Photon Factory Activity Report 2014 #32 (2015) B

BL-10A/2013G046

High-pressure behavior of cuprospinel CuFe2O4:
Influence of the Jahn-Teller effect on the spinel structure
Atsushi Kyono1,* Yuki Nakamoto2, Masafumi Sakata2 and Masato Kato1
1
University of Tsukuba, Tsukuba 305-8572, Japan2
Osaka University, Osaka 560-8531, Japan
1 Introduction
Understanding the crystal structures of spinels under
non-ambient conditions is an important step in the effort
to correlate geophysical phenomena. Upon compression,
the Jahn-Teller effect becomes active and yields atetrahedral angular distortion, which induces the cubic-totetragonal transition of spinels. [1, 2] Here, we report ahigh-pressure single-crystal synchrotron X-ray diffraction
study of synthetic cuprospinel, which exhibits a strong
Jahn-Teller effect at octahedrally coordinated Cu2+ in thespinel structure. In the present study, the structuralfeatures of pressure-induced Jahn-Teller distortion in
cuprospinel are described.
2 Experiment
Pure cuprospinel powder CuFe2O4 was synthesizedfrom a mixture of Fe2O3 and Cu, which was first heated in
an open quartz tube with a flowing oxygen gas stream at
1000oC for 24 h. The powder sample was then taken outof the furnace and sealed under vacuum in a quartz vessel.
It was subsequently placed back into the furnace andheated again at 1300oC for 36 h, resulting in euhedral
single crystals with size up to 300 m.
The high-pressure single-crystal synchrotron X-ray
diffraction study was carried out using a high-resolution,
vertical-type four-circle diffractometer and scintillation
counter installed at BL-10A at the Photon Factory, HighEnergy Accelerator Research Organization (KEK),Tsukuba, Japan. Unit cell parameters of the sample weredetermined by a least-squares refinement of the setting
angles of 15–25 centered reflections with 20° < 2θ < 30°.
A total of 7 high-pressure data collections (P = 0.0, 1.8,2.7, 3.8, 4.6, 5.9, and 6.8 GPa) were carried out.
3 Results and Discussion
The unit cell volume of cuprospinel decreases
continuously from 590.8(6) Å3 to 579.5(8) Å3 up to 3.8
GPa. Least-squares fitting to a third-order BirchMurnaghan equation of state yields the zero-pressurevolume V0 = 590.7(1) Å3 and bulk modulus K0 =188.1(4.4) GPa with K’ fixed at 4.0. The structural
formula determined by electron microprobe analysis andsite occupancy refinement is represented asT(Fe3+0.90Cu2+0.10)M(Fe3+1.10Fe2+0.40Cu2+0.50)O4. Most of the
Cu2+ are preferentially distributed onto the octahedrallycoordinated (M) site of the spinel structure. With pressure,the arrangement of the oxygen atoms around the M cation
approaches a regular octahedron. This leads to an increasein the electrostatic repulsion between the coordinatingoxygen ions and the 3dz2 orbital of MCu2+. At 4.6 GPa, acubic-tetragonal phase transition is indicated by asplitting of the a axis of the cubic structure into a smallera axis and a longer c axis, with unit cell parameters a =5.882(1) Å and c = 8.337(1) Å. The tetragonal structurewith space group I41/amd was refined to R1 = 0.0332 andwR2 = 0.0703 using 38 observed reflections. At the M site,
the two M-O bonds parallel to the c-axis direction of theunit cell are stretched with respect to the four M-O bonds
parallel to the ab-plane, which leads to an elongatedoctahedron along the c-axis. The cubic-to-tetragonaltransition induced by the Jahn-Teller effect at Cu2+ is
attributable to this distortion of the CuO6 octahedron andinvolves Cu 3dz2 orbital, ab initio quantum chemical
calculations support the observation (Fig. 1). At thetetrahedrally coordinated (T) site, on the other hand, thetetrahedral O-T-O bond angle increases from 109.47 ° to111.7(7) °, which generates a compressed tetrahedral
geometry along the c-axis. As a result of the competing
distortions between the elongated octahedron and thecompressed tetrahedron, the a unit cell parameter isshortened with respect to the c unit cell parameter, giving
a c/a’ ratio (a’ = 2 a) slightly greater than unity as
referred to cubic lattice (c/a’ = 1.002). The c/a’ value
increases to 1.007 with pressure, suggesting further
distortions of the elongated octahedron and compressed
tetrahedron.

Fig. 1: Electronic orbitals in the tetragonally distorted
cuprospinel at 4.6 GPa, calculated by ab initio method.
References[1] Kyono et al. Am. Mineral., 96, 1193, (2011).
[2] Kyono et al. Phy. Chem. Mineral., 39, 131, (2012).
* kyono@geol.tsukuba.ac.jp