Competing Magnetic Superexchange Interactions in Layered Vanadyl Phosphate Hydrates

We present a comprehensive investigation of the layered vanadyl phosphate hydrates Ni(VO)₂(PO₄)₂·4H₂O (NiVP) and Co(VO)₂(PO₄)₂·4H₂O (CoVP), combining anisotropic magnetic susceptibility, heat capacity measurements, Density Functional Theory (DFT) calculations, and X-ray charge density analysis. Magnetization measurements reveal a ferromagnetic response along the crystallographic <i>c</i>-axis and an antiferromagnetic response within the <i>ab</i>-plane.

Both compounds exhibit the presence of magnetocrystalline anisotropy, consistent with an easy-plane anisotropic magnetic state in NiVP, while in CoVP, strong spin–orbit coupling leads to a more complex behaviour that prevents a straightforward determination of the moment orientation. Prior to the onset of three-dimensional magnetic ordering at <i>T</i>_<i>N</i> ≈ 4 K, both systems develop significant short-range spin correlations, as evidenced by deviations from the Curie–Weiss behaviour and a reduced λ-type anomaly in heat capacity, indicating a gradual crossover from low-dimensional correlations to three-dimensional ordering.