Vibrational behavior of the phosphates ions in dittmarite-type compounds M′M″PO4·H2O (M′ = K+, NH4+; M″ = Mn2+, Co2+, Ni2+)

The IR and Raman spectra of the isostructural M′M″PO4·H2O compounds (M′ = K+, NH4+; M″ = Mn2+, Co2+, Ni2+) are reported and discussed with respect to the normal vibrations of the PO43− ions. The vibrational behavior of PO43− is in agreement with its low site symmetry Cs in the lattices—the symmetric ν1 and ν2 modes are activated in the IR spectra and the degeneration of the asymmetric ν3 and ν4 modes is lifted. A relatively large unit-cell group splitting is observed for ν1 in both the IR and Raman spectra and for ν3 in Raman spectra. It has been established that the mean wavenumbers of the PO stretches (ν¯PO) are not affected by the M2+ ions present, but they are lower for the NH4-series than for the K-one (predominant influence of both the smaller repulsion potential and the hydrogen bonds in the NH4-lattices over the influence of the M+–O interactions). The extent of the energetic distortion of the PO43− ions has been estimated based on the spectroscopic data for the site group splitting of the asymmetric modes (Δν3 and Δν4), the separation between the highest and the lowest wavenumbered PO stretches (Δνmax) and the intensity of ν1 in the IR spectra. The data provide an evidence that the PO43− ions in KM″PO4·H2O are more distorted regarding the PO bond lengths than those in NH4M″PO4·H2O, 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).