Decoupled structural and non-collinear magnetic phase transitions in Fe(ND3)2PO4

Interest in Fe(ND3)2PO4, one of the first three-dimensional structurally linked solids forming Fe–ND3 bonds, has been renewed recently due to the observation of structural and magnetic phase transitions. This paper reports powder neutron diffraction experiments which show that Fe(ND3)2PO4 crystallizes at room temperature in an orthorhombic (Pnma) crystal structure, but that below Tt = 226(5) K the crystal progressively adopts a monoclinic (P21/n) structure via a continuous phase transition, as observed from heat capacity measurements in the temperature range 250–100 K. In addition, neutron diffraction experiments suggest that the magnetic structure is non-collinear with an incommensurate propagation vector k→inc=(1/2-τ,0,τ), with τ ∼ 0.04 reciprocal lattice units (rlus), and that the magnitude of the Fe(III) magnetic moments is close to μFe = 4.5 μB at T = 2 K, describing a helimagnetic arrangement. To the best of our knowledge, this is an unprecedented magnetic ordering in an iron phosphate lattice. We argue that the proposed magnetic ordering can be explained by means of a model of superexchange interactions involving Fe–O–P–O–Fe paths.