Magnetic properties of two hydrothermally synthesized iron(iii) phosphates: Fe(NH3)2PO4 and Fe(NH4)(HPO4)2

The open-framework compounds Fe(NH3)2PO4 and Fe(NH4)(HPO4)2 were synthesized by hydrothermal reactions in the H3PO4–(NH2)2CO–FeCl3–H2O system. The electron paramagnetic resonance spectra for Fe(NH3)2PO4 show an isotropic signal with g = 2, characteristic of high spin Fe(III) ions in an octahedral environment. Below 22 K the Fe(NH3)2PO4 compound exhibits a long-range antiferromagnetic ordering in which the magnetic interactions involve Fe–O–P–O–Fe superexchange pathways. Fe(NH4)(HPO4)2 exhibits an antiferromagnetic behaviour with the presence of a significant ferromagnetic component from 18 to 3 K. In this temperature range one component of the resonance signal undergoes a shift from 336 mT (g = 2) toward magnetic fields near to zero, indicating the existence of low-dimensional ferromagnetic interactions. The presence of that ferromagnetic component was attributed to the frustration of one third of the total Fe(III) spins in the compound. Below 3 K, the effect of the long-range interactions causes an antiparallel arrangement of the magnetic moments inducing another magnetic transition to an antiferromagnetic phase.