Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH

The structural transformation of high pH Na-rich buserite (NaBu) to H-excha nged hexagonal birnessite (HBi) at low pH was studied by simulation of expe rimental X-ray diffraction patterns. Four HBi samples were prepared by equi libration of NaBu at constant pH in the range pH 5-2. The samples differ fr om each other by the presence or one (at pH 2 and 3) or two (at pH 4 and 5) phases, and by the structural heterogeneity of these phases which decrease s with decreasing pH. The sample obtained at pH 5 is a 4:1 physical mixture of a 1H phase (a = 4.940 Angstrom. b = a/root 3 = 2.852 Angstrom, c = 7.23 5 Angstrom, beta = 90 degrees, gamma = 90 degrees) and of a 1M phase (a = 4 .940 Angstrom, b = a/root 3 = 2.852 Angstrom, c= 7.235 Angstrom, beta = 119 .2 degrees, gamma = 90 degrees) in which successive layers are shifted with respect to each other by +a/3 along the a axis as in chalcophanite. Both t he 1H and 1M phases contain very few well-defined stacking faults at pH 5. At pH 3, the sample is a 8:5 physical mixture of a 1H phase containing 15% of monoclinic layer pairs and of a 1M phase containing 40% of orthogonal la yer pairs. Any further decrease of the pH leads to the formation of a singl e defective 1H phase. This III phase contains 20% and 5% of monoclinic laye r pairs at pH 3 and 2, respectively. Independent of pH, all phases contain 0.833 Mn-layer cations, 0.167 vacant layer sites, and 0.167 interlayer Mn c ations located either above or below layer vacancies per octahedron. A stru ctural formula is established at each pH. The origin of the observed phase and structural heterogeneities has been an alyzed. 1H and 1M phases are assumed to inherit their specific structural a nd crystal chemical features from the two distinct NaBu modifications. NaBu type I, with a high proportion of Mn-layer(4+) cations, is thought to be r esponsible for the monoclinic layer stacking because this configuration all ows Mn cations from adjacent layers to be as far as possible from each othe r, thus minimizing the electrostatic repulsion between these high charge ca tions. In contrast, NaBu type Il has a high interlayer charge induced by Mn -layer(3+) for Mn-layer(4+) substitutions. Consequently, the 1H phase has a high amount of interlayer protons and achieves compensation of the unfavor able overlap of layer and interlayer Mn cations, in projection on the nb-pl ane, by the presence of strong hydrogen bondings between layers, The higher proportion of defined stacking faults in both 1H and 1M phases at pn 3 com pared to pH 5 can be attributed to the increase in reaction rate with decre asing pH. At lower pH (3 and 2) the formation or strong hydrogen bonds betw een adjacent layers controls the layer stacking mode and leads to the forma tion of a unique 1H phase. The proportion of well-defined stacking faults i n this phase decreases from pH 3 to 2.