K. Kuma et al., CRYSTAL-STRUCTURES OF SYNTHETIC 7-ANGSTROM AND 10-ANGSTROM MANGANATESSUBSTITUTED BY MONOVALENT AND DIVALENT-CATIONS, Mineralogical Magazine, 58(392), 1994, pp. 425-447
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.