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

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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.

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