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Title: Time-Resolved Structural Analysis of K- and Ba-Exchange Reactions in Synthetic Na-birnessite using Synchrotron X-ray Diffraction

Abstract

Time-resolved Rietveld refinements using synchrotron X-ray diffraction (XRD) have documented real-time changes in unit-cell parameters in response to cation substitution in synthetic Na-birnessite. Potassium- and Ba-birnessite, like Na-birnessite, were found to have triclinic symmetry. Rietveld analyses of the XRD patterns for K- and Ba-exchanged birnessite revealed decreases in the a, c, and {beta} unit-cell parameters, with a decrease of 1.7 and 0.5%, respectively, in unit-cell volume relative to Na-birnessite. Fourier electron difference syntheses revealed that the changes in the configuration of the interlayer species, and the charge, size, and hydration of the substituting cations, serve as the primary controls on changes in unit-cell parameters. Split electron density maxima with centers at (0 0 0.5) were present for Na, K, and Ba end-members; however, with increased substitution of K{sup +} for Na{sup +}, the axis connecting the split-site maxima rotated from an orientation parallel to the b-axis to along the a-axis. Substitution of Ba{sup 2+} for Na{sup +} did not result in rotation, but splitting of the interlayer site was more pronounced.

Authors:
; ; ; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
929891
Report Number(s):
BNL-80473-2008-JA
Journal ID: ISSN 0003-004X; AMMIAY; TRN: US0806669
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: American Mineralogist; Journal Volume: 92
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; CATIONS; CONFIGURATION; ELECTRON DENSITY; ELECTRONS; HYDRATION; ORIENTATION; POTASSIUM; ROTATION; SIZE; SYMMETRY; SYNCHROTRON RADIATION; VOLUME; X-RAY DIFFRACTION; national synchrotron light source

Citation Formats

Lopano,C., Heaney, P., Post, J., Hanson, J., and Komarneni, S. Time-Resolved Structural Analysis of K- and Ba-Exchange Reactions in Synthetic Na-birnessite using Synchrotron X-ray Diffraction. United States: N. p., 2007. Web. doi:10.2138/am.2007.2242.
Lopano,C., Heaney, P., Post, J., Hanson, J., & Komarneni, S. Time-Resolved Structural Analysis of K- and Ba-Exchange Reactions in Synthetic Na-birnessite using Synchrotron X-ray Diffraction. United States. doi:10.2138/am.2007.2242.
Lopano,C., Heaney, P., Post, J., Hanson, J., and Komarneni, S. Mon . "Time-Resolved Structural Analysis of K- and Ba-Exchange Reactions in Synthetic Na-birnessite using Synchrotron X-ray Diffraction". United States. doi:10.2138/am.2007.2242.
@article{osti_929891,
title = {Time-Resolved Structural Analysis of K- and Ba-Exchange Reactions in Synthetic Na-birnessite using Synchrotron X-ray Diffraction},
author = {Lopano,C. and Heaney, P. and Post, J. and Hanson, J. and Komarneni, S.},
abstractNote = {Time-resolved Rietveld refinements using synchrotron X-ray diffraction (XRD) have documented real-time changes in unit-cell parameters in response to cation substitution in synthetic Na-birnessite. Potassium- and Ba-birnessite, like Na-birnessite, were found to have triclinic symmetry. Rietveld analyses of the XRD patterns for K- and Ba-exchanged birnessite revealed decreases in the a, c, and {beta} unit-cell parameters, with a decrease of 1.7 and 0.5%, respectively, in unit-cell volume relative to Na-birnessite. Fourier electron difference syntheses revealed that the changes in the configuration of the interlayer species, and the charge, size, and hydration of the substituting cations, serve as the primary controls on changes in unit-cell parameters. Split electron density maxima with centers at (0 0 0.5) were present for Na, K, and Ba end-members; however, with increased substitution of K{sup +} for Na{sup +}, the axis connecting the split-site maxima rotated from an orientation parallel to the b-axis to along the a-axis. Substitution of Ba{sup 2+} for Na{sup +} did not result in rotation, but splitting of the interlayer site was more pronounced.},
doi = {10.2138/am.2007.2242},
journal = {American Mineralogist},
number = ,
volume = 92,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • In this study, we applied time-resolved synchrotron X-ray diffraction (TRXRD) to develop kinetic models that test a proposed two-stage reaction pathway for cation exchange in birnessite. These represent the first rate equations calculated for cation exchange in layered manganates. Our previous work has shown that the substitution of K, Cs, and Ba for interlayer Na in synthetic triclinic birnessite induces measurable changes in unit-cell parameters. New kinetic modeling of this crystallographic data supports our previously postulated two-stage reaction pathway for cation exchange, and we can correlate the kinetic steps with changes in crystal structure. In addition, the initial rates ofmore » cation exchange, R ({angstrom}{sup 3} min{sup -1}), were determined from changes in unit-cell volume to follow these rate laws: R = 1.75[K{sup +}{sub (aq)}]{sup 0.56}, R = 41.1[Cs{sup +}{sub (aq)}]{sup 1.10}, R = 1.15[Ba{sup 2+}{sub (aq)}]{sup 0.50}. Thus, the exchange rates for Na in triclinic birnessite decreased in the order: Cs >> K > Ba. These results are likely a function of hydration energy differences of the cations and the preference of the solution phase for the more readily hydrated cation.« less
  • We have explored the exchange of Cs for interlayer Na in birnessite using several techniques, including transmission electron microscopy (TEM) and time-resolved synchrotron X-ray diffraction (XRD). Our goal was to test which of two possible exchange mechanisms is operative during the reaction: (1) diffusion of cations in and out of the interlayer or (2) dissolution of Na-birnessite and reprecipitation of Cs-birnessite. The appearance of distinct XRD peaks for Na- and Cs-rich phases in partially exchanged samples offered support for a simple diffusion model, but it was inconsistent with the compositional and crystallographic homogeneity of (Na,Cs)-birnessite platelets from core to rimmore » as ascertained by TEM. Time-resolved XRD revealed systematic changes in the structure of the emergent Cs-rich birnessite phase during exchange, in conflict with a dissolution and reprecipitation model. Instead, we propose that exchange occurred by sequential delamination of Mn oxide octahedral sheets. Exfoliation of a given interlayer region allowed for wholesale replacement of Na by Cs and was rapidly followed by reassembly. This model accounts for the rapidity of metal exchange in birnessite, the co-existence of distinct Na- and Cs-birnessite phases during the process of exchange, and the uniformly mixed Na- and Cs-compositions ascertained from point analyses by selected area electron diffraction and energy dispersive spectroscopy of partially exchanged grains.« less