Vibrational behavior of the phosphates ions in dittmarite-type compounds M′M″PO 4·H 2O (M′ = K +, NH 4+; M″ = Mn 2+, Co 2+, Ni 2+)

Abstract

The IR and Raman spectra of the isostructural M′M″PO 4·H 2O compounds (M′ = K +, NH 4 +; M″ = Mn 2+, Co 2+, Ni 2+) are reported and discussed with respect to the normal vibrations of the PO 4 3− ions. The vibrational behavior of PO 4 3− is in agreement with its low site symmetry C s in the lattices—the symmetric ν 1 and ν 2 modes are activated in the IR spectra and the degeneration of the asymmetric ν 3 and ν 4 modes is lifted. A relatively large unit-cell group splitting is observed for ν 1 in both the IR and Raman spectra and for ν 3 in Raman spectra. It has been established that the mean wavenumbers of the P O stretches ( ν ¯ PO ) are not affected by the M 2+ ions present, but they are lower for the NH 4-series than for the K-one (predominant influence of both the smaller repulsion potential and the hydrogen bonds in the NH 4-lattices over the influence of the M +–O interactions). The extent of the energetic distortion of the PO 4 3− ions has been estimated based on the spectroscopic data for the site group splitting of the asymmetric modes (Δ ν 3 and Δ ν 4), the separation between the highest and the lowest wavenumbered P O stretches (Δ ν max) and the intensity of ν 1 in the IR spectra. The data provide an evidence that the PO 4 3− ions in KM″PO 4·H 2O are more distorted regarding the P O bond lengths than those in NH 4M″PO 4·H 2O, but their angular distortion is the same in both series. The trends for the energetic distortion of the phosphate ions found from the spectroscopic data correspond to the data for their geometric distortion deduced from the values of the distortion indices DI(PO) and DI(OPO).