INFRARED-SPECTRA OF THE MONOHYDRATES OF MANGANESE(III) PHOSPHATE AND MANGANESE(III) ARSENATE - RELATION TO THE COMPOUNDS OF THE KIESERITE FAMILY

The IR spectra of the title compounds suggest that the hydrogen bondin g is stronger in MnAsO4 . H2O than in MnPO4 . H2O. The similarity of t he IR spectra of these two compounds with the spectra of magnesium sul fate monohydrate (kieserite) and the members of its family (compounds with the general formula MIIRO4 . H2O, where MII is Mg, Mn, Fe, Co, Ni or Zn, and R is S or Se) suggests a close structural similarity betwe en them, in agreement with the published diffraction data. Both spectr oscopic and diffraction studies indicate that, due to a synergetic eff ect of the highly charged and smaller Mn3+ cation and the increased pr oton-acceptor ability of the PO4/AsO4 oxygens, the hydrogen bonds are stronger in MnPO4 . H2O and MnAsO4 . H2O than in the compounds of the kieserite class. The most notable spectral difference between MnPO4 . H2O and MnAsO4 . H2O, on the one hand, and MnSO4 . H2O, on the other, is seen in the region of the delta(HOH) vibrations. (C) 1997 Elsevier Science B.V.