Powder x-ray diffraction study of a cryptomelane-type manganic acid and its alkali cation exchanged forms
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Sridhar Komarneni Materials Research Laboratory and Department of Agronomy, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 13 November 1992; accepted 30 July 1993)
Cryptomelane-type manganic acid (CMA), H 2 Mn 8 Oi 6 • 2.4H2O, with a theoretical cation exchange capacity of 2.70 meq/g, was prepared and its cation exchange properties were studied as a function of XM which is defined by the ratio of uptake in meq/g to the theoretical capacity. Plots of the corrected selectivity coefficients log K " vs the fractional exchange XM for alkali metal ions on a 2 X 2 type tunnel-structured manganic_acid showed a gradual decrease in small XM regions and a steep decrease in large X M regions. The maximum extents of exchange (XM) were 0.92 for Li + , 0.74 for Na + and K + , 0.67 for Rb + , and 0.44 for Cs + . These maximums have been attributed to steric limitation as well as a limit on the expansion of the crystal lattice. Some x-ray diffraction (XRD) data of alkali cation exchanged forms were in good agreement with the XRD data of synthetic alkali CMA phases as given in the cards by the Joint Committee on Powder Diffraction Standards (JCPDS). The lattice parameter a0 has been found to depend on the exchanged amounts and the nature of the cations involved, while the CQ value remained almost the same irrespective of the amount of exchange or the nature of the cations.
I. INTRODUCTION Naturally occurring manganese dioxide minerals are known to have various crystal structures.1"3 Their synthetic analogues have been intensively studied so as to utilize their selective cation-exchange properties.4"8 Recently a new type of MnC>2, A—MnC
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