Vibrational analysis of iron and zinc phosphate conversion coating constituents

The FT-MIR/FT-FIR and NIR-FT-Raman spectra of orthorhombic alpha-Zn3(PO4)2 x 4H2O (alpha-hopeite) and monoclinic Zn2Fe(PO4)2 x 4H2O (phosphophyllite), including deuterated samples, have been measured in the polycrystalline state at room temperature and below. The distribution of vibrational levels was related to the results of complete unit-cell group analyses. The number of uncoupled OD stretching modes of alpha-hopeite (isotopically dilute samples) strongly exceeds that expected from the number of hydrogen positions of the structure reported. In contrast, unequivocal assignment of the four hydrogen bonds of phosphophyllite has been performed. The distortion of the phosphate tetrahedra, as revealed from both site group and unit-cell group splitting of the PO stretching modes, is found to be almost equal in both compounds, in accordance with the identical tetrahedral linkage scheme.