Magnetic properties of M(PO3)3 (M = Fe, Mo). A comparative neutron diffraction study

Abstract

The isostructural M(PO₃)₃ (M = Fe, Mo) metaphosphates were synthesised by ceramic methods with control of the M/P ratio in the reaction medium. Neutron powder diffraction experiments, heat capacity measurements and Mössbauer spectrometry were carried out on polycrystalline samples. Both compounds are ordered as three-dimensional Heisenberg antiferromagnets with J/k values of ca. 0.3 K. The magnetic susceptibility in the Fe(PO₃)₃ phase depends strongly on the magnetic field at temperatures less than 10 K suggesting the presence of some ferromagnetic contributions in the ordered state whereas in the Mo(PO₃)₃ metaphosphate field dependence of the magnetic susceptibility even at 10 mT was not detected. The heat-capacity measurements exhibit three-dimensional magnetic ordering (λ-type) peaks at 4 and 7.8 K for the molybdenum and iron phases respectively. The measurements above T_N suggest the presence of a small short-range order in both phases. However, it is worth mentioning that Fe(PO₃)₃ shows the presence of a significant magnetic anomaly below the ordering temperature. The Mössbauer measurements in the iron metaphosphate indicate the presence of three different high-spin Fe(III) ions in the structure. The magnetic structures of both compounds are consistent with the existence of predominant antiferromagnetic superexchange interactions via |PO₄| tetrahedra between the metal(III) ions. The magnetic moments are arranged in (100) and (001) ferromagnetic layers for the molybdenum and iron metaphosphate respectively. The neighbouring layers are disposed antiparallel cancelling their components and stabilising the antiferromagnetic structure in both compounds. Magnetostructural correlations indicate that only super-superexchange interactions via one or two phosphate groups are present in both Fe and Mo compounds. The values of the magnetic exchange pathways [M–O–P and O–P–O] are characteristic of antiferromagnetic couplings. Notwithstanding this, the existence of competition between different magnetic interactions through super-superexchange pathways can cause magnetic frustration giving rise to the presence of a weak ferromagnetic component in Fe(PO₃)₃.