Recent COMPRES-supported High P-T 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@vt.edu & nross@vt.eduAcknowledgments: 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 -AlOODat ambient conditions using powder neutron diffraction; To study the evolution of the O-H(D) bonds of -AlOODas 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 Earth’s 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 KT=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]. (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 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 -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) http://www.crystal.vt.edu/compres/