At room temperature, the normal oxide spinels NiCr 2O 4 and CuCr 2O 4 are tetragonally distorted and crystallize in the I4 1/amd space group due to cooperative Jahn-Teller ordering driven by the orbital degeneracy of tetrahedral Ni2 + (t24) and Cu2 + (t25). Upon cooling, these compounds undergo magnetic ordering transitions; interactions are somewhat frustrated for NiCr 2O 4 but not for CuCr 2O 4. We employ variable-temperature high-resolution synchrotron x-ray powder diffraction to establish that at the magnetic ordering temperatures there are further structural changes, which result in both compounds distorting to an orthorhombic structure consistent with the Fddd space group. NiCr 2O 4 exhibits additional distortion, likely within the same space group, at a yet-lower transition temperature of T=30 K. The tetragonal to orthorhombic structural transition in these compounds appears to primarily involve changes in NiO 4 and CuO 4 tetrahedra.
|Physical Review B - Condensed Matter and Materials Physics
|Published - Aug 6 2012
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics