Structure, spectra, and stability of solid bismuth carbonates

Abstract

A previously unreported basic bismuth carbonate, (Bi0)4(0H)2C03, has been identified as an intermediate product in the interconversion of a-BiZ03 and synthetic bismutite, (BiO)?CO,, in aqueous carbonate solutions. It has a narrow stability field, between C 0 2 partial pressures of 10-55".0 Pa and IO-' .""' Pa, in the presence of dilute aqueous solution at 25OC. Gibbs energies of formation, calculated from these partial pressures, are AG"((B~O),(OH)~CO,, s, 298.15 K) = -1678 ? 9 kJ mol-' and A l ~" ( ( ~i O ) 2 C 0 3 . s. 298.15 K) = -945 -f 7 kJ mol-'. The two carbonates have been compared by thermogravimetric analysis, X-ray powder diffractometry, and infrared and Raman spectrosFopy. Thc unit cell of (BiO),(OH)ZC03 is dimensionally orthorhombic, with n = 10.772(1), b = 5.4898(5), c = 14.757(1) A , Z = 4, but its true symmctry is probably triclinic. A structural model for (BiO),(OH)2C03, and two modified models for (Bi0)2C03, are proposed. The possible natural occurrence of (BiO),(OH)2C03 is discussed.