Spectroscopic and magnetic properties of three M(PO3)3 (M=Cr and Mo) metaphosphates

Abstract

Three metaphosphates with formula M(PO₃)₃ (M=Cr and Mo) and Cr₂(P₆O₁₈) have been synthesized and characterized. IR data allow establishment of a good correlation between the vibrational frequencies and the chain or ring structures of the M(PO₃)₃ (M=Cr, Mo) metaphosphates and the Cr₂(P₆O₁₈) hexametaphosphate, respectively. The reflectance diffuse spectra show bands belonging to the Cr^III and Mo^III cations in octahedral symmetry. The values of 10Dq and Racah parameters B and C have been calculated. The percentage of reduction calculated for the B parameter (≈70%) is indicative of a significant degree of covalence in the M–O (M=Cr, Mo) bonds. The EPR spectra of the metaphosphate compounds have been explained on the basis of the existence of more than one different environment for the metallic cations. In doped aluminium metaphosphates with Cr^III and Mo^III ions the EPR data indicate the presence of one centre with |D|>hν and two centres with |D|<hν. Magnetic measurements show an antiferromagnetic behaviour for all compounds. In the case of the Cr₂(P₆O₁₈) hexametaphosphate the existence of a weak ferromagnetism intrinsic to the sample has been observed at ca. 3 K.