Infrared and Raman spectra of magnesium ammonium phosphate hexahydrate ( struvite) and its isomorphous analogues. IV. FTIR spectra of protiated and partially deuterated nickel ammonium phosphate hexahydrate and nickel potassium phosphate hexahydrate

Abstract

The Fourier transform infrared spectra recorded from room temperature down to the boiling temperature of liquid nitrogen of two struvite analogues, nickel ammonium phosphate hexahydrate (NiNH 4PO 4·6H 2O) and nickel potassium phosphate hexahydrate (NiKPO 4·6H 2O) and their partially deuterated analogues were analyzed and compared to the previously studied spectra of struvite and its potassium analogue. In the stretching mode region of the water molecules and ammonium ions, a broad asymmetric and deuteration sensitive band appears, which is an indication for strong hydrogen bonds in the structure of NiNP. In the LNT difference spectra of samples with low deuterium content (≈2–3% D), several bands appear between 2520 and 2080 cm −1 due to vibrations of isotopically isolated HDO molecules and NH 3D + ions. The most significant difference between the two studied spectra is between 2350 and 2250 cm −1 where additional bands from ND stretching modes of isotopically isolated NH 3D + ions are expected in the spectrum of NiNP. In the region of ν 3(PO 4) modes one strong, slightly asymmetric, temperature-sensitive band appears above 1000 cm −1 in both spectra. The analysis of the RT and LNT spectra of the protiated and partially deuterated compounds, the band at around 575 cm −1 is assigned to the ν 4(PO 4) modes and the remaining ones as due to librational and translational modes of the water molecules.