| A group of four chromium phosphate compounds: Cr(PO3)3, Cr4(P2O7)3, Cr2P4O13 and β-CrPO4 was investigated using the IR and EPR techniques. The Infra-Red spectra allowed us to distinguish two specific vibration bands: a band corresponding to a stretching vibration of the P-O bond (ca. 1400 ÷ 700 cm-1) as well as a band corresponding to the bending vibrations of the O-P-O groups and vibrations of the Cr-O bonds (ca. 700 ÷ 300 cm-1). Despite a low crystallographic symmetry (a monoclinic space group), the Cr2P4O13 phosphate possessed some structural symmetry elements which simplified the IR spectrum. The EPR results showed the Cr3+ ions could not be treated as isolated magnetic ions. In case of all the analyzed samples, trivalent chromium ions created antiferromagnetic exchange coupled pairs or greater antiferromagnetic clusters. The strength of the antiferromagnetic interaction between the species was strongly dependent on the neighboring ions. It seems that PO4 tetrahedra play a role of mediators in exchange interactions (superexchange). |
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