Disordered Crystal Structure and Anomalously High Solubility of Radium Carbonate

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 K sp 0) is −7.5(1) (2σ) (s = 0.05 g·L–1). This is significantly higher than the log10 K sp 0 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).