HR: 1340h
AN: MR23B-0071    [Abstracts]
TI: A High Pressure Post-Perovskite Phase Transition in NaMgF₃--a MgSiO₃ Analog Material
AU: * Martin, C
EM: chmartin@ic.sunysb.edu
AF: Geosciences Department, Stony Brook University, Stony Brook, NY 11794-2100 United States
AU: Liu, H
EM: hliu@hpcat.aps.anl.gov
AF: HPCAT, Bld. 434E Advanced Photon Source Argonne National Laboratory, Argonne, IL 60439-4803 United States
AU: Crichton, W
EM: crichton@esrf.fr
AF: European Synchrotron Radiation Facility, 6 rue Jules Horowitz BP220, GRENOBLE, 38043 France
AU: Parise, J B
EM: john.parise@sunysb.edu
AF: Geosciences Department, Stony Brook University, Stony Brook, NY 11794-2100 United States
AB: Since Murakami et al. (2004) identified a perovskite (pv, Pbnm) to post-perovskite (ppv, Cmcm) structural phase transition in MgSiO₃, the transition has been reported to occur in many oxides at ultra-high pressures (>60 GPa). The layered ppv structure is rapidly shaping a better understanding of seismic anisotropy in the controversial D" region of the lower mantle. While the ppv unit cell may be derived from indexing of the powder pattern, the structure adopted at high pressure is experimentally ill-constrained due to compromised powder diffraction statistics typically obtained from small sample volumes at extreme conditions in the diamond anvil cell. NaMgF₃, a structural analog material to MgSiO₃ pv, exhibits a large compressibility and presents the possibility of reducing the pv-ppv transition pressure, allowing for improved powder statistics from a larger sample volume. In accordance with our previous theoretical and experimental evidence (Liu et al., 2005; Parise et al., 2004), we have observed a phase transition in NaMgF₃ during two recent independent high pressure trials utilizing monochromatic x-ray diffraction and in-situ laser heating in the diamond anvil cell at pressures as low as 30 GPa. From our analysis thus far, we have found the unit cell of the high pressure phase cannot be indexed according to pv (Pbnm) or close permutations of ppv (Cmcm) unit cells predicted for NaMgF₃ or unit cells observed for ppv MgSiO₃ and MgGeO₃. In addition, we have precluded a breakdown to high pressure phases of NaF and MgF₃ as an explanation for the observed data. Upon pressure release, we observe diffraction peaks from the high pressure phase in the absence of pv NaMgF₃, suggesting the high pressure structure is quenchable to ambient conditions. The results of the work in progress will be presented at the meeting.
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005