Polymorphism and Anion Rotational Disorder in the Alkaline Earth Carbonates

Abstract

X-ray diffraction methods have been developed for the study of materials at high temperatures under controlled atmospheres, and an investigation of the high temperature phases of the alkaline earth carbonates has been made. BaCO3 was found to change from orthorhombic (aragonite type) to hexagonal (calcite type) at 803°C, and to cubic (sodium chloride type) at 976°C. SrCO3 was found to change from orthorhombic (aragonite type) to hexagonal (calcite type) at 912°C. A few experiments with mixtures, which go through similar transitions, are described. These transitions are related to the rotational activity of the carbonate ions and consideration of the nature of the transitions leads to a description of the probable rotational behavior of these ions in the various phases.