Experimental evidence of static disorder of carbonate ions in Ba-doped calcite

Abstract

Incorporation of structurally incompatible Ba2+ into calcite by crystallization via amorphous calcium carbonate induces extinction of 113 reflection in X-ray diffraction patterns even at room temperature. Recently, this phenomenon has been hypothesized as deriving from the static disorder of CO32− in Ba-doped calcite, creating a marked increase in the unit-cell volume. To clarify this hypothesis by experiments, behaviors of CO32− and local structures in Ba-doped calcite samples were investigated. Powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) indicate that CO32− ions in Ba-doped calcite are disordered, even at 91K. 13C MAS NMR spectra suggest that Ba atoms in calcite are substituted randomly for Ca atoms. The Ca K-edge X-ray absorption fine structure (XAFS) and the IR absorption spectra indicated reduction of CO32− symmetry with increasing Ba content. These local structure changes by incorporation of incompatible Ba2+ into calcite cause static disorder of CO32− at room temperature.