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 ? = 4.724 Å. 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, ?FI = 4.19 ± 0.02 ?B 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 vec kAF ? (1/16,0,1/16) and a magnetic moment for the Fe3+ ions of ?AF = 4.41 ± 0.03 ?B 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.

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