Title: Crystal Structure and Thermodynamic Stability of Ba/Ti-Substituted Pollucites for Radioactive Cs/Ba Immobilization

Abstract

As an analogue of the mineral pollucite (CsAlSi₂O₆), CsTiSi₂O_6.5 is a potential host phase for radioactive Cs. However, as ¹³⁷Cs and ¹³⁵Cs transmute to ¹³⁷Ba and ¹³⁵Ba, 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_xBa_(1-x)/2TiSi₂O_6.5 and Cs_xBa_1-xTiSi₂O_7-0.5x, (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_xBa_(1-x)/2TiSi₂O_6.5 samples are phase-pure, Cs_xBa_1-xTiSi₂O_7-0.5x samples contain Cs_3x/(2+x)Ba_(1-x)/(2+x)TiSi₂O_6.5 pollucites (i.e., also two-Cs-to-one-Ba substitution) and a secondary phase, fresnoite (Ba₂TiSi₂O₈). Thus, the Cs_xBa_1-xTiSi₂O_7-0.5x series is energetically less favorable than Cs_xBa_(1-x)/2TiSi₂O_6.5. To study the stability systematics of Cs_xBa_(1-x)/2TiSi₂O_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 excessmore » Ba), thereby providing viable ceramic waste forms for all the Ba decay products.« less

Authors:

Xu, Hongwu ^[1]; Chavez, Manuel E. ^[1]; Mitchell, Jeremy N. ^[1]; Garino, Terry J. ^[2]; Schwarz, Haiqing L. ^[2]; Rodriguez, Mark A. ^[2]; Rademacher, David X. ^[2]; Nenoff, Tina Maria ^[2]

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+ Show Author Affiliations

1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
2. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

Publication Date:

Thu Apr 23 00:00:00 EDT 2015

Research Org.:

Sandia National Lab. (SNL-NM), Albuquerque, NM (United States); Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)

Sponsoring Org.:

USDOE Office of Nuclear Energy (NE), Fuel Cycle Technologies (NE-5); USDOE

OSTI Identifier:

1183100

Alternate Identifier(s):

OSTI ID: 1321745; OSTI ID: 1401387

Report Number(s):

SAND2014-17035J; LA-UR-14-25570
Journal ID: ISSN 0002-7820; 536967; TRN: US1600455

Grant/Contract Number:  

AC04-94AL85000; AC52-06NA25396; DE‐AC52‐06NA25396

Resource Type:

Accepted Manuscript

Journal Name:

Journal of the American Ceramic Society

Additional Journal Information:

Journal Volume: 98; Journal Issue: 8; Journal ID: ISSN 0002-7820

Publisher:

American Ceramic Society

Country of Publication:

United States

Language:

English

Subject:

07 ISOTOPE AND RADIATION SOURCES; 36 MATERIALS SCIENCE; Material Science