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.

Infrared and Raman spectra of magnesium ammonium phosphate hexahydrate (struvite) and its isomorphous analogues. X. Vibrational spectra of magnesium rubidium arsenate hexahydrate and magnesium thallium arsenate hexahydrate

“…It should be also noted that in both studied compounds, the 1(AsO4) modes appear at higher frequencies than the 3(AsO4) vibrations. The same spectral feature has been established in the vibrational spectra of orthorhombic MgNH4AsO4•6H2O and MgKAsO4•6H2O [8,9] and the cubic form of MgCsAsO4•6H2O [13], whereas for Mg2KH(AsO4)•15H2O it was found that both 1(AsO4) and 3(AsO4) modes have practically the same wavenumbers at around 830 cm -1 [11]. This finding is consistent with spectroscopic studies on different arsenate compounds (natural and synthetic) [27][28][29][30][31].…”

“…However, our spectroscopic data [9] revealed that this polymorphous transition does not significantly affect the appearance of the LNT IR spectra of the potassium analogue, which are still very much alike the ones obtained for MgRbAsO4•6H2O and Mg-TlAsO4•6H2O. The identical conclusion has been deduced for the spectra of the other related com-pounds with formula M I M II XO4•6H2O (M I = NH4, K, Rb, Cs, Tl; M II = Mg, Co, Ni; X= P, As) no matter if they crystallize in the orthorhombic, hexagonal, cubic or monoclinic system and/or correspondingly they have four, two or one crystallographically different water molecules of crystallization in their structure [8,9,13,[18][19][20][21][22][23][24][25]. The only difference that is evident in these spectra is in the region of the HOH librations and is dependent on the number of the crystallographically different water molecules of crystallization that exist in the structure and the type of the anion (phosphate or arsenate).…”

“…It should be mentioned here that in the spectra of the cubic form of MgCsAsO4•6H2O [13], the lowest frequency band in this region is observed at 1520 cm -1 which is a frequency lower than the one of the bending HOH vibration of gaseous water (1595 cm -1 ). Such bands have been observed in the spectra of the compounds with formula M I M II XO4•6H2O (M I = NH4, K, Rb, Cs, Tl; M II = Mg, Co, Ni; X= P, As) and they can be explained as due to second order transitions, Fermi type of resonance interactions (resonance interactions of overtones of lower frequency modes with δ(HOH) vibrations) or to coupling with low frequency lattice modes [8][9][10][11][12][13]. All these mechanisms are generally physically feasible and none of them (or their combination) should be neglected.…”

“…A closer look in the stretching HOH region in the RT and LNT spectra of the titled compounds (Figs. 2-4) and the spectra of the compounds with formula M I M II XO4•6H2O (M I = NH4, K, Rb, Cs, Tl; M II = Mg, Co, Ni; X= P, As), no matter the crystal system [8,9,13,[18][19][20][21][22][23][24][25], in the region between 3800 cm -1 and 2200 cm -1 in the IR spectra and between 3500 cm -1 and 2200 cm -1 in the Raman spectra, one wide and asymmetric band with many shoulders is observed that is more intensive in the IR spectra and is sensitive to deuteration (Figs. 2-4).…”

“…With this contribution we continue the study of the vibrational spectra of arsenate crystal hydrates with the general formula M I MgAsO4•6H2O (M I = NH4, K, Rb, Tl) having struvite-type or closely related structures which have been a subject of our research in the last fifteen years [8][9][10][11][12][13]. Here, we have focused on rubidium and thallium representatives, MgRbAsO4•6H2O and MgTlAsO4•6H2O that are very little known.…”