Crystal structure and thermoelastic properties of (Mg _0.91 Fe _0.09 )SiO ₃ postperovskite up to 135 GPa and 2,700 K

Abstract

Intriguing seismic observations have been made for the bottom 400 km of Earth's mantle (the D؆ region) over the past few decades, yet the origin of these seismic structures has not been well understood. Recent theoretical calculations have predicted many unusual changes in physical properties across the postperovskite transition, perovskite (Pv) 3 postperovskite (PPv), that may provide explanations for the seismic observations. Here, we report measurements of the crystal structure of (Mg 0.91Fe0.09)SiO3-PPv under quasi-hydrostatic conditions up to the pressure (P)-temperature (T) conditions expected for the core-mantle boundary (CMB). The measured crystal structure is in excellent agreement with the first-principles calculations. We found that bulk sound speed (V ⌽) decreases by 2.4 ؎ 1.4% across the PPv transition. Combined with the predicted shear-wave velocity (V S) increase, our measurements indicate that lateral variations in mineralogy between Pv and PPv may result in the anticorrelation between the V ⌽ and VS anomalies at the D؆ region. Also, density increases by 1.6 ؎ 0.4% and Grü neisen parameter decreases by 21 ؎ 15% across the PPv transition, which will dynamically stabilize the PPv lenses observed in recent seismic studies.equation of state ͉ mantle ͉ phase transition ͉ bulk sound speed ͉ Grü neisen parameter T he DЉ region is believed to play an important role for the dynamics of the mantle and the core. The recent discovery of the postperovskite (PPv) transition (1-3) at the P-T conditions relevant to the DЉ region has provided new opportunities to understand the seismic observations and dynamic processes in the region. First-principles calculations (1, 4, 5) have predicted drastic changes in some geophysically important properties across the PPv transition (6, 7). The unusual changes have been attributed to the fundamental differences in crystal structure between perovskite (Pv), a 3D network structure with cornersharing SiO 6 octahedra, and PPv, a 2D layered structure with both corner and edge sharing SiO 6 octahedra (1,4,8,9). Therefore, measurements of the crystal structure provide a fundamental test for the predicted properties of PPv. However, synthesis of an appropriate single crystal for PPv in its stability field is extremely challenging for current techniques, making Rietveld refinement the only plausible method for studying the crystal structure. Currently only a single Rietveld refinement (10) exists for MgSiO 3 -PPv at 116 GPa and 300 K (Table 1).Some theoretical studies (1, 4) have predicted that bulk sound speed (V ⌽ ) decreases across the PPv transition whereas shearwave velocity (V S ) increases. Shieh et al. (Table 2), suggesting a large increase in V ⌽ across the PPv transition. However, no pressure medium was used in this study. The larger amount of Fe in Mao et al. (12) may cause the difference, yet a first-principles calculation showed that Fe has little effect on the bulk modulus of PPv (13).We have measured x-ray diffraction patterns of (Mg 0.91 Fe 0.09 )SiO 3 -PPv under q...

Postperovskite phase equilibria in the MgSiO 3 –Al 2 O 3 system

“…In contrast to the Pv structures, no marked displacement of Al was found in the A site in pPv, which is because of the smaller Mg cite in pPv (12). No significant structural change was therefore found in pPv associated with the Al incorporation (Table 1), and the calculated crystallographic parameters of the pPv phases are in excellent agreement with the values of recent Rietveld refinements (24).…”

“…This may also affect the high-P,T pPv stability, but the effects in a multicomponent system are poorly constrained (17,18,24,33). The solution mechanisms of iron under the ultrahigh-pressure condition, where the low-spin state with smaller ionic radius is predominant, are also unclear (34), although some low-pressure experiments report that iron may also influence the site preference of Al 3ϩ in Pv (35).…”

High-Fe (Mg, Fe)O inclusion in diamond apparently from the lowermost mantle

“…Experiments show that the most abundant minerals in the lower mantle are expected to be magnesium silicate perovskite, (Mg, Fe)SiO 3 (∼75%), and periclase-wüstite solid solutions, (Mg, Fe)O (∼20%) (Ringwood, 1989), with much smaller amounts of other phases such as CaSiO 3 perovskite. Near the core-mantle boundary a post-perovskite phase may replace perovskite (Shim et al, 2008;, depending on pressuretemperature conditions, but the oxide phase is stable throughout the lower mantle.…”

Single-crystal structure determination of (Mg,Fe)SiO3postperovskite

“…Table 3 lists the selected bond lengths and interatomic angles in the structure of ppv at high P and room T in comparison with the results from powder diffraction data in (Mg 1-x , Fe x )SiO 3 (0≤ x ≤ 0.1) (2,8,10,19). The Fe content in the ppv phase was refined to be x = 0.07 when the Fe-Mg ratio was used as a variable in the refinement.…”

“…However, preparation of suitable singlecrystal ppv samples for conventional single-crystal XRD technique has not been possible. The ppv phase crystallizes into a polycrystalline form at high P-T conditions and becomes amorphous upon release of pressure back to the ambient conditions (9,10). Therefore, structural studies of the ppv phase have been limited to powder diffraction techniques.…”