Structural and magnetic properties of Cu-Ni-Cr spinel oxides

The compounds CuCr2O4 and NiCr2O4 crystallize at room temperature in a tetragonal distorted spinel structure, s.g. I41/amd, with axes ratio c/a<1 and >1, respectively. The distortion is caused by the Jahn-Teller ions Cu2+ and Ni2+ which flatten or elongate their surrounding oxygen tetrahedron. CuCr2O4 and NiCr2O4 form a complete solid solution series Cu1-x Ni x Cr2O4 where for 0.825<x<0.875 members with orthorhombic symmetry were found. Using neutron powder diffraction and thermal analysis methods several members of the solid solution series were investigated. On cooling, all samples showed a temperature-dependent crystallographic phase transition from cubic to tetragonal symmetry between 865 K (CuCr2O4) and 310 K (NiCr2O4). The phase Cu0.15Ni0.85Cr2O4 undergoes a second crystallographic transition to orthorhombic symmetry, space group Fddd, at T=300 K. The neutron diffraction experiments as well as SQUID measurements reveal magnetic ordering of the ions between 150 and 50 K which partially occurs as a two-step mechanism