Pressure-Induced Structure Change With Electron-Lattice Interaction in Oxide Minerals Bearing Transition Elements
Abstract
Many minerals bearing transition elements in the crust and mantle undergo structural changes with electron- lattice interactions under high pressures. Crystal structures together with physical properties of earth materials under non-ambient conditions are an important step to understand geophysical phenomena. Miniature diamond-anvil cell (DAC) and synchrotron radiation facilities enable us to execute the single-crystal diffraction studies and to elucidate electron deformation density distribution. Structure changes under compression, such as lattice distortion, decomposition and phase transitions by the lattice instability due to electronic state change can be elucidated by Maximum Entropy Method (MEM) using X-ray single-crystal diffraction intensities. For the structural studies at high pressures, the conventional difference Fourier synthesis cannot provide accurate electron density distribution because of the termination effect of the Fourier series, because of the limited diffraction angle of DAC. Electron density distribution analysis by MEM overcomes the difficulty and presents much more reliable electron distribution. Fe3-xTixO4, continuous solid solutions have a spinel structure (Fd3m). Using synchrotron radiation we performed high-pressure single-crystal structure analyses up to 15GPa and powder diffraction experiments up to 60GPa. The cubic-to-tetragonal (I41/amd) transition pressure decreases with increasing Ti content. MEM map clearly show the transition was induced from the tetragonal distortion due to the Jahn-Teller effect of Fe2+ at the tetrahedral site. Spinels with less than x=0.6 do not transform to tetragonal but directly to orthorhombic CaTi2O4-type (Bbmm) structure. Structure refinements of Fe2TiO4 were also executed at low-temperatures down to -170C at ambient pressure. The transition to the low-temperature tetragonal phase takes place at - 110C. The c/a ratio is 0.9982(4) at 11.43GPa and 0.9967(5) at 170C. MEM and difference Fourier maps of Fe2TiO4 reveal the degeneracy of eg orbit group at the tetrahedral site for IVFe2 under high-pressure or low- temperature condition prefers electronic state to dx2-y2, instead of dz2 inducing the tetragonal distortion with c/a<1. Another high-pressure phase of orthorhombic structure of Fe2TiO4 was found at 30GPa and the structure was determined to be CaTi2O4-type structure. In addition, we found a further higher-pressure polymorph for specimens with x=0.8 and 1.0 at 59GPa and 45GPa, respectively.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMMR24A..01Y
- Keywords:
-
- 1042 Mineral and crystal chemistry (3620);
- 3620 Mineral and crystal chemistry (1042);
- 3924 High-pressure behavior;
- 3954 X-ray;
- neutron;
- and electron spectroscopy and diffraction