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 MKPO4·6H2O (M = Mg; Ni) and PO43- ions isolated in MgNH4AsO4·6

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 MgKPO4·6H2O and NiKPO4·6H2O and phosphate doped in MgNH4AsO4·6H2O in different degrees were recorded. All three compounds are isostructural with struvite, MgNH4PO4·6H2O, space group Pmn21, forming substitutional mixed crystals with Cs symmetry of the anions. Analysis of the region of ν4(NH4), ν3(PO4) and ν4(PO4) modes of LNT FT-IR difference spectra of analogues with a small content of NH4+ and PO43- revealed the expected decrease of Td symmetry of free NH4+ and PO43- ions to Cs site symmetry. Due to the Cs site symmetry of the anions, the degeneration of the ν4(NH4), ν3PO4) and ν4PO4) 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.