HR: 0800h
AN: T41B-1180    [Abstracts]
TI: X-ray Diffraction and Vibrational Spectroscopy Studies on Brucite-type $\beta$-Cadmium Hydroxide to 36 GPa.
AU: * Rekhi, S
EM: srekhi@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Shim, S
EM: sangshim@MIT.EDU
AF: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Martin, M
EM: MCMartin@lbl.gov
AF: The Advanced Light Source Division, Lawrence Berkeley National Lab, Berkeley, CA 94720 United States
AU: Jeanloz, R
EM: jeanloz@berkeley.edu
AF: University of California, 302 McCone Hall, Berkeley, CA 94720 United States
AB: The brucite-structured hydroxides serve as a good model system to understand the compression of complex layered magnesium hydrous silicates at high pressure. We have studied the pressure behavior of $\beta$-Cd(OH)$_2$ using x-ray diffraction at APS and SSRL, and Raman and infrared-absorption spectroscopy at MIT and ALS, respectively, from 0 to 36 GPa at room temperature. The x-ray measurements show that the $c/a$ ratio decreases steeply (-0.0117/GPa) up to 7 GPa, decreases much gently (-0.0014/GPa) between 7 and 20 GPa, and drops by 1.8 % at 22 GPa. Between 22 and 27 GPa, the $c/a$ ratio does not change with pressure. A fit of pressure-volume data at 0-20 GPa yields $K_0$ = 38(2) GPa and $K_0'$=11(1). The $K_0'$ value is the highest among all the measured for brucite-type hydroxides ($K_0'$= 4-5.2). Assuming a discontinuity at 6-9 GPa, a separate P-V data fit of 0-6 GPa and 9-20 GPa yields $K_0$ = 49(2) GPa and 105(7) GPa, respectively for $K_0'=4$. The Raman-active lattice modes [$E_g$ and $A_{1g}$] of $\beta$-Cd(OH)$_2$ stiffen linearly between 0 and 17 GPa with a discontinuity at 10 GPa, and disappear at 17 GPa. The OH-stretching mode ($A_{1g}$) of $\beta$-Cd(OH)$_2$ softens linearly with pressure without discontinuity and with smallest slope (-3.62 cm$^{-1}$/GPa) among the studied brucite type hydroxides. Unlike $A_{1g}$(OH), the OH-asymmetric stretching mode ($A_{2u}$) softens at 0-8 GPa (-2.15 cm$^{-1}$/GPa) and becomes almost pressure independent above 10 GPa. Both $A{1g}$ and $A_{2u}$ OH-modes disappear at pressure above 20 GPa. Similar to other brucite-type materials, the $c$-axis of $\beta$-Cd(OH)$_2$ is much more compressible to 7 GPa and becomes comparable to that of the $a$-axis above 7 GPa. In addition, $\beta$-Cd(OH)$_2$ becomes very incompressible above 7 GPa. Together with these X-ray observations, the discontinuity at 10 GPa in the shifts of the lattice and $A_{2u}$(OH) modes indicate a displacement or rearrangement of hydrogen in the crystal structure of $\beta$-Cd(OH)$_2$ at 7-10 GPa. A phase change takes place from crystalline to amorphous states at 20 GPa.
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 1010 Chemical evolution
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting