Disordered Crystal Structure and Anomalously High Solubility of Radium Carbonate

Radium-226 carbonate was synthesized from radium-barium sulfate (²²⁶Ra_0.76Ba_0.24SO₄) 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_sp⁰) is -7.5(1) (2σ) (s = 0.05 g·L^-1). This is significantly higher than the log₁₀ K_sp⁰ 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).