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

✍ Scribed by M. Tovar; R. Torabi; C. Welker; F. Fleischer

• Publisher: Elsevier Science
• Year: 2006
• Tongue: English
• Weight: 489 KB
• Volume: 385-386
• Category: Article
• ISSN: 0921-4526
• DOI: 10.1016/j.physb.2006.05.181

✦ Synopsis

The compounds CuCr 2 O 4 and NiCr 2 O 4 crystallize at room temperature in a tetragonal distorted spinel structure, s.g. I4 1 /amd, with axes ratio c/ao1 and 41, respectively. The distortion is caused by the Jahn-Teller ions Cu 2+ and Ni 2+ which flatten or elongate their surrounding oxygen tetrahedron. CuCr 2 O 4 and NiCr 2 O 4 form a complete solid solution series Cu 1Àx Ni x Cr 2 O 4 where for 0.825oxo0.875 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 (CuCr 2 O 4 ) and 310 K (NiCr 2 O 4 ). The phase Cu 0.15 Ni 0.85 Cr 2 O 4 undergoes a second crystallographic transition to orthorhombic symmetry, space group Fddd, at T ¼ 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 twostep mechanism.