HR: 0800h
AN: MR11A-0897    [Abstracts]
TI: From Technological Materials to Mantle Minerals: Perovskites at High Pressures and Temperatures
AU: * Yu, R
EM: rcyu@berkeley.edu
AF: Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720-4767 United States
AU: * Yu, R
EM: rcyu@berkeley.edu
AF: Institute of Physics, Chinese Academy of Sciences, P.O.Box, 603, Beijing, 100080 China
AU: Zhang, W
EM: zhangw@aphy.iphy.ac.cn
AF: Institute of Physics, Chinese Academy of Sciences, P.O.Box, 603, Beijing, 100080 China
AU: Lee, V E
EM: vlee@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720-4767 United States
AU: Grocholski, B
EM: brent@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720-4767 United States
AU: McWilliams, R S
EM: rmcw@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720-4767 United States
AU: Montgomery, W
EM: wmg@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720-4767 United States
AU: Speziale, S
EM: speziale@uclink.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720-4767 United States
AU: Meng, Y
EM: ymeng@hpcat.aps.anl.gov
AF: HPCAT, Advanced Photon Source, Argonne National Laboratory, Bld. 434E, Argonne, IL 60439 United States
AU: Jin, C
EM: cqjin@aphy.iphy.ac.cn
AF: Institute of Physics, Chinese Academy of Sciences, P.O.Box, 603, Beijing, 100080 China
AU: Jeanloz, R
EM: jeanloz@berkeley.edu
AF: Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720-4767 United States
AB: Strongly correlated electron states, intermediate between the freely mobile electrons of simple metals and the fully pinned (localized) electrons of insulators, are poorly understood yet define key properties of many oxides, from crystal structure and cation valence to magnetic ordering and electrical conductivity. Pressure can be especially effective in characterizing strongly-correlated systems by tuning the interplay between crystal structure (interatomic distances and coordinations) and electronic degrees of freedom (density of state, mobility, magnetic superexchange). In particular, we show that pressure - counter-intuitively - suppresses the coupling of electronic states to lattice vibrations and structural distortions (Jahn-Teller effect) in an important class of oxides. We have studied orthorhombic perovskite-structured ({\it Pbnm}) LaMnO$_{3}$ at high pressures and temperatures, using a laser-heated diamond anvil cell with synchrotron x-ray diffraction at the Advanced Photon Source HP-CAT beamline. This material is a structural analog of mantle perovskites, and is closely related to compounds exhibiting colossal magnetoresistance (CMR). Alternating long and short Mn-O distances in the ab plane of the LaMnO$_{3}$ crystal structure are due to a cooperative Jahn-Teller (JT) distortion removing the degeneracy of the eg orbitals in the {\it t$^{3}$$_{2g}$e$^{1}$$_{g}$} electron configuration of the Mn$^{3+}$ ions, and stabilizing the {\it 3d(3z$^{2}$-r$^{2}$)} type with respect to the {\it 3d(x$^{2}$-y$^{2}$)} type orbitals. The insulating character of LaMnO$_{3}$ has been attributed to either i) the strong Jahn-Teller electron-phonon coupling localizing the {\it e$_{g}$} electrons as polarons, or ii) electron-electron interactions resulting in a Mott-insulator. The latter is exemplified by Fe$_{2}$O$_{3}$ hematite, which was previously thought to take on the perovskite structure but now is instead known to undergo a Mott transition at high pressure. Compression to 11 GPa at room temperature decreases the degree of crystal-structural distortion of LaMnO$_{3}$, implying a suppression of the Jahn-Teller effect in accord with previous observations [Loa, et al., 2001]. Laser-heating to $\sim$2000 K at 11 GPa causes the appearance of two new diffraction peaks that are not due to the Pbnm structure, and further suppresses Jahn-Teller distortion. Both temperature and pressure thus appear to suppress the electronically-induced crystal-structural distortion (Jahn-Teller effect), and LaMnO$_{3}$ appears to be a Mott insulator like Fe$_{2}$O$_{3}$ at low pressure.
DE: 3900 MINERAL PHYSICS
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting