Magnetic structures of the B and C type Cr(PO3)3 metaphosphates

Abstract

The B and C type Cr(PO₃)₃ metaphosphates were synthesised by ceramic methods with control of the Cr/P ratio in the reaction medium. Neutron powder diffraction experiments and heat-capacity measurements were carried out on polycrystalline samples. The C-type [Cr(PO₃)₃] compound is ordered as a three-dimensional Heisenberg antiferromagnet with a J/k value of 0.3 K whereas the B-type [Cr₂P₆O₁₈] phase shows that the magnetic susceptibility depends strongly on the magnetic field at temperatures less than 4 K suggesting the presence of some ferromagnetic contributions in the ordered state. The heat-capacity measurements of C-type Cr(PO₃)₃ exhibit a three-dimensional magnetic ordering (λ-type) peak at 4.2 K. The absence of an appreciable anomaly in the magnetic heat capacity obtained above T_N suggests a small short-range order in this Cr(PO₃)₃ metaphosphate. The magnetic structures of both compounds are consistent with the existence of predominant antiferromagnetic superexchange interactions via |PO₄| tetrahedra between the Cr(III) ions. In the C-type phase, the magnetic moments arranged in (100) ferromagnetic layers are antiparallel with those of the neighbouring layers stabilising an antiferromagnetic structure. The magnetic moments lie in the xz plane pointing 13° out of the x axis. In the B-type phase, the presence of a weak ferromagnetic component is observed. Magnetostructural correlations in both chromium compounds indicate that only super-superexchange interactions via one or two phosphate groups are present. The [Cr–O–P and O–P–O] angles of the magnetic exchange pathways show typical values of antiferromagnetic couplings. The competition between different magnetic interactions in Cr(PO₃)₃ through super-superexchange pathways is not strong enough to cause magnetic frustration as observed in other metaphosphates.