Decomposition of Silver Carbonate; the Crystal Structure of Two High-Temperature Modifications of Ag₂CO₃

High-resolution powder diffraction was used to study the thermal transformation of silver carbonate. A sample of Ag(2)CO(3) was heated in a capillary under 4.5 atm CO(2) pressure. The decomposition temperature of silver carbonate to silver oxide is thereby increased, allowing high-resolution synchrotron X-ray powder diffraction patterns of the two high-temperature phases of Ag(2)CO(3) to be collected. The structure of the low-temperature (lt) phase was confirmed, and the structures of the two high-temperature phases were determined by direct methods and refined using the Rietveld method: lt-Ag(2)CO(3) (295 K) P2(1)/m, z = 2, a = 4.8521(2) A, b = 9.5489(4) A, c = 3.2536(1) A, beta = 91.9713(3) degrees; beta-Ag(2)CO(3) (453 K) P31c, z = 6, a = 9.1716(4) A, c = 6.5176(3) A; alpha-Ag(2)CO(3) (476 K) P6 macro 2m, z = 3, a = 9.0924(4) A, c = 3.3249(1) A. In addition, thermal expansion properties, anisotropic microstrain distributions, and thermal transformations of the three silver carbonate phases and silver oxide are described.