The cooperative Jahn-Teller effect and anti-isostructural phases in Ni1−xCoxCr2O4 solid solutions: Synchrotron X-ray diffraction study

A low-temperature high-resolution powder diffraction study has been carried out on a system of Ni1−xCoxCr2O4 solid solutions with x≤0.1 down to 5 K. The tetragonal phase (I41/amd) with c/a<1 was experimentally observed in Ni0.9Co0.1Cr2O4 below ∼64 K. The new phase is anti-isostructural to the well-known tetragonal phase of NiCr2O4. A unified phenomenological model has been applied to structural phase transitions in Ni1−xCoxCr2O4 solid solutions. It shows that the appearing of such a phase is justified from a structural point of view. Analysis of atomic displacements confirms predictions of the symmetry-based model-free consideration.