Energy calculations for the lattice dynamics of NH₄IO₃

The lattice dynamics of ammonium iodate have been studied experimentally and theoretically for centre-zone phonons. The frequencies of normal modes have been measured by Raman spectroscopy. The experimental results have been used to obtain the parameters of a simplified rigid-ion model. It is shown that the strongest bonds are the linear I-O and N-H covalent interactions, while the low-frequency external NH₄-IO₆ mode is mainly ionic in character. The effective ionic charge of the NH₄ group is - 0.82 e.