Vibrational spectra of Mg₂KH(XO₄)₂·15H₂O (X=P, As) containing dimer units [H(XO₄)₂]

Infrared and Raman spectra of Mg₂KH(PO₄)₂·15H₂O and Mg₂KH(AsO₄)₂·15H₂O and a series of their partially deuterated analogues were recorded and analyzed. Compounds of the type Mg₂KH(XO₄)₂·15H₂O (X=P, As) are little-known and a rare case of phosphate and arsenate salts containing dimer units [H(XO₄)₂] in the crystal structure. The analysis of their IR spectra (recorded at room and liquid nitrogen temperature) and Raman spectra showed that the spectral characteristics of the XO₄ groups connected in a dimer through a proton are not consistent with the presence of X-O-H covalent linkage and C₁ crystallographic symmetry of the XO₄ groups. The observation of a singlet Raman band for the ν₁(XO₄) mode as well as the absence of substantial splitting of the ν₃(XO₄) modes and IR activation of the ν₁(XO₄) mode suggest that the dimer units [H(XO₄)₂] are most probably symmetric rather than non-symmetric ones. It was found that, in the vibrational spectra of Mg₂KH(AsO₄)₂·15H₂O, both ν₁(AsО₄) and ν₃(AsО₄) modes have practically the same wavenumber around 830cm^-1. It was also established that the ν₄(PО₄) modes in the deuterated hydrogendiphosphate compound are strongly coupled, most probably with HDO and/or D₂O librations. As a whole, the spectral picture of Mg₂KH(XO₄)₂·15H₂O (X=P, As) very much resembles that observed for the struvite type compounds with the formula KMgXO₄·6H₂O (X=P, As) which do not contain X-OH groups. This means that vibrations of the dimers [H(XO₄)₂] play a relatively small part in the general spectral appearance.