类硬锰矿隧道结构片段（隧道尺寸为 2 × 2）。
