Infrared and Raman spectra of magnesium ammonium phosphate hexahydrate (struvite) and its isomorphous analogues. VII: Spectra of protiated and partially deuterated hexagonal magnesium caesium phosphate hexahydrate

The Fourier transform infrared and Raman spectra of the struvite analogue, hexagonal magnesium caesium phosphate hexahydrate, MgCsPO4·6H2O (hP50) and of its partially deuterated analogues were recorded from room temperature (RT) down to the boiling temperature of liquid nitrogen (LNT). The existence of strong hydrogen bonds between water molecules and PO43− ions is supported by the appearance of a broad band from 3600 to 2200 cm−1 in the O–H stretching region of the vibrational spectra. In the region of the OD stretching vibrations of isotopically isolated HDO molecules of the analogue with a small deuterium content (≈5% D), at least two bands (from the expected three) are observed in the difference LNT infrared spectrum. In the region of ν3(PO4) modes of the infrared spectra, a broad and asymmetric band (at around 1000 cm−1) is found, while in the region of the ν4(PO4) bending vibration and of the external modes of the water molecules, several bands can be seen. The intense band at 945 cm−1 in the Raman spectra can with certainty be attributed to the ν1(PO4) mode. On the basis of a careful analysis of the RT and LNT spectra of the protiated compound, as well as those of its partially deuterated analogues, the asymmetric band at around 550 cm−1 could be assigned to the components of the ν4(PO4) mode, the bands between 470 and 430 cm−1 to the ν2(PO4) vibrations and the remaining ones as due to pure or coupled librational and translational modes of the water molecules. The external modes of the phosphate ions and those of the water molecules are mixed.