Infrared and Raman spectra of magnesium ammonium phosphate hexahydrate (struvite) and its isomorphous analogues. Part VI: FT-IR spectra of isomorphously isolated species. NH4+ ions isolated in MKPO 4·6H 2O (M = Mg; Ni) and PO43- ions isolated in MgNH 4AsO 4·6H 2O

Examination of mixed crystals, especially those with isomorphously isolated ions, has proved very useful in spectra-structure correlation studies. Room (RT) and low temperature (LNT) FT-IR spectra of ammonium doped in MgKPO ₄·6H ₂O and NiKPO ₄·6H ₂O and phosphate doped in MgNH ₄AsO ₄·6H ₂O in different degrees were recorded. All three compounds are isostructural with struvite, MgNH ₄PO ₄·6H ₂O, space group Pmn2 ₁, forming substitutional mixed crystals with C_s symmetry of the anions. Analysis of the region of ν ₄(NH ₄), ν ₃(PO ₄) and ν ₄(PO ₄) modes of LNT FT-IR difference spectra of analogues with a small content of NH4+ and PO43- revealed the expected decrease of T_d symmetry of free NH4+ and PO43- ions to C_s site symmetry. Due to the C_s site symmetry of the anions, the degeneration of the ν ₄(NH ₄), ν ₃PO ₄) and ν ₄PO ₄) modes is raised and, hence, three components are observed in the difference FT-IR spectra. This conclusion can not be derived only from studies of no-doped polycrystalline samples of struvite type compounds.