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

The open-framework compounds Fe(NH3)(2)PO4 and Fe(NH4)(HPO4)(2) were synthe sized by hydrothermal, reactions in the H3PO4-(NH2)(2)CO-FeCl3-H2O System. The electron paramagnetic resonance spectra for Fe(NH3)(2)PO4 show an isotr opic signal with g=2, characteristic of high spin Fe(III) ions in an octahe dral environment. Below 22 K the Fe(NH3)(2)PO4 Compound exhibits a long-ran ge antiferromagnetic ordering in which the magnetic interactions involve Fe -O-P-O-Fe superexchange pathways. Fe(NH4)(HPO4)(2) exhibits an antiferromag netic behaviour with the presence of a significant ferromagnetic component from 18 to 3 K. In this temperature range one component of the resonance si gnal undergoes a shift from 336 mT (g = 2) toward magnetic fields near to z ero, indicating the existence of low-dimensional ferromagnetic interactions . The presence of that ferromagnetic component was attributed to,the frustr ation of one third of the total Fe(III) spins in the compound. Below 3 K, t he effect of the long-range interactions causes an antiparallel arrangement of the magnetic moments inducing another magnetic transition to an antifer romagnetic phase.