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Disordered Crystal Structure and Anomalously High Solubility of Radium Carbonate

Matyskin, Artem V. ; Ebin, Burçak ; Allard, Stefan ; Torapava, Natallia ; Eriksson, Lars ; Persson, Ingmar ; Brown, Paul L. and Ekberg, Christian (2023) In Inorganic Chemistry 62(30). p.12038-12049
Abstract

Radium-226 carbonate was synthesized from radium-barium sulfate (226Ra0.76Ba0.24SO4) at room temperature and characterized by X-ray powder diffraction (XRPD) and extended X-ray absorption fine structure (EXAFS) techniques. XRPD revealed that fractional crystallization occurred and that two phases were formed─the major Ra-rich phase, Ra(Ba)CO3, and a minor Ba-rich phase, Ba(Ra)CO3, crystallizing in the orthorhombic space group Pnma (no. 62) that is isostructural with witherite (BaCO3) but with slightly larger unit cell dimensions. Direct-space ab initio modeling shows that the carbonate oxygens in the major Ra(Ba)CO3 phase are highly disordered. The... (More)

Radium-226 carbonate was synthesized from radium-barium sulfate (226Ra0.76Ba0.24SO4) at room temperature and characterized by X-ray powder diffraction (XRPD) and extended X-ray absorption fine structure (EXAFS) techniques. XRPD revealed that fractional crystallization occurred and that two phases were formed─the major Ra-rich phase, Ra(Ba)CO3, and a minor Ba-rich phase, Ba(Ra)CO3, crystallizing in the orthorhombic space group Pnma (no. 62) that is isostructural with witherite (BaCO3) but with slightly larger unit cell dimensions. Direct-space ab initio modeling shows that the carbonate oxygens in the major Ra(Ba)CO3 phase are highly disordered. The solubility of the synthesized major Ra(Ba)CO3 phase was studied from under- and oversaturation at 25.1 °C as a function of ionic strength using NaCl as the supporting electrolyte. It was found that the decimal logarithm of the solubility product of Ra(Ba)CO3 at zero ionic strength (log10 Ksp0) is −7.5(1) (2σ) (s = 0.05 g·L-1). This is significantly higher than the log10 Ksp0 of witherite of −8.56 (s = 0.01 g·L-1), supporting the disordered nature of the major Ra(Ba)CO3 phase. The limited co-precipitation of Ra2+ within witherite, the significantly higher solubility of pure RaCO3 compared to witherite, and thermodynamic modeling show that the results obtained in this work for the major Ra(Ba)CO3 phase are also applicable to pure RaCO3. The refinement of the EXAFS data reveals that radium is coordinated by nine oxygens in a broad bond distance distribution with a mean Ra-O bond distance of 2.885(3) Å (1σ). The Ra-O bond distance gives an ionic radius of Ra2+ in a 9-fold coordination of 1.545(6) Å (1σ).

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publishing date
type
Contribution to journal
publication status
published
subject
in
Inorganic Chemistry
volume
62
issue
30
pages
12 pages
publisher
The American Chemical Society (ACS)
external identifiers
  • pmid:37477287
  • scopus:85166382165
ISSN
0020-1669
DOI
10.1021/acs.inorgchem.3c01513
language
English
LU publication?
no
id
6f7404b9-5bf7-41e6-bdc5-d8e32c04ef71
date added to LUP
2023-11-02 14:24:31
date last changed
2024-04-19 03:30:23
@article{6f7404b9-5bf7-41e6-bdc5-d8e32c04ef71,
  abstract     = {{<p>Radium-226 carbonate was synthesized from radium-barium sulfate (<sup>226</sup>Ra<sub>0.76</sub>Ba<sub>0.24</sub>SO<sub>4</sub>) at room temperature and characterized by X-ray powder diffraction (XRPD) and extended X-ray absorption fine structure (EXAFS) techniques. XRPD revealed that fractional crystallization occurred and that two phases were formed─the major Ra-rich phase, Ra(Ba)CO<sub>3</sub>, and a minor Ba-rich phase, Ba(Ra)CO<sub>3</sub>, crystallizing in the orthorhombic space group Pnma (no. 62) that is isostructural with witherite (BaCO<sub>3</sub>) but with slightly larger unit cell dimensions. Direct-space ab initio modeling shows that the carbonate oxygens in the major Ra(Ba)CO<sub>3</sub> phase are highly disordered. The solubility of the synthesized major Ra(Ba)CO<sub>3</sub> phase was studied from under- and oversaturation at 25.1 °C as a function of ionic strength using NaCl as the supporting electrolyte. It was found that the decimal logarithm of the solubility product of Ra(Ba)CO<sub>3</sub> at zero ionic strength (log<sub>10</sub> K<sub>sp</sub><sup>0</sup>) is −7.5(1) (2σ) (s = 0.05 g·L<sup>-1</sup>). This is significantly higher than the log<sub>10</sub> K<sub>sp</sub><sup>0</sup> of witherite of −8.56 (s = 0.01 g·L<sup>-1</sup>), supporting the disordered nature of the major Ra(Ba)CO<sub>3</sub> phase. The limited co-precipitation of Ra<sup>2+</sup> within witherite, the significantly higher solubility of pure RaCO<sub>3</sub> compared to witherite, and thermodynamic modeling show that the results obtained in this work for the major Ra(Ba)CO<sub>3</sub> phase are also applicable to pure RaCO<sub>3</sub>. The refinement of the EXAFS data reveals that radium is coordinated by nine oxygens in a broad bond distance distribution with a mean Ra-O bond distance of 2.885(3) Å (1σ). The Ra-O bond distance gives an ionic radius of Ra<sup>2+</sup> in a 9-fold coordination of 1.545(6) Å (1σ).</p>}},
  author       = {{Matyskin, Artem V. and Ebin, Burçak and Allard, Stefan and Torapava, Natallia and Eriksson, Lars and Persson, Ingmar and Brown, Paul L. and Ekberg, Christian}},
  issn         = {{0020-1669}},
  language     = {{eng}},
  number       = {{30}},
  pages        = {{12038--12049}},
  publisher    = {{The American Chemical Society (ACS)}},
  series       = {{Inorganic Chemistry}},
  title        = {{Disordered Crystal Structure and Anomalously High Solubility of Radium Carbonate}},
  url          = {{http://dx.doi.org/10.1021/acs.inorgchem.3c01513}},
  doi          = {{10.1021/acs.inorgchem.3c01513}},
  volume       = {{62}},
  year         = {{2023}},
}