Crystal structures of synthetic 7 Å and 10 Å manganates substituted by mono- and divalent cations

Abstract

The crystal structures of synthetic 7 .~ and 10 Jk manganates, synthetic birnessite and buserite, substituted by mono-and divalent cations were investigated by X-ray and electron diffractions. The monoclinic unit cell parameters of the subcell of lithium 7 A manganate, which is one of the best ordered manganates, were obtained by computing the X-ray powder diffraction data: a = 5.152 A, b = 2.845 A, c = 7.196 A, 13 = 103.08 ~ On the basis of the indices obtained by computing the X-ray diffraction data of Li 7 A manganate, monovalent Na, K and Cs and divalent Be, Sr and Ba 7 A manganates were interpreted as the same monoclinic structure with 13 = 100-103 ~ as that of Li 7 A manganate, from their X-ray diffraction data. In addition, divalent Mg, Ca and Ni 10 A manganates were also interpreted as the same monoclinic crystal system with 13 = 90-94 ~ The unit cell parameters, especially a, c and 13, change possibly with the type of substituent cation probably because of the different ionic radius, hydration energy and molar ratio of substituent cation to manganese. However, these diffraction data, except for those of Sr and Ba 7 A and Ca and Ni 10 A manganates, reveal only some parts of the host manganese structure with the edge-shared [MnO6] octahedral layer. On the other hand, one of the superlattice reflections observed in the electron diffractions was found in the X-ray diffraction lines for heavier divalent cations Sr and Ba 7 A and Ca and Ni 10 A manganates. The reflection presumably results from the substituent cation position in the interlayer which is associated with the vacancies in the edge-shared [MnO6] layer and indicates that the essential vacancies are linearly arranged parallel to the b-axis. Furthermore, the characteristic superlattice reflection patterns for several cations, Li, Mg, Ca, Sr, Ba and Ni, manganates were interpreted that the substituent cations are regularly distributed in the interlayer according to the exchange percentage of substituent cation to Na + . In contrast, the streaking in the a-direction observed strongly in the electron diffractions for heavier monovalent cations, K and Cs, manganates probably results from the disordering of their cations in the a-direction in the interlayer.

Structure of heavy-metal sorbed birnessite: Part 2. Results from electron diffraction

“…Giovanoli et al (1970) described super-cell reflections of high-pH Na-rich synthetic birnessite, hereafter referred to as NaBi, and attributed them to the ordering of vacant layer octahedra capped by interlayer cations as in chalcophanite (Wadsley 1955;Post and Appleman 1988). The same interpretation was proposed by Chukhrov et al (1978) for a Carich birnessite sample, and by Kuma et al (1994) for synthetic NaBi samples exchanged with different monovalent and divalent cations. On the contrary, Post and Veblen (1990) suggested that high-pH synthetic birnessites consist of vacancy-free layers and, therefore, that super-cell reflections more likely result from an ordered distribution of interlayer cations.…”

“…Selected-area electron diffraction (SAED) has often been used, sometimes to complement X-ray diffraction (XRD), to study the structure of birnessite, and especially to reveal the super-cell periodicities in these phyllomanganates (Post and Veblen 1990;; Kuma et al 1994;Drits et al 1997Drits et al , 1998. However, the origin of super-cell reflections observed in SAED patterns of natural and synthetic birnessites remained unclear for a long time.…”

A consistent model for surface complexation on birnessite (−MnO2) and its application to a column experiment

“…The disproportionation of 2 Mn 3ϩ atoms to Mn 4ϩ and Mn 2ϩ creates a vacancy in the octahedral layer, while Mn 2ϩ goes into solution or into the interlayer. The amount of water present in the birnessite crystal lattice depends on the ambient conditions (temperature, humidity) and on the interlayer cation (Paterson et al, 1994;Kuma et al, 1994). With either 1 or 2 layers of water molecules in the interlayer region, the c-axis spacing of birnessite is 7 or 10 Å. Na-birnessite (buserite) has a 10 Å spacing in water, which decreases to 7 Å upon drying.…”

Structure of heavy-metal sorbed birnessite: Part 1. Results from X-ray diffraction

“…Recent crystal chemical studies of synthetic buserite and birnessite by complementary diffraction and spectroscopic methods have provided important new insight on the structure of these compounds (Chukhrov et al 1985;Post and Veblen 1990;Manceau and Charlet 1992;Kuma et al 1994;Drits et al 1997Drits et al , 1998aManceau et al 1997;Silvester et al 1997;Lanson et al 2000). This new information will serve as a basis for the present detailed crystal chemical studies of Me-sorbed birnessite (MeBi).…”