Infrared and Raman spectra of cubic form of magnesium caesium arsenate hexahydrate

Abstract

Fourier transform infrared (FT-IR) spectra recorded at room temperature (RT) and at the boiling temperature of liquid nitrogen (LNT), as well as Raman spectra recorded at room temperature for the cubic polymorph of magnesium cesium arsenate hexahydrate (MgCsAsO4·6H2O) and its partially deuterated analogues, were interpreted with respect to the normal modes of the water molecules and the arsenate ions and water librations. The spectral characteristics of MgCsAsO4·6H2O were compared to the cubic form of the phosphate analogue. A spectral similarity between the two isostructural salts was established, except for the obvious differences due to the nature of the anions (AsO43– vs PO43–). The spectroscopic data for the uncoupled OD stretching mode of the matrix-isolated HDO molecules revealed that the hydrogen bonds formed in the arsenate salt were stronger than those in the phosphate. In the Raman spectrum of the protiated compound, only one very intensive band at 811 cm–1 was observed in the region of the stretching vibrations of the AsO43– ion, which was insensitive to deuteration. In accordance with the expectation, one band appeared in the same spectral range in the infrared spectra of the protiated and highly deuterated sample at 792 cm–1 and 810 cm–1, respectively, which can be attributed with certainty to the asymmetric stretching ν3(AsO4) modes.