Synthesis and magnetic properties of a synthetic analog of yakubovichite, CaNi₂Fe(PO₄)₃

The crystal structure of CaNi₂Fe(PO₄)₃ belongs to the well-known α-CrPO₄ structural type. Phosphates with this structural architecture are considered promising candidates for alkaline-ion batteries. Within the crystal structure of CaNi₂Fe(PO₄)₃, iron and nickel atoms occupy independent octahedral sites. Nickel-centered octahedra share edges to form Ni₂O₇ dimers, which, in turn, link via vertices with FeO₆ octahedra to construct a three-dimensional framework. The presence of two different transition metal cations may give rise to rare magnetic states. Although a synthetic analogue of CaNi₂Fe(PO₄)₃ was previously obtained via a sol–gel method by Ouaatta S. (2017), its magnetic behavior remained uninvestigated.

In this work, CaNi₂Fe(PO₄)₃ was synthesized using a solid-state reaction method. A stoichiometric mixture of high-purity FeC₁₂H₂₂O₁₄·2H₂O, CaC₁₂H₂₂O₁₄, NiO, and (NH₄)₂HPO₄ was preliminarily annealed at 873 K to remove organic components, followed by a two-stage annealing process at 1273 K with intermediate grinding. Phase purity and chemical composition were checked by X-ray powder diffraction (XRD) and Energy-Dispersive X-ray spectroscopy (EDX). Magnetic susceptibility measurements indicate that CaNi₂Fe(PO₄)₃ undergoes a transition to a magnetically long-range-ordered state at T_c = 15.4 K with residual magnetization. This is confirmed by the presence of a hysteresis loop measured at 2 K. Analysis of the temperature dependence of the magnetic susceptibility suggests the dominance of antiferromagnetic interactions in CaNi₂Fe(PO₄)₃. The weak ferromagnetism of CaNi₂Fe(PO₄)₃ motivates further study of the magnetic exchange interactions to elucidate the fundamental mechanism and applicative potential of the title compound.