Revision 4 1 2 3 High-pressure behavior of cuprospinel CuFe2O4: 4 Influence of the Jahn-Teller effect on the spinel structure 5 6 Atsushi Kyono1,*, Stephen A. Gramsch2, Yuki Nakamoto3, Masafumi Sakata3, 7 Masato Kato1, Tomoya Tamura1, and Takamitsu Yamanaka2 8 9 1Division of Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, 10 University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8572, Japan 11 2Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., 12 Washington DC, 20015-1305, USA 13 3Center for Science and Technology under Extreme Conditions, Osaka University, 14 Osaka 560-8531, Japan 15 16 17 18 19 Correspondence author: A. Kyono 20 Correspondence e-mail: [email protected] 21 Phone: +81-29-853-7176, Fax: +81-29-853-7887 22 pg. 1 Revision 4 23 ABSTRACT 2+ 24 The Jahn-Teller effect at Cu in cuprospinel CuFe2O4 was investigated using high-pressure 25 single-crystal synchrotron X-ray diffraction techniques at beamline BL10A at the Photon Factory, 26 KEK, Japan. Six data sets were collected in the pressure range from ambient to 5.9 GPa at room 27 temperature. Structural refinements based on the data were performed at 0.0, 1.8, 2.7, and 4.6 GPa. 28 The unit cell volume of cuprospinel decreases continuously from 590.8(6) Å3 to 579.5(8) Å3 up to 3.8 29 GPa. Least-squares fitting to a third-order Birch-Murnaghan equation of state yields the zero-pressure 3 30 volume V0 = 590.7(1) Å and bulk modulus K0 = 188.1(4.4) GPa with K’ fixed at 4.0. The structural 31 formula determined by electron microprobe analysis and site occupancy refinement is represented as T 3+ 2+ M 3+ 2+ 2+ 2+ 32 (Fe 0.90Cu 0.10) (Fe 1.10Fe 0.40Cu 0.50)O4. Most of the Cu are preferentially distributed onto the 33 octahedrally coordinated (M) site of the spinel structure. With pressure, the arrangement of the oxygen 34 atoms around the M cation approaches a regular octahedron. This leads to an increase in the 2 M 2+ 35 electrostatic repulsion between the coordinating oxygen ions and the 3dz orbital of Cu . At 4.6 GPa, 36 a cubic-tetragonal phase transition is indicated by a splitting of the a axis of the cubic structure into a 37 smaller a axis and a longer c axis, with unit cell parameters a = 5.882(1) Å and c = 8.337(1) Å. The 38 tetragonal structure with space group I41/amd was refined to R1 = 0.0332 and wR2 = 0.0703 using 38 39 observed reflections. At the M site, the two M-O bonds parallel to the c-axis direction of the unit cell 40 are stretched with respect to the four M-O bonds parallel to the ab-plane, which leads to an elongated 41 octahedron along the c-axis. The cubic-to-tetragonal transition induced by the Jahn-Teller effect at 2+ 2 42 Cu is attributable to this distortion of the CuO6 octahedron and involves Cu 3dz orbital, ab initio 43 quantum chemical calculations support the observation. At the tetrahedrally coordinated (T) site, on 44 the other hand, the tetrahedral O-T-O bond angle increases from 109.47 ° to 111.7(7) °, which 45 generates a compressed tetrahedral geometry along the c-axis. As a result of the competing distortions 46 between the elongated octahedron and the compressed tetrahedron, the a unit cell parameter is 47 shortened with respect to the c unit cell parameter, giving a c/a’ ratio (a’ = 2 a) slightly greater than 48 unity as referred to cubic lattice (c/a’ = 1.002). The c/a’ value increases to 1.007 with pressure, 49 suggesting further distortions of the elongated octahedron and compressed tetrahedron. pg. 2 Revision 4 50 51 Keywords: cuprospinel, CuFe2O4, high-pressure, single-crystal synchrotron x-ray diffraction, 52 Jahn-Teller effect 53 pg. 3 Revision 4 54 INTRODUCTION 55 The mineral spinel (MgAl2O4) adopts a slightly distorted, close-packed arrangement of oxygen 56 atoms with magnesium and aluminum atoms distributed among the two types of interstitial sites with 57 tetrahedral (T) and octahedral (M) coordination, respectively. In general, the spinel unit cell is 58 composed of 32 oxygen atoms arranged in a cubic close-packed array, which forms 64 T sites and 32 59 M sites. In the normal spinel, divalent cations (A2+) occupy one-eighth of the available T sites in the 60 close-packed array, while trivalent cations (B3+) occupy half of the available M sites, leading to the T 2+ M 3+ 61 formula (A ) (B )2O4, with eight formula units in the unit cell (Z = 8). In the inverse spinel, on the 62 other hand, half of the B3+ cations occupy the T site and the remaining A2+ and B3+ cations occupy the T 3+ M 2+ 3+ 63 M site, resulting in the formula (B ) (A B )2O4. The spinel structure is extremely flexible with 64 regard to the variety of cations it can incorporate into both sites, as well as their distribution in the 65 structure. At present, approximately 200 different oxides with the spinel structure have been already 66 reported in the JCPDS-ICDD database (ICDD 2013). In particular, transition metal cations may be 67 accomodated on both the T and M sites without any noticeable structural modification. In the unit cell 68 of the normal spinel (space group Fd 3 m), the T and M sites are restricted to the 8a (1/8, 1/8, 1/8) and 69 16d (1/2, 1/2, 1/2) Wyckoff positions, respectively, while the oxygen irons are located in the Wyckoff 70 position 32e at (u, u, u). Hence, the spinel structure is completely described by the cubic unit cell 71 parameter a and the oxygen positional parameter u only (e.g., Lavina et al. 2002). With u = 1/4, the 72 oxygen irons are on the perfect cubic closest packing positions, but u generally deviates slightly from 73 this ideal value. If displacement of the oxygen positions occurs, it is always along the [111] direction, 74 which is aligned on the nearest T site. Although the spinel structure is cubic, tetragonal spinelloid 75 phases can occur in some cases as a result of the Jahn-Teller effect imposed by the electronic states of 76 some transition metal cations. Figure 1 shows the relationship between electronic configurations and 77 the Jahn-Teller effect for transition metal cations in the T and M sites. If an orbital energy level is 78 occupied in an asymmetric manner, then an orbitally degenerate state exists for that coordination 79 environment. The Jahn-Teller theorem states that the coordination environment must distort so as to 80 lower the symmetry and remove the degeneracy (Dunitz and Orgel, 1957; Ballhausen 1962). Because pg. 4 Revision 4 81 the metal-ligand overlaps are strongest for the eg orbitals of an octahedral complex, the Jahn-Teller 82 effect is much more noticeable for 3d4 high spin (HS), 3d7 low spin (LS), and 3d9 electronic 83 configurations, as shown in Fig. 1 (e.g., [6]Mn3+(HS), [6]Cr2+(HS), [6]Co2+(LS), [6]Cu2+). Often, these 84 distortions are permanent and such distortions of an ideal coordination environment are generally 85 known as "static" Jahn-Teller distortions. Similar considerations apply for the 3d3(HS), 3d4(HS), 86 3d5(LS), 3d6(LS), 3d8, and 3d9 configurations in a tetrahedral environment (e.g., [4]Cr3+(HS), 87 [4]V2+(HS), [4]Mn3+(HS), [4]Cr2+(HS), [4]Fe3+(LS), [4]Mn2+(LS), [4]Co3+(LS), [4]Fe2+(LS), [4]Ni2+, [4]Cu2+). 88 The interacting orbitals of the cations in the T site do not overlap effectively with those of the oxygen 89 irons, so the interactions are relatively weaker than in the M site. When Jahn-Teller active cations such 90 as Ni2+, Cu2+, and Mn3+ occupy either the T or M site in the spinel structure, a tetragonal phase occurs 91 as a result of the Jahn-Teller distortion. Spinels with a strong Jahn-Teller distortion at the T site are the 92 tetragonally distorted nichromite NiCr2O4 (Crottaz et al. 1997; Ivanov et al. 2001; Suchomel et al. 93 2012; Bush et al. 2013), cupric chromite CuCr2O4 (Crottaz et al. 1997; Dollase and O’Neill 1997; 94 Kennedy and Zhou 2008; Shoemaker and Seshadri, 2010; Suchomel et al. 2012), and cupper 95 dirhodium tetraoxide CuRh2O4 (Dollase and O’Neill 1997; Ismunandar et al. 1999). On the other hand, 96 those with a strong Jahn-Teller distortion at the M site are tetragonally distorted cuprospinel CuFe2O4 97 (Prince and Treuting 1956; Gabal et al. 2011; Balagurov et al. 2013), hetaerolite ZnMn2O4 (Åsbrink et 98 al. 1999; Choi et al. 2006), hausmannite MnMn2O4 (Bosi et al. 2002, 2010), and iwakiite MnFe2O4 99 (Matsubara et al. 1979). All of the tetragonally distorted spinels resulting from a Jahn-Teller distortion 100 at either the T or M site have space group symmetry I41/amd, except for iwakiite (P42/nnm). These 101 tetragonal spinelloids are of particular interest relative to the temperature effects on spin-orbital 102 interactions as well as the influence of the Jahn-Teller effect on the crystal symmetry. 103 Understanding the crystal structures of earth materials under non-ambient conditions is an 104 important step in the effort to correlate geophysical phenomena. The general pressure range of the 105 cubic-tetragonal phase transitions in spinels suggests the possibility that such transformations may 106 occur in the Earth's mantle.