Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A42: Focus Session: Planetary Materials I |
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Sponsoring Units: DMP DCOMP Chair: Renata Wentzcovitch, University of Minnesota Room: Baltimore Convention Center 345 |
Monday, March 13, 2006 8:00AM - 8:36AM |
A42.00001: Electronic structure and phase transition in iron bearing minerals Invited Speaker: First-principles calculations are playing an important role in the development of our understanding of Earth interior. A proper treatment of iron bearing minerals is fundamental in this respect. Unfortunately standard DFT approaches such as the local density (LDA) or generalized gradient (GGA) approximations fail in describing even qualitative features of even simple iron minerals, such as the insulating nature and magnetic structure of many oxides. DFT+U approximation has demonstrated to improve significantly the physical description of transition metal and rare earth compounds. In order to make DFT+U a really ``ab-initio'' approach, an internally consistent determination of the involved U parameter is however needed. In alternative computationally more demanding, but supposedly more accurate, approaches such as PBE0 or B3LYP Hartree-Fock-DFT hybrid functionals can be explored. I'll report on my recent research on these subjects, focussing on iron oxide, magnisium-wustite high-spin-low-spin phase transition, and ematite. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 8:48AM |
A42.00002: High-to-low spin transition in iron in Magnesiow\"{u}stite: elastic properties Cesar R.S. da Silva, Taku Tsuchiya, Renata M. Wentzcovitch, Stefano de Gironcoli The high-to-low spin transition in iron in Magnesiow\"{u}stite (Mw), Mg$_{(1-x)}$Fe$_{x}$,O, is accompanied by considerable volume reduction. This changes the elastic properties of Mw across this transition. Using an LDA+U method with consistently calculated Hubbard U, we investigate the elastic signature of this transition. We find temperature sensitive changes in elasticity across this transition. In Earth's lower mantle, this transition should occur continuously and leave behind an unnoticeable sign. [Preview Abstract] |
Monday, March 13, 2006 8:48AM - 9:00AM |
A42.00003: Spin state of ferrous iron in perovskite Ryan Requist, Koichiro Umemoto, Renata Wentzcovitch Diamond anvil cell experiments at pressures up to 145 GPa have shown evidence for a high spin to low spin transition in iron in magnesium silicate perovskite (MgSiO$_3$.) The spin transition will influence the optical absorption spectrum and elastic properties of this mineral. We present density functional calculations of (Mg,Fe)SiO$_3$ with ferrous iron substituting for magnesium at a concentration of 12.5\%. The calculations use the local density approximation with Hubbard term (LDA+U) and supercells containing up to 160 atoms. We describe the pressure dependence of the iron spin state. Research supported by NSF/EAR 0230319, NSF/ITR 0428774 and 0426757, VLab, NSF/ITR 0325218, ITAMIT. [Preview Abstract] |
Monday, March 13, 2006 9:00AM - 9:12AM |
A42.00004: Ab Initio Study of Thermodynamics of Fe and Spin Transitions in the Lower Mantle Dane Morgan, Amelia Berta, Kristin Persson, Gerbrand Ceder Recent experiments have demonstrated spin transitions in Fe in both the rocksalt ferropericlase (Mg,Fe)O and perovskite (Mg,Fe)SiO$_3$ phases at lower mantle pressures. The spin transitions have potentially profound implications for the materials properties of the lower mantle. However, the coupling of thermodynamic temperature effects and the spin transition is still poorly understood. In this talk we present an ab initio based thermodynamic model for Fe spin transitions in lower mantle phases. We build a free energy model which includes configurational, vibrational, magnetic, and electronic contributions. The resulting free energy expressions are used to construct a phase diagram for the lower mantle ferropericlase which includes the impact of Fe spin transitions. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:48AM |
A42.00005: Effects of the Spin Transition of Iron in Magnesiow\"{u}stite-(Mg,Fe)O: Applications to the Earth's Lower Mantle Invited Speaker: Magnesiow\"{u}stite [(Mg,Fe)O] is the second most abundant mineral in the Earth's lower mantle. Here I will discuss the spin states of iron in magnesiow\"{u}stite and the isolated effects of the electronic transitions on the elastic, thermodynamic, magnetic, and vibrational properties of magnesiow\"{u}stite under high pressures and high temperatures. Pressure-induced electronic spin transitions of iron from high-spin to low-spin states have been recently observed to occur in magnesiow\"{u}stite under high pressures using high-pressure X-ray emission spectroscopy and synchrotron M\"{o}ssbauer spectroscopy. Based on the synchrotron M\"{o}ssbauer studies of (Mg$_{0.75}$,Fe$_{0.25})$O, the simultaneous disappearance of the quadrupole splitting and the drop of the isomer shift at above 62 GPa are consistent with a high-spin to low-spin electronic transition of iron in the sample between 62 and 70 GPa. Addition of FeO in MgO stabilizes the high-spin state to higher pressures and the high-spin to low-spin transition of iron in magnesiow\"{u}stite results in an abnormal compressional behaviour between the high-spin and the low-spin states$^{1}$. Moreover, there are also significant changes in particular physical properties of magnesiow\"{u}stite such as force constant across the electronic spin transition. Here I have combined results from a variety of high-pressure techniques to understand the effects of the electronic transition on the physical properties of magnesiow\"{u}stite and to explore possible geophysical consequences of the transition in the Earth's lower mantle. $^{1}$J. F. Lin, V. V. Struzhkin, S. D. Jacobsen, M. Hu, P. Chow, J. Kung, H. Liu, H. K. Mao, and R. J. Hemley, Spin transition of iron in magnesiow\"{u}stite in Earth's lower mantle, \textit{Nature,} 436, 377-380, 2005. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:00AM |
A42.00006: Electronic Spin State and Elasticity of (Mg, Fe)(Si, Al)O3-perovskite at high pressure Li Li, Donald Weidner, John Brodholt, Stephen Stackhouse, Maria Alfredsson, David Price We investigate the effect of pressure on the electronic spin state of ferric iron in Al-bearing MgSiO$_{3}$-perovskite using first-principle computations (Density Functional Theory with the Generalized Gradient Approximation). We also calculate the single crystal elastic moduli ($c_{ij})$ for (Mg, Fe$^{3+})$(Si, Al)O$_{3}$ perovskite to understand the effect of chemical variations and spin state transitions of the Fe$^{3+}$ ions on these properties. Ferric iron (6.25 mol{\%}) and Al (6.25 mol{\%}) substitute for Mg and Si respectively. Our results show that spin state transition from high spin (HS) to low spin (LS) occurs on the Fe$^{3+}$ ions at high pressure, while there is no stability field for the intermediate spin state. Fe$^{3+}$ alone can be responsible for the spin state transition. The models witness a transition pressure ranging from 97-126 GPa. Differential stress can change the pressure for the spin collapse. These results are one explanation to the reported experimental observations that the spin transition occurs over a wide pressure range. We find that ferric iron lowers the elastic moduli relative to the Al charge-coupled substitution. The spin state of the iron for this composition has a relatively small effect ($<$ 0.5{\%} variation) on both bulk modulus and shear modulus. Replace this text with your abstract body. [Preview Abstract] |
Monday, March 13, 2006 10:00AM - 10:12AM |
A42.00007: Theory of magnesium silicates with bearing Fe and Al in the lower mantle and the Earth's D'' layer Feiwu Zhang, Artem Oganov Although iron and aluminum incorporation into Earth's mantle minerals are expected to have important effects, little is known about Fe valence and spin state in such major phases as MgSiO$_{3}$ perovskite (\textit{Pv}, main mineral of the lower mantle) and post-perovskite (\textit{PPv}, main mineral of the Earth's D'' layer). Here, we perform \textit{ab initio} simulations, indicating that $Fe^{2+}\to Fe(metal)+Fe^{3+}$ in \textit{Pv} {\&} \textit{PPv}. A new detailed microscopic picture of MgSiO$_{3}$ \textit{Pv} {\&} \textit{PPv} is presented, including the complexity of Fe-Al incorporation, the spin transitions in Fe$^{3+}$ and lack of such in Fe$^{2+}$, and the effects of Fe$^{2+}$, Fe$^{3+}$, Al$^{3+}$ on the stability of \textit{PPv}. These theory results explain and reconcile the recent diverse experimental results on the iron spin transition in MgSiO$_{3}$ and provide basis for future geodynamical and petrological studies of the mantle and the D'' layer. [Preview Abstract] |
Monday, March 13, 2006 10:12AM - 10:24AM |
A42.00008: High Pressure studies on nanoparticles of gamma Fe$_{2}${O}$_{3}$ Arun Bommannavar, Maddury Somayazulu, Vaman Naik, Ratna Naik Compressibility of the gamma phase of Fe$_{2}$O$_{3}$ (Maghemite) nano-particles was studied using angle dispersive x-ray diffraction on the micro-diffraction beamline at HPCAT of the Advanced Photon Source. Nano-particles of three different sizes (3 and10 and 20 nm) were studied up to 31 GPa using a diamond anvil cell equipped with c-BN seats. Two samples were synthesized by treating sulfonated divinyl benzene polystyrene resin matrix with aqueous solutions of (1) FeCl2,(2) FeCl$_{3}$. The particle size of $\gamma $-Fe$_{2}$O$_{3}$ prepared using FeCl3 was $\sim $ 3 nm and with FeCl2 was $\sim $ 10 nm. The 20 nm particle size sample was bought commercially. The bulk modulii for 10 nm and 20 nm samples were 212 (5) and 207 (5) GPa which are close to the bulk value of 203 (10) GPa, whereas 3.4 nm sample shows a higher value of 240 (5) GPa. Transition pressure (P$_{tr})$ at which maghemite transforms to hematite varies with particle size and was estimated to be 10 (2) GPa, 17 (2) GPa and 27 (2) GPa for 3 nm, 10 nm and 20 nm particle size samples, respectively. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 10:36AM |
A42.00009: Composition Dependence of Pressure-Induced Spin Transitions in the (Mg,Fe)SiO3 Perovskite and (Mg,Fe)O Rocksalt System Amelia Berta, Kristin Persson, Gerbrand Ceder, Dane Morgan Recent experimental results suggest that Fe undergoes a high-spin to low-spin transition in both the rocksalt and perovskite phases at lower mantle pressures. These spin transitions may have a profound impact on the properties of lower mantle phases. In this work the critical spin-transition pressures for Fe in perovskite (Mg,Fe)SiO$_{3}$ and rocksalt (Mg,Fe)O are calculated using \textit{ab initio} methods. We focus in particular on the alloy nature of the material, studying the spin-transition pressure for varying concentrations of Fe. The results show that as the concentration of Fe increases, the transition pressure decreases in the perovskite. This is directly opposite the trend observed for spin transition pressures found in rocksalt (Mg,Fe)O. The difference in trends in spin-transition pressure is explained by the difference in physics between the two structures. [Preview Abstract] |
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