HR: 1340h
AN: MR13C-1400    [Abstracts]
TI: Structural Properties of MgSiO3 Perovskite, Twin Walls and Their Binding to Oxygen Vacancies at High Pressures from Force Field Simulations
AU: * Goncalves-Ferreira, L
EM: lgon04@esc.cam.ac.uk
AF: University of Cambridge, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom
AU: Redfern, S A
EM: satr@esc.cam.ac.uk
AF: University of Cambridge, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom
AU: Artacho, E
EM: emilio@esc.cam.ac.uk
AF: University of Cambridge, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom
AB: Aiming to understand possible origins of seismic wave attenuation and variations in the quality factor (Q) of minerals at the Earth's interior, it has been accepted that the friction in the motion of twin domain walls under applied stress leads to a reduction in Q [1]. To interpret this behaviour one must first appreciate the influence of pressure on wall motion and dynamics. Experimental observations of twin wall motion at high pressure are difficult at present. For this reason, we have undertaken a computational study of wall structure and dynamics in MgSiO₃ perovskite, in the absence or presence of defects, as a function of pressure between 0 and 140 GPa. This work is based on previous studies on CaTiO₃ perovskite [2,3] (symmetry Pbnm) with 7800 atoms and orthorhombic periodic boundary conditions. It has been geometrically adapted to lower mantle's MgSiO₃ perovskite. The system comprises two (100) ferroelastic twin walls separated by 34.2 Å, relaxed within a variable supercell of 26x10x6 unit cells, with the rigid ion set of interatomic potentials with partial ionic charges employed by Alfredsson et al., 2005 [4]. The degree of distortion in the system is manifest in the octahedral tilting angles along each of the crystallographic directions. The octahedral tilting orientations close to the walls are well described by the behaviour of two order parameters: the main one, defining the wall, goes as α \hspace{1mm} tanh(2(x - x')/w) , where α = 11.14° is the bulk value for the tilt, and w = 11.11 Å is the wall width. The secondary order parameters describe a slight breather at the wall. As wall dynamics are influenced by the presence of oxygen vacancies, we have calculated their binding energies to the described twin walls as a function of pressure. We find that this binding strongly depends on the vacancy disposition. For a vacancy between Si atoms in an axis perpendicular to the wall the binding energy is 1.09 eV at zero pressure. For vacancies along Si-Si axes parallel to the wall, the binding is reduced by 43%. This behaviour is analogous to what reported for CaTiO₃ [3]. The dependence on pressure of the binding, however, is quite different, with a clear increase in binding for higher pressures, reaching a binding energy of 1.22 eV (perpendicular) and 0.94 eV (parallel) at 140 GPa.
\noindent {\small [1] Harrison RJ and Redfern SAT, Phys. Earth Planet. Inter. 134, 253 (2002)}. \noindent {\small [2] Calleja M, Dove M and Salje EKH, J. Phys.: Condens. Matter 15, 2301 (2003)}. \noindent {\small [3] Goncalves-Ferreira L, Redfern SAT and Artacho E, American Geophysical Union, Fall Meeting 2006, abstract #MR11A-0095 (2006) and to be published}. \noindent {\small [4] Alfredsson M, Brodholt JP, Dobson DP, Oganov AR, Catlow CRA, Parker SC and Price GD, Phys. Chem. Miner. 31, 671 (2005)}.
DE: 3904 Defects
DE: 3924 High-pressure behavior
DE: 5144 Wave attenuation
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting