Recent COMPRESsupported High PT Neutron Diffraction Experiments

1
Recent COMPRES-supported High P-T Neutron
Diffraction Experiments
Husin Sitepu and Nancy L. Ross Crystallography
Laboratory, Department of Geosciences, Virginia
Tech, 4044 Derring Hall, Blacksburg, VA
24060-0420, USA. sitepu_at_vt.edu nross_at_vt.edu

Acknowledgments
The authors would like to acknowledge COMPRES
(Consortium for Materials Properties Research in
Earth Sciences under NSF Cooperative Agreement
EAR 01-35554) for financial support Thanks to
ISIS, LANSCE and ILL for providing the beam time
on the projects
High Pressure Neutron Diffraction Study of ?-AlOOD
Investigation of Mg-Al site disorder in shocked
MgAl2O4 spinel using neutron diffraction
Husin Sitepu Carine Vanpeteghem Virginia Tech
Eiji Ohtani, A. Suzuki, Asami Sano Tohoku Univ
Oliver Tschauner University of Las Vegas
Husin Sitepu Virginia Tech Paul D. Asimow Tom
J. Ahrens California Tech Ron Smith ISIS

• Objectives
• To determine the position of H(D) atoms in
  ?-AlOOD at ambient conditions using powder
  neutron diffraction
• To study the evolution of the O-H(D) bonds of
  ?-AlOOD as a function of pressure with the
  Paris-Edinburgh cell.

• The proposed experiment shall provide an
  understanding of the effect of dynamic
  compression on Mg-Al disorder in spinel.
• The results can be used to model shock conditions
  of spinel-bearing meteorites and impacted
  rocks, and contribute to a principal
  understanding of shock- induced disorder effects
  in solids on an atomic level.
• Medium-resolution high-intensity POLARIS
  diffractometer at ISIS was used, because it
  provides neutron powder diffraction data of
  high statistical quality at high-Q necessary to
  refine the site occupancies.

Background The high-pressure phase of aluminum
hydroxide, ?-AlOOH, is of interest because it is
a potential reservoir of water in the Earths
lower mantle. The crystal structure of ?-AlOOH
is orthorhombic (space group P21nm) and is
similar to the CaCl2 modification of SiO2.
Vanpeteghem et al. 1 showed that this phase is
the least compressible hydrous phase known so far
with KT252 GPa (with K' 4). The purpose of this
work is to determine the positions of H in the
structure and to determine whether there are any
changes in the O-H bonds with pressure that might
lead to a phase transition as predicted by
Tsuchiya et al. 2.
(a)
(b)
1. Vanpeteghem C.B., Ohtani E. and Kondo T.
(2002) Geophysical Research Letters 29,
14224. 2. Tsuchiya J., Tsuchiya T., Tsuneyuki
S. and Yamanka T. (2002) Geophysical Research
Letters 29,15417.
Structure parameters
(b)
(a)
RWP 2.20 RP 3.79 R(F2) 4.45 ?2 1.471
RWP 1.74 RP 1.41 R(F2) 3.30 ?2 5.743
Figure 2. The agreement between the measured and
calculated neutron diffraction patterns for
?-AlOOD (a) at ambient conditions and (b) at
the highest pressure of 9GPa.

• The results at ambient conditions show that
• The refined structural parameters along with
  the inter atomic distances agree reasonably well
  with Suzuki et als results 3.
• The AlO6 are quite distorted as seen by range
  of Al-O distances and divergence of O-Al-O
  angles from 90? (see Figure 4).
• Isotropic displacement parameters and location
  of H(D) determined for first time.
• D-O2 1.013Å
• O2-D . . O1 2.559Å
• O2-D-O1 176.85?
•  
• The results show good agreement with first
  principle calculations (Tsuchiya et al 2) and
  are comparable to ?-CrOOD.

Figure 1. Comparison of the pristine structure
(a) before and (b) after the shock
experiment, where the mainly difference is
the temperature factors.

• Preliminary results show that
• the site occupancies are very intriguing and
  suggest that spinel is changing character from
  normal to inverse ( maximal site disorder)
• the change in intensities is so pronounced
• the isotropic temperature factors increase
  dramatically for shocked sample.
• So, it can be concluded that pressure and/or
  dynamic compression enhance site disorder. The
  short, 1 ?s, time scale of the shock experiment
  allows for Mg-Al inter diffusion (remarkable!).

Figure 3. The structure of ?-AlOOD viewed onto
(001)
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