CRYSTAL-STRUCTURES OF SYNTHETIC 7-ANGSTROM AND 10-ANGSTROM MANGANATESSUBSTITUTED BY MONOVALENT AND DIVALENT-CATIONS

The crystal structures of synthetic 7 angstrom and 10 angstrom mangana tes, synthetic birnessite and buserite, substituted by mono- and dival ent cations were investigated by X-ray and electron diffractions. The monoclinic unit cell parameters of the subcell of lithium 7 angstrom m anganate, which is one of the best ordered manganates, were obtained b y computing the X-ray powder diffraction data: a = 5.152 angstrom, b = 2.845 angstrom, c = 7.196 angstrom, beta = 103.08-degrees. On the bas is of the indices obtained by computing the X-ray diffraction data of Li 7 angstrom manganate, monovalent Na, K and Cs and divalent Be, Sr a nd Ba 7 angstrom maganates were interpreted as the same monoclinic str ucture with beta = 100-103-degrees as that of Li 7 angstrom manganate, from their X-ray diffraction data. In addition, divalent Mg, Ca and N i 10 angstrom manganates were also interpreted as the same monoclinic crystal system with beta = 90-94-degrees. The unit cell parameters, es pecially a, c and beta, change possibly with the type of substituent c ation probably because of the different ionic radius, hydration energy and molar ratio of substituent cation to manganates. However, these d iffraction data, except for those of Sr and Ba 7 angstrom and Ca and N i 10 angstrom 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 dif fractions was found in the X-ray diffraction lines for heavier divalen t cations Sr and Ba 7 angstrom and Ca and Ni 10 angstrom manganates. T he reflection presumably results from the substituent cation position in the interlayer which is associated with the vacancies in the edge-s hared [MnO6] layer and indicates that the essential vacancies are line arly 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 ar e regularly distributed in the interlayer according to the exchange pe rcentage of substituent cation to Na+. In contrast, the streaking in t he a-direction observed strongly in the electron diffractions for heav ier monovalent cations, K and Cs, manganates probably results from the disordering of their cations in the a-direction in the interlayer.