Infrared and Raman Spectra of Magnesium Ammonium Phosphate Hexahydrate (Struvite) and Its Isomorphous Analogues

Abstract

The three investigated compounds — KMgPO₄·6H₂O (KMP), NH₄MgPO₄·6H₂O (NMP) and NH₄MgAsO₄·6H₂O (NMA) — are characterized by the existence, in their structure, of quite strong O_w⋯O hydrogen bonds (of which some are among the shortest of this type ever found in crystalline hydrates, the O_w⋯O distances being slightly above 260 pm). Reflecting the strength of the hydrogen bonds, a broad and structured feature extending from ≈ 4000 down to ≈ 2000 cm⁻¹ is found in the infrared spectra. Although not as rich in detail as the infrared spectra, the Raman ones are similar in appearance. Rather surprisingly, the spectral picture is not very different in the case of the two phosphate compounds (KMgPO₄·6H₂O and NH₄MgPO₄·6H₂O) although N-H stretching bands are expected in the same region of the spectrum of NMP. Thus, obviously, the intensity of the feature in the O-H/N-H stretching region is due mainly to modes of the water molecules whereas the NH₄ stretches contribute to a lesser degree to the shape and intensity. The comparison of the spectra in the O-D/N-D stretching region confirms this.