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@vt.edu & nross@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 sitepu@vt.edunross@vt.edusitepu@vt.edunross@vt.edu Thanks to ISIS, LANSCE and ILL for providing the beam time on the projects http://www.crystal.vt.edu/compres/ High Pressure Neutron Diffraction Study of  -AlOOD Oliver Tschauner University of Las Vegas & Husin Sitepu Virginia Tech Paul D. Asimow & Tom J. Ahrens California Tech & Ron Smith ISIS Investigation of Mg-Al site disorder in shocked MgAl 2 O 4 spinel using neutron diffraction Husin Sitepu & Carine Vanpeteghem Virginia Tech & Eiji Ohtani, A. Suzuki, Asami Sano Tohoku Univ  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,  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. 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!). Background: The high-pressure phase of aluminum hydroxide,  -AlOOH, is of interest because it is a potential reservoir of water in the Earth’s lower mantle. The crystal structure of  -AlOOH is orthorhombic (space group P2 1 nm) and is similar to the CaCl 2 modification of SiO 2. Vanpeteghem et al. [1] showed that this phase is the least compressible hydrous phase known so far with K T =252 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]. Objectives: (i)To determine the position of H(D) atoms in  -AlOODat ambient conditions using powder neutron diffraction; (i)To determine the position of H(D) atoms in  -AlOOD at ambient conditions using powder neutron diffraction; (ii)To study the evolution of the O-H(D) bonds of  -AlOODas a function of pressure with the Paris-Edinburgh cell. (ii)To study the evolution of the O-H(D) bonds of  -AlOOD as a function of pressure with the Paris-Edinburgh cell. (a) (b) 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. (a) (b) Figure 1. Comparison of the pristine structure (a) before and (b) after the shock experiment, where the mainly difference is the temperature factors. [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: R WP = 1.74 R P = 1.41 R(F 2 )= 3.30  2 = 5.743 R WP = 2.20 R P = 3.79 R(F 2 )= 4.45  2 = 1.471 The results at ambient conditions show that:  The refined structural parameters along with the inter atomic distances agree reasonably well with Suzuki et al’s 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 The results show good agreement with first principle calculations (Tsuchiya et al [2]) and are comparable to  -CrOOD. Figure 3. The structure of  -AlOOD viewed onto (001)