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 MgNH4AsO4·6H2O

Abstract 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 2 O and NiKPO 4 ·6H 2 O and phosphate doped in MgNH 4 AsO 4 ·6H 2 O in different degrees were recorded. All three compounds are isostructural with struvite, MgNH 4 PO 4 ·6H 2 O, space group Pmn 2 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 NH 4 + and PO 4 3 - revealed the expected decrease of T d symmetry of free NH 4 + and PO 4 3 - 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 3 PO 4 ) and ν 4 PO 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.