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

Polycrystalline samples of struvite-type MgRbAsO 4 ·6H 2 O and MgTlAsO 4 ·6H 2 O in both protiated and deuterated forms have been prepared for the first time by a precipitation method. Detailed analysis of their infrared (IR) and Raman (R) spectra recorded at room temperature (RT) and liquid nitrogen temperature (LNT) has been carried out and assignment of the vibrational bands has been proposed. The observed spectral pictures confirm the isostructurality of the two arsenate salts in agreement with the structural data. Similarly to other struvite-type and structure-related arsenate salts, the n 1 (AsO 4 ) modes appear at higher frequencies than the n 3 (AsO 4 ) vibrations. Low intensity and temperature sensitive bands that could be assigned as due to stretching Mg–O vibrations and n 4 (AsO 4 ) modes are observed below 500 cm –1 in the LNT IR spectra of the studied compounds, at 472 cm –1 and 445 cm –1 for the rubidium analogue and at 470 cm –1 and 440 cm –1 for the thallium analogue. In the far-infrared and Raman spectra many bands are observed between 400 cm –1 and 200 cm –1 , most of which are sensitive to deuteration, suggesting that they are not pure but coupled and can be related to n(Mg–Ow) modes. On the other hand, a band around 405 cm –1 in the spectra of the protiated compounds most probably is due to n 2 (AsO 4 ) mode because of its frequency position, intensity and slight sensitivity to deuteration.