Coupling of the Out-of-Plane Bending Mode in Nitrates and Carbonates of the Aragonite Structure

Abstract

The infrared absorption of polycrystalline samples of KNO₃ and BaCO₃ in the form of Nujol mulls has been observed between 11.3 and 12.5 microns under conditions such that the rotational lines of the NH₃ spectrum separated by 1.5 cm^—1 could be clearly resolved. When the N¹⁵ or C¹³ content is increased to the order of 50 percent, fine structure is observed which is not present e.g., in NaNO₃, which possesses a crystal structure similar to calcite. The fine structure is interpreted by postulating a force constant which couples the out‐of‐plane vibrations of nearest neighbors in the aragonite‐type lattice. Very satisfactory agreement between the calculated and observed spectra is then found; in the case of BaCO₃, the vibrations of CO₃ ions which occur in isotopic isolation, as pairs, triplets, and quadruplets are resolved. The significance of the coupling constant is discussed; in principle, it may be used to evaluate the dipole derivative for the mode under consideration.