High-pressure behavior of cuprospinel CuFe2O4: Influence of the Jahn-Teller effect on the spinel structure
- Univ. of Tsukuba, Ibaraki (Japan)
- Carnegie Inst. of Washington, Washington, D.C. (United States)
- Osaka Univ., Osaka (Japan)
The Jahn-Teller-effect at Cu2+ in cuprospinel CuFe2O4 was investigated using high-pressure, single crystal synchrotron x-ray diffraction (XRD) techniques at beamline BL10A at the Photon Factory, KEK, Japan. Six data sets were collected in the pressure range from ambient to 5.9 GPa at room temperature. Structural refinements based on the data were performed at 0.0, 1.8, 2.7, and 4.6 GPa. The unit cell volume of cuprospinel decreases continuously from 590.8 (6) Å3 to 579.5 (8) Å3 up to 3.8 GPa. Leastsquares fitting to a third-order Birch-Murnaghan equation of state yields zero-pressure volume V0 = 590.7 (1) Å3 and bulk modulus K0 = 188.1 (4.4) GPa with K’ fixed at 4.0. The crystal chemical composition determined by electron-probe analysis and x-ray site-occupancy refinement is represented as [Cu0.526Fe0.474][6][Cu0.074Fe1.926]O4. Most of the Cu2+ are preferentially distributed onto the tetrahedral (T) site of the spinel structure. At 4.6 GPa, a cubic-tetragonal phase transition is indicated by a splitting of the a axis of the cubic structure into a smaller a axis and a longer c axis, with unit cell parameters a = 5.882 (1) Å and c = 8.337 (1) Å. The tetragonal crystal structure with space group I41/amd was refined to R1 = 0.0182 and wR2 = 0.0134 using observed 35 x-ray reflections. At the T site, the tetrahedral O-T-O bond angles along the c-axis direction of the unit cell decreases slightly from 109.47 ° to 108.7 (4) °, which generates a stretched tetrahedral geometry along the c-axis. The cubic-totetragonal transition induced by the Jahn-Teller effect at Cu2+ is attributable to the angular distortion at the tetrahedral site. At the octahedral (M) site, on the other hand, the two M-O bonds parallel to the caxis are shortened with respect to the four M-O bonds parallel to the ab-plane, which are lengthened as a result of the phase transition, leading to a compressed octahedral geometry along the c-axis. With the competing distortions between the stretched tetrahedron and the compressed octahedron along the c-axis, the a unit cell parameter is shortened with respect to the c unit cell parameter, giving a c/a ratio slightly greater than unity as referred to cubic lattice (c/a = 1.002). The c/a value increases to 1.007 with pressure, suggesting a further evolution of the stretched tetrahedron and the compressed octahedron. The variation of c/a ratio of the cuprospinel is similar to that observed in the tetragonally distorted cuprospinel with Cu2+ fully occupying the octahedral site of the structure.
- Research Organization:
- Carnegie Inst. of Science, Argonne, IL (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA)
- Grant/Contract Number:
- NA0002006
- OSTI ID:
- 1335652
- Journal Information:
- American Mineralogist, Vol. 100, Issue 8-9; ISSN 0003-004X
- Publisher:
- Mineralogical Society of AmericaCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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