Vibrational spectra of MgKH(XO)·15HO (X=P, As) containing dimer units [H(XO)].

Infrared and Raman spectra of MgKH(PO)·15HO and MgKH(AsO)·15HO and a series of their partially deuterated analogues were recorded and analyzed. Compounds of the type MgKH(XO)·15HO (X=P, As) are little-known and a rare case of phosphate and arsenate salts containing dimer units [H(XO)] 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 XO groups connected in a dimer through a proton are not consistent with the presence of X-O-H covalent linkage and C crystallographic symmetry of the XO groups. The observation of a singlet Raman band for the ν(XO) mode as well as the absence of substantial splitting of the ν(XO) modes and IR activation of the ν(XO) mode suggest that the dimer units [H(XO)] are most probably symmetric rather than non-symmetric ones. It was found that, in the vibrational spectra of MgKH(AsO)·15HO, both ν(AsО) and ν(AsО) modes have practically the same wavenumber around 830cm. It was also established that the ν(PО) modes in the deuterated hydrogendiphosphate compound are strongly coupled, most probably with HDO and/or DO librations. As a whole, the spectral picture of MgKH(XO)·15HO (X=P, As) very much resembles that observed for the struvite type compounds with the formula KMgXO·6HO (X=P, As) which do not contain X-OH groups. This means that vibrations of the dimers [H(XO)] play a relatively small part in the general spectral appearance.