Crystal Chemistry and Phase Transitions in Substituted Pollucites along the CsAlSi2O6-CsTiSi2O6.5 Join: A Powder Synchrotron X-ray Diffractometry Study |
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Authors: | Hongwu Xu Alexandra Navrotsky M Lou Balmer Yali Su |
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Affiliation: | Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616;Pacific Northwest National Laboratory, Richland, Washington 99352 |
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Abstract: | The crystal structures for a suite of substituted pollucites with the compositions CsTi x Al1– x Si2O6+0.5 x , 0 ≤ x ≤ 1, have been determined from Rietveld analysis of powder synchrotron XRD data. Our results indicate that the pollucite end member (CsAlSi2O6) has a tetragonal structure (space group I 41/ a ), whereas all other compositions are cubic (space group Ia 3 d ). The increased symmetry for the titanium-substituted structures is presumably due to the incorporation of additional O2− anions (needed for compensating the charge imbalance between Ti4+ and Al3+), which effectively holds open the expanded cubic framework. In situ cooling experiments of the substituted phase CsTi0.1Al0.9Si2O6.05 reveal a displacive transformation to the tetragonal structure at ∼230 K. This transformation is tricritical in nature and is analogous to the tetragonal-to-cubic transition in pollucite on heating. |
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Keywords: | pollucite waste disposal cesium/cesium compounds X-ray methods crystals/crystallization |
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