Abstract: The NH4Fe(HPO4)2 and its deuterated form, ND4Fe(DPO4)2, were investigated in detail from powder neutron diffraction data allowing a precise determination of the hydrogen positions and low temperature magnetic structures. Below TC = 17.82 K they order with a ferrimagnetic structure and the magnetic moments lying in the crystallographic plane ac. As the temperature is lowered to Tt = 3.52 K the system undergoes a magnetic phase transition to an equal moment antiphase structure characterized by the propagation vector close to vec kAF ? (1/16,0,1/16) and a magnetic moment for the Fe3+ ions of 4.8 ?B at 1.89 K. The low symmetry of this crystal structure and the complex pattern of competing superexchange pathways are responsible for the existence of this double magnetic phase transition.

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