The Jahn-Teller-Effect of Cu2+ on the Spinel Structure at High Pressure

Abstract:

The Jahn-Teller-effect of Cu²⁺ on the spinel structure (cuprospinel CuFe₂O₄) was investigated using the single-crystal synchrotron X-ray diffraction (XRD) technique at the beamline BL10A of PF KEK, Japan. The XRD data were collected in the pressure range from 0.0 to 5.9 GPa and the structural refinements based on single-crystal diffraction data were performed at 0.0, 1.8, 2.7, and 4.6 GPa. The unit-cell parameters were determined from the least-squares refinement of d-values of the observed 20 reflections. The a unit-cell parameter decreases continuously from 8.3908 (5) Å to 8.311 (7) ų up to 5.9 GPa. At 4.6 GPa, however, the crystal structure started to distort and the a axis of the cubic phase splits to a smaller a axis (a_x = 8.3367 (6) Å ) and a longer c axis (a_z = 8.340 (1) Å). This change indicates a phase transition from a cubic (space group Fd-3m, Z = 8) to a tetragonal structure (space group I4₁/amd, Z = 4). The crystal structure after the phase transition has a body-centered tetragonal lattice with lattice constants a = 5.8949 (6) Å and c = 8.340 (1) Å. The crystal structure at 4.6 GPa was refined with a full-matrix least-squares refinement to R1 = 0.0148, wR2 = 0.0117 using observed 35 X-ray reflections. The tetrahedral (T) site is filled with 52.6% Cu²⁺ and 47.4% Fe³⁺ cations, whereas, the octahedral (M) site is with 3.7% Cu²⁺ and 96.3% Fe³⁺ cations. At the tetrahedral site, the tetrahedral O-T-O bond angles along the c-axis direction of the unit cell decreases slightly from 109.5° to 108.8 (1)°, which generates a 'stretched' tetrahedral geometry. The cubic-to-tetragonal transition is caused by the Jahn-Teller effect of Cu²⁺ cation giving rise to the angular distortion at the tetrahedral environment. In the octahedral site, on the other hand, the two M-O bonds parallel to the c-axis are more shortened than the four M-O bonds parallel to the the ab-plane. Consequently, the octahedral coordination changes a 'squeezed' octahedral geometry along the c-axis. With pressure, the anisotropic deformation of both the tetrahedral and octahedral coordinations leads to tetragonal compression of the unit cell (the c/a ratio < 1).