HR: 0830h
AN: T11C-0412    [PDF]
TI: Phase Transformation of Al$_{2}$O$_{3}$ Under High Pressure and High Temperature and its Effect on Ruby Pressure Scale
AU: Prewitt, C
EM: c.prewitt@gl.ciw.edu
AF: University of Arizona, Gould-Simpson Building,1040 E. Fourth St., Tucson, AZ 85721 United States
AU: * Lin, J
EM: j.lin@gl.ciw.edu
AF: Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington DC, DC 20015 United States
AU: Dera, P
EM: p.dera@gl.ciw.edu
AF: Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington DC, DC 20015 United States
AU: Nagayoshi, S
EM: sata@jamstec.go.jp
AF: Japan Marine Science and Technology Center, 2-15 Natsushima Natsushima-cho Yokosuka, KANAGAWA, 0061 Japan
AU: Gregoryanz, E
EM: e.gregoryanz@gl.ciw.edu
AF: Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington DC, DC 20015 United States
AU: Mao, H
EM: mao@gl.ciw.edu
AF: Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington DC, DC 20015 United States
AU: Hemley, R
EM: hemley@gl.ciw.edu
AF: Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington DC, DC 20015 United States
AB: Corundum ($\alpha$-Al$_{2}$O$_{3}$) is an important material in geophysics, high-pressure physics, and ceramic science. The pressure-induced shift of the Cr$^{3+}$ fluorescence wavelength of ruby (Cr$^{3+}$ doped $\alpha$-Al$_{2}$O$_{3}$) is widely used as a pressure calibrant in diamond anvil cell experiments. Theoretical calculations predict that the corundum transforms to the Rh$_{2}$O$_{3}$(II) structure (space group: Pbcn) and then to the Pbnm-perovskite structure under high pressures (Marton and Cohen, 1994; Thomson et al., 1996). The phase transformation from the corundum structure to the Rh$_{2}$O$_{3}$(II) structure was reported to occur at about 100 GPa in the high-pressure X-ray diffraction experiments after high-temperature laser heating at about 1000 K (Funamori and Jeanloz, 1997), but other high-pressure X-ray diffraction experiments to 175 GPa at 300 K did not observe such a phase transformation (Jephcoat et al., 1988). Recent shock-wave experiments on corundum show that two transitions occurred at 79 GPa and 250 GPa (Hama and Suito, 2002), consistent with the theoretical calculations (Thomson et al., 1996). To understand the crystal structure of the high-pressure phase and the effect of the phase transformation on the ruby pressure scale, we have studied Al$_{2}$O$_{3}$ with in situ X-ray diffraction in a laser-heated diamond anvil cell up to 130 GPa and 2300 K. A phase transformation in Al$_{2}$O$_{3}$ was observed to occur above 100 GPa and at high temperatures. The powder diffraction lines of the high-pressure phase are consistent with that of the Rh$_{2}$O$_{3}$(II) structure model. The refined crystal structure of the high-pressure phase will be discussed in this paper. Moreover, the ruby fluorescence spectra of the quenched Al$_{2}$O$_{3}$ samples under ambient conditions show significant red shifts; i.e. the R$_{1}$ peak of the quenched sample occurs at 696 nm while the R$_{1}$ peak for the sample before pressurizing occurs at 692 nm. The fossilized pressure indicates that chromium atoms have been re-distributed in the Al$_{2}$O$_{3}$ structure during laser heating. The evidence also suggests that a phase transformation in ruby at high pressures after high-temperature laser heating may affect the ruby calibration scale.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3900 MINERAL PHYSICS
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
DE: 3999 General or miscellaneous
SC: Tectonophysics [T]
MN: 2003 Fall Meeting