Neutron powder diffraction investigation in ammonium iron(III) bis (hydrogenphosphate)

Abstract

The deuterated form of ammonium iron(III) bis (hydrogenphosphate), ND₄Fe(DPO₄)², was investigated in detail from neutron powder diffraction data with a wavelength λ = 4.724 ˚A. The material undergoes two successive magnetic phase transitions which are associated with the Fe³+ magnetic moments. One at TC = 17.82 ± 0.05 K is attributed to the ferrimagnetic order with the magnetic moments, µF I = 4.19 ± 0.02 µB at 4 K, lying on the crystallographic plane ac. The other transition is found to be at Tt = 3.52 ± 0.05 K due to an antiferromagnetic arrangement, with an equal moment antiphase structure that is characterized by a long–period propagation vector close to →kAF ≈ (1/16, 0, 1/16) and a magnetic moment for the Fe³+ ions of µAF = 4.41 ± 0.03 µB at 1.5 K. The low symmetry of its triclinic crystal structure and the complex pattern of competing superexchange pathways seem to be responsible for the existence of this double magnetic phase transition.