Abstract: Combining neutron diffraction, magnetization measurements up to 330 kOe and specific-heat data, we have studied in detail both the crystal and magnetic structures of triclinic ND4Fe(DPO4)2 and NH4Fe(HPO4)2 compounds. The low symmetry of this structure gives rise to a complex pattern of competing superexchange interactions between the magnetic moments of two types of Fe3+ sites (with different site symmetry) that are responsible for the existence of two magnetic phase transitions. Below TC=17.82±0.05?K ND4Fe(DPO4)2 orders ferrimagnetically with the magnetic moments lying in the crystallographic plane ac. As the temperature is lowered to Tt=3.52±0.05?K the compound undergoes a magnetic phase transition to an equal moment antiphase structure characterized by the propagation vector close to ?kAF?(1/16,0,1/16) and a magnetic moment for the Fe3+ ions of 4.8??B at 1.89 K. In addition, a two-step metamagnetic process is observed in the magnetization measurements at 2 K, where the antiphase ordering is destroyed under a field of only 2 kOe and the compound recovers the high-temperature ferrimagnetic ordering at around 20 kOe. The stability of this ferrimagnetic phase under magnetic field is only broken when the strength of the field reaches values as large as 180 kOe, and the magnetic moments begin to rotate to reach the full-induced ferromagnetic structure. A mean-field model has been used to account for the magnetization process leading to an estimation of the molecular-field coefficient of ?2.86?K and the value of the critical magnetic field of 535 kOe to attain the full-induced ferromagnetic phase.

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