Very low H–O–H bending frequencies. IV. Fourier transform infrared spectra of synthetic dittmarite

Abstract

The Fourier transform infrared spectra of MgNH 4PO 4·H 2O (the synthetic analogue of the mineral dittmarite) and of a series of its partially deuterated analogues have been studied as a part of our continuous work on compounds exhibiting very low water bending frequencies. Although, the presence of ammonium bands makes the assignments in this case more difficult than for the potassium analogues, the isomorphism between the compounds of the MNH 4PO 4·H 2O type (M=Mg, Co, Ni, Mn and one of the polymorphs of CdNH 4PO 4·H 2O) and those which contain potassium instead of ammonium as well as the careful analysis of the spectra warrant the conclusion that in the presently studied compound the water bending mode appears at a frequency which is far more than 100 cm −1 lower than in the gaseous water. The spectra clearly show that the ammonium ions in the structure are involved in quite strong hydrogen bonds, a characteristic which is a precondition for a material to behave as a protonic conductor.