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Possible structural polymorphism in Al-bearing magnesiumsilicate post-perovskite

¹ High Pressure Science and Engineering Center, Department of Physics, University of Nevada, Las Vegas, Nevada 89154, U.S.A.
² Division of Geology and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
³ Department of Physics, New Mexico State University, Las Cruces, New Mexico 88003, U.S.A.
⁴ HPCAT, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.
⁵ Center for Condensed Matter Science and Technology, Research Academy of Science and Technology, Harbin 150080, China
⁶ Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, U.S.A.
⁷ Physics Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, U.S.A.

View larger version (15K): [in this window] [in a new window]   FIGURE 1. View along the [001] direction of the relaxed structures from the ab initio calculations. The two types of layers are indicated by light and dark shading of the SiO6-octahedra in each layer. Magnesium atoms are shown as black circles and unit cells are indicated by thin solid lines. (a) 1 x 1 (perovskite); (b) 2 x 1; (c) 3 x 1; (d) 4 x 1; (e) 5 x 1; (f) 2 x 2; (g) 3 x 3; (h) x 0 (CaIrO3). We also give the calculated fractional coordinates of the 2 x 1 and the 3 x 1 type structures as they are used to model the observed diffraction patterns. 2 x 1: Mg: 0.057, , 0.538; 0.269, , 0.083; 0.730, , 0.917; Si: 0, 0, 0; 0.330, 0.502, 0.555; 0.380, , 0.820; O: 0.792, 0.560, 0.140; 0.207, 0.060, 0.860; 0.306, , 0.429; 0.458, 0.555, 0.243; 0.644, , 0.313. 3 x 1: Mg: 0.042, , 0.514; 0.303, , 0.073; 0.559, , 0.989; 0.799, , 0.895; Si: 0, 0, 0; , 0, ; 0.247, 0.501, 0.583; O: 0.985, , 0.111; 0.230, , 0.455; 0.480, , 0.368; 0.7352, , 0.287; 0.845, 0.559, 0.125; 0.906, 0.443, 0.647; 0.343, 0.556, 0.283; 0.407, 0.444, 0.801.

View larger version (75K): [in this window] [in a new window]   FIGURE 2. (a) Diffraction patterns from Sample 2 at 93 GPa. The diffraction is from the KBr pressure medium, from MgSiO3-perovskite, and from post-perovskite-like material. In any case, the diffraction features are Debye fringes from a powdered material with minor granularity. (b) Sample 1 at 115 GPa. This sample exhibits noticeable texture in a fine-grained sample aggregate. The intensity distribution is therefore not homogeneous along each Debye fringe but modulated. Another obvious consequence of texture is the alternating sequence of sections of enhanced and oppressed diffraction intensity along the sequence of 2 angles of observed diffraction.

View larger version (12K): [in this window] [in a new window]   FIGURE 3. Rietveldrefinements Samples 1 and 2. Black crosses = Observed signal; red line = modeled diffraction pattern; tick marks = 2-angles of observed reflections. (a) Sample 2: wavelength was 0.34531 Å. The whole pattern, including background (not shown here) was fitted. For MgSiO3-perovskite we find an RF2 of 0.09, for the 3 x 1 kinked post-perovskite RF2 is 0.24, and for KBr 0.04. The fitted cell parameters of the 3 x 1 phase are 9.608(16) x 6.043(13) x 4.248(8) Å3, β = 98.689(14)°, the cell parameters of pervoskite are 4.442(1) x 4.651(1) x 6.438(1) Å3, and the cell parameter for KBr is 2.7979(3) Å. The standard deviation is given in brackets. (b) Sample 1: wavelength was 0.3888 A. Background was subtracted in advance of Rietveld refinement because of a strong modulation from excitation halos around the intense peaks for the pressure medium. The weighted profile refinement parameter wRp was 0.09 and 2 = 31.3. The RF2 for the 2 x 1 phase was 0.25. The cell parameters are 6.804(5) x 6.289(7) x 4.161(4) Å3, β = 96.17(9)°. The cell parameter for NaCl is 2.727(1) Å and for perovskite 4.445(3) x 4.598(4) x 6.372(3) Å3.

View larger version (12K): [in this window] [in a new window]   FIGURE 4. Raman spectra of the two samples at the pressures of 95 and 115 GPa. The Raman spectra were collected right after the diffraction experiment without changing pressure or additional heating. Both spectra are dominated by Raman peaks above 700 cm–1 where perovskite does not have measurable Raman scattering intensity (Serghiou et al. 1998; Karki et al. 2002). These strong features are not single broad peaks but are composed of several overlapping peaks which appear as shoulders in the spectra, in particular in the spectrum of Sample 1 (lower spectrum). The lower frequency part of the spectrum is complex and involves more Raman peaks than expected for the CaIrO3-type structure (see text). Sample 1 (lower spectrum) exhibits marked texture (Fig. 2b) and the observed Raman peak intensities are not expected to represent an average. The upper spectrum (Sample 2) is from a powdered sample.

View larger version (2K): [in this window] [in a new window]   APPENDIX FIGURE 1. Correlation of observed reflections in reciprocal space. We show the array of a total of 10 reflections in two different parallel projections. The reflections establish two intersecting sequences of reciprocal lattice vectors thus spanning a two-dimensional reciprocal lattice. The observable portion of reciprocal space is essentially two-dimensional since the sample containing Mao-Bell type diamond anvil cell could not be rotated during the experiment. The two-dimensional character of the available set of reflections does not permit rigorous indexing. We, therefore, considered the 2 angles of the observed reflections only and compared them to predicted values of perovskite, CaIrO3-type post-perovskite and kinked post-perovskites as shown in Table 1.