ALBERT

Description: A structural change in Fe 2 SiO 4 spinel (ringwoodite) has been found by synchrotron powder diffraction study and the structure of a new high-pressure phase was determined by Monte-Carlo simulation method and Rietveld profile fitting of X-ray diffraction data up to 64 GPa at ambient temperature. A transition from the cubic spinel structure to a body centered orthorhombic phase ( I -Fe 2 SiO 4 ) with space group Imma and Z = 4 was observed at approximately 34 GPa. The structure of I -Fe 2 SiO 4 has two crystallographically independent FeO 6 octahedra. Iron resides in two different sites of sixfold coordination: Fe1 and Fe2, which are arranged in layers parallel to (101) and (011) and are very similar to the layers of FeO 6 octahedra in the spinel structure. Silicon is located in the sixfold coordination in I -Fe 2 SiO 4 . The transformation to the new high-pressure phase is reversible under decompression at ambient temperature. A martensitic transformation of each slab of the spinel structure with translation vector $$ 〈 \overrightarrow{1/8}\hspace{0.17em}\overrightarrow{1/8}\hspace{0.17em}\overrightarrow{1/8} 〉 $$ generates the I -Fe 2 SiO 4 structure. Laser heating of I -Fe 2 SiO 4 at 1500 K results in a decomposition of the material to rhombohedral FeO and SiO 2 stishovite. Fe K β X-ray emission measurements at high pressure up to 65 GPa show that the transition from a high spin (HS) to an intermediate spin (IS) state begins at 17 GPa in the spinel phase. The IS electron spin state is gradually enhanced with pressure. The Fe 2+ ion at the octahedral site changes the ion radius under compression at the low spin, which results in the changes of the lattice parameter and the deformation of the octahedra of the spinel structure. The compression curve of the lattice parameter of the spinel is discontinuous at ~20 GPa. The spin transition induces an isostructural change.

Description: The Jahn-Teller effect at Cu 2+ in cuprospinel CuFe 2 O 4 was investigated using high-pressure single-crystal synchrotron X-ray diffraction 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) to 579.5(8) Å 3 up to 3.8 GPa. Least-squares fitting to a third-order Birch-Murnaghan equation of state yields the zero-pressure volume V 0 = 590.7(1) Å 3 and bulk modulus K 0 = 188.1(4.4) GPa with K ' fixed at 4.0. The structural formula determined by electron microprobe analysis and site occupancy refinement is represented as T (Fe 3+ 0.90 Cu 2+ 0.10 ) M (Fe 3+ 1.10 Fe 2+ 0.40 Cu 2+ 0.50 )O 4 . Most of the Cu 2+ are preferentially distributed onto the octahedrally coordinated ( 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 increase in the electrostatic repulsion between the coordinating oxygen ions and the 3 d z 2 orbital of M Cu 2+ . 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 structure with space group I 4 1 / amd was refined to R 1 = 0.0332 and wR 2 = 0.0703 using 38 observed reflections. At the M site, the two M -O bonds parallel to the c -axis direction of the unit cell are stretched with respect to the four M -O bonds parallel to the a-b plane, which leads to an elongated octahedron along the c -axis. The cubic-to-tetragonal transition induced by the Jahn-Teller effect at Cu 2+ is attributable to this distortion of the CuO 6 octahedron and involves Cu 3 d z 2 orbital, ab initio quantum chemical calculations support the observation. At the tetrahedrally coordinated ( T ) site, on the other hand, the tetrahedral O- T -O bond angle increases from 109.47° to 111.7(7)°, which generates a compressed tetrahedral geometry along the c -axis. As a result of the competing distortions between the elongated octahedron and the compressed tetrahedron, the a unit-cell parameter is shortened with respect to the c unit-cell parameter, giving a c / a ' ratio ( $${a}^{\prime }=\sqrt{2}\hspace{0.17em}a$$ ) 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.