Decoupled structural and non-collinear magnetic phase transitions in Fe(ND_3)_2PO_4

摘要

Interest in Fe(ND_3)_2PO_4, one of the first three-dimensional structurally linked solids forming Fe-ND_3 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(ND_3)_2PO_4 crystallizes at room temperature in an orthorhombic (Pnma) crystal structure, but that below T_t = 226(5) K the crystal progressively adopts a monoclinic {P2_1n) 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 — tau, 0,tau), with tau ~ 0.04 reciprocal lattice units (rlus), and that the magnitude of the Fe(III) magnetic moments is close to mu Fe = 4.5 mu B 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.