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Crystal Structure and Thermodynamic Stability of Ba/Ti‐Substituted Pollucites for Radioactive Cs/Ba Immobilization
Author(s) -
Xu Hongwu,
Chavez Manuel E.,
Mitchell Jeremy N.,
Garino Terry J.,
Schwarz Haiqing L.,
Rodriguez Mark A.,
Rademacher David X.,
Nenoff Tina M.
Publication year - 2015
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/jace.13608
Subject(s) - crystallography , crystal structure , rietveld refinement , materials science , phase (matter) , x ray crystallography , boron , crystallization , analytical chemistry (journal) , chemistry , diffraction , organic chemistry , physics , chromatography , optics
As an analogue of the mineral pollucite (CsAlSi 2 O 6 ), CsTiSi 2 O 6.5 is a potential host phase for radioactive Cs. However, as 137 Cs and 135 Cs transmute to 137 Ba and 135 Ba, respectively, through the beta decay, it is essential to study the structure and stability of this phase upon Cs → Ba substitution. In this work, two series of Ba/Ti‐substituted samples, Cs x Ba (1− x )/2 TiSi 2 O 6.5 and Cs x Ba 1− x T iS i 2 O 7−0.5 x , ( x = 0.9 and 0.7), were synthesized by high‐temperature crystallization from their respective precursors. Synchrotron X‐ray diffraction and Rietveld analysis reveal that while Cs x Ba (1− x )/2 TiSi 2 O 6.5 samples are phase‐pure, Cs x Ba 1− x TiS i 2 O 7−0.5 x samples contain Cs 3 x /(2+ x ) Ba (1− x )/(2+ x ) TiSi 2 O 6.5 pollucites (i.e., also two‐Cs‐to‐one‐Ba substitution) and a secondary phase, fresnoite (Ba 2 TiSi 2 O 8 ). Thus, the Cs x Ba 1− x TiS i 2 O 7−0.5 x series is energetically less favorable than Cs x Ba (1− x )/2 TiSi 2 O 6.5 . To study the stability systematics of Cs x Ba (1− x )/2 TiSi 2 O 6.5 pollucites, high‐temperature calorimetric experiments were performed at 973 K with or without the lead borate solvent. Enthalpies of formation from the constituent oxides (and elements) have thus been derived. The results show that with increasing Ba/(Cs + Ba) ratio, the thermodynamic stability of these phases decreases with respect to their component oxides. Hence, from the energetic viewpoint, continued Cs → Ba transmutation tends to destabilize the parent silicotitanate pollucite structure. However, the Ba‐substituted pollucite co‐forms with fresnoite (which incorporates the excess Ba), thereby providing viable ceramic waste forms for all the Ba decay products.