Magnetic susceptibility, specific heat and magnetic structure of CuNi2(PO4)2

The CuNi₂(PO₄)₂ phosphate has been synthesized by the ceramic method at 800 °C in air. The crystal structure consists of a three-dimensional skeleton constructed from MO₄ (M^IICu and Ni) planar squares and M₂O₈ dimers with square pyramidal geometry, which are interconnected by (PO₄)³⁻ oxoanions with tetrahedral geometry. The magnetic behavior has been studied on powdered sample by using susceptibility, specific heat and neutron diffraction data. The bimetallic copper(II)-nickel(II) orthophosphate exhibits a three-dimensional magnetic ordering at, approximately, 29.8 K. However, its complex crystal structure hampers any parametrization of the J-exchange parameter. The specific heat measurements exhibit a three-dimensional magnetic ordering (λ-type) peak at 29.5 K. The magnetic structure of this phosphate shows ferromagnetic interactions inside the Ni₂O₈ dimers, whereas the sublattice of Cu(II) ions presents antiferromagnetic couplings along the y-axis. The change of the sign in the magnetic unit-cell, due to the 1/2, 0, 1/2] propagation vector determines a purely antiferromagnetic structure.