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Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses
Published on: June 7, 2018
In situ X-ray diffraction study of cesium exchange in synthetic umbite
Christopher S Fewox1, Abraham Clearfield, Aaron J Celestian
1Department of Chemistry, Texas A & M University, P.O. Box 30012, College Station, Texas 77842-3012, USA.
Inorganic Chemistry
|March 8, 2011
Summary
Cesium ions (Cs(+)) exchange into umbite-(HK) in a two-step process, first filling larger channels then migrating to smaller ones. This ion exchange induces a structural transformation from monoclinic to orthorhombic symmetry.
Area of Science:
- Materials Science
- Crystallography
- Inorganic Chemistry
Background:
- The umbite framework, H(1.22)K(0.84)ZrSi(3)O(9)·2.16H(2)O (umbite-(HK)), features a structure with potential for ion exchange.
- It contains two distinct ion exchange sites within tunnels parallel to the a-axis.
Purpose of the Study:
- To investigate the in situ ion exchange mechanism of Cs(+) into the umbite-(HK) framework.
- To characterize the structural changes and final product composition during Cs(+) exchange.
Main Methods:
- Time-resolved X-ray diffraction at the National Synchrotron Light Source.
- Rietveld structure refinements to model cation occupancy.
- Solid-state (133)Cs Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy.
Main Results:
- A two-step Cs(+) exchange process was observed: initial occupation of larger channels (Exchange Site 2) followed by migration to smaller channels (Exchange Site 1).
- A structural phase transition from monoclinic (P2(1)/c) to orthorhombic (P2(1)2(1)2(1)) symmetry occurred when Cs(+) occupancy in the smaller cavity exceeded 0.50.
- The final product, umbite-(CsK), has the formula H(0.18)K(0.45)Cs(1.37)ZrSi(3)O(9)·0.98H(2)O with an orthorhombic unit cell.
Conclusions:
- The ion exchange behavior of umbite-(HK) is dependent on the size and accessibility of its internal channels.
- The structural transformation is a key feature of Cs(+) incorporation, leading to a new crystalline phase.
- Solid-state NMR indicates minimal electronic differences between the two exchange sites in the final structure.
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