Metal-water interactions and hydrogen bonding in dittmarite-type compounds M′M″PO4·H2O (M′ = K+, NH4+; M″ = Mn2+, Co2+, Ni2+)

The vibrational behavior of the uncoupled νOD modes and of the water librations in dittmarite-type compounds M′M″PO4·H2O (M′ = K+, NH4+; M″ = Mn2+, Co2+, Ni2+) is analyzed in terms of the influence of two types of metal−water interactions (M+−H2O and M2+−H2O), the hydrogen bonding and the repulsion potential of the lattices. The M+−H2O interaction is found to be the main factor, which influences νOD. The strong K+−H2O interaction weakens in a higher degree the intramolecular O−H bonds than the corresponding M2+−H2O interactions (M2+ = Mn, Co, Ni). As a result νOD is shifted to lower wavenumbers in the potassium series than in ammonium one, irrespective of the synergetic effect of M2+, the hydrogen bond lengths and the repulsion potential of the lattices. The analysis of the spectroscopic data evidences for the dominating influence of the M2+−H2O interaction on the wagging mode. The blue shift of νwag strictly follows the increasing synergetic effect of M2+, i.e. νM′/Mn < νM′/Co < νM′/Ni, in all cases, irrespective of the strength of the M+−H2O interactions, the hydrogen bond lengths and the higher repulsion potential of the K-lattices. The rocking mode is insensitive to the replacement of the M2+ ions. Some relations between the spectroscopic and structural data are given.