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7.0
Mar 29, 2016
03/16
Mar 29, 2016
by
Gaurav Shukla; Renata M. Wentzcovitch
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The spin crossover of iron in Fe$^{3+}$-bearing bridgmanite, the most abundant mineral of the Earth's lower mantle, is by now a well-established phenomenon, though several aspects of this crossover remain unclear. Here we investigate effects of disorder, iron concentration, and temperature on this crossover using ab initio LDA + U$_{sc}$ calculations. The effect of concentration and disorder are addressed using complete statistical samplings of coupled substituted configurations in super-cells...
Topics: Materials Science, Condensed Matter, Strongly Correlated Electrons
Source: http://arxiv.org/abs/1603.08947
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2.0
Mar 29, 2016
03/16
Mar 29, 2016
by
Gaurav Shukla; Matteo Cococcioni; Renata M. Wentzcovitch
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We report \textit{ab initio} (LDA + U$_{sc}$) calculations of thermoelastic properties of ferric iron (Fe$^{3+}$)- and aluminum (Al)-bearing bridgmanite (MgSiO$_3$ perovskite), the main Earth forming phase, at relevant pressure and temperature conditions and compositions. Three coupled substitutions, namely, [Al]$_{Mg}$-[Al]$_{Si}$, [Fe$^{3+}$]$_{Mg}$-[Fe$^{3+}$]$_{Si}$, and [Fe$^{3+}$]$_{Mg}$-[Al]$_{Si}$ have been investigated. Aggregate elastic moduli and sound velocities are successfully...
Topics: Materials Science, Condensed Matter, Strongly Correlated Electrons
Source: http://arxiv.org/abs/1603.08899
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13
Mar 24, 2015
03/15
Mar 24, 2015
by
Gaurav Shukla; Mehmet Topsakal; Renata M. Wentzcovitch
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MgGeO$_3$-perovskite is known to be a low-pressure analog of MgSiO$_3$-perovskite in many respects, but especially in regard to the post-perovskite transition. As such, investigation of spin state changes in Fe-bearing MgGeO$_3$ might help to clarify some aspects of this type of state change in Fe-bearing MgSiO$_3$. Using DFT+U calculations, we have investigated pressure induced spin state changes in Fe$^{2+}$ and Fe$^{3+}$ in MgGeO$_3$ perovskite and post-perovskite. Owing to the relatively...
Topics: Condensed Matter, Materials Science
Source: http://arxiv.org/abs/1503.07194
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11
Mar 19, 2015
03/15
Mar 19, 2015
by
Gaurav Shukla; Zhongqing Wu; Han Hsu; Andrea Floris; Matteo Cococcioni; Renata M. Wentzcovitch
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We present LDA+U calculations of high temperature elastic properties of bridgmanite with composition (Mg$_{(1-x)}$Fe$_{x}^{2+}$)SiO$_3$ for $0\le{x}\le0.125$. Results of elastic moduli and acoustic velocities for the Mg-end member (x=0) agree very well with the latest high pressure and high temperature experimental measurements. In the iron-bearing system, we focus particularly on the change in thermoelastic parameters across the state change that occurs in ferrous iron above $\sim$30 GPa,...
Topics: Condensed Matter, Materials Science
Source: http://arxiv.org/abs/1503.05983
5
5.0
Oct 27, 2014
10/14
Oct 27, 2014
by
Han Hsu; Renata M. Wentzcovitch
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Spin crossover of iron is of central importance in solid Earth geophysics. It impacts all physical properties of minerals that altogether constitute $\sim 95$ vol\% of the Earth's lower mantle: ferropericlase [(Mg,Fe)O] and Fe-bearing magnesium silicate (MgSiO$_3$) perovskite. Despite great strides made in the past decade, the existence of intermediate-spin (IS) state in ferrous iron (Fe$^{2+}$) (with total electron spin $S=1$) and its possible role in the pressure-induced spin crossover in...
Topics: Physics, Geophysics, Strongly Correlated Electrons, Materials Science, Condensed Matter
Source: http://arxiv.org/abs/1410.7200
2
2.0
Oct 21, 2014
10/14
Oct 21, 2014
by
Koichiro Umemoto; Burak Himmetoglu; Jian-Ping Wang; Renata M. Wentzcovitch; Matteo Cococcioni
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We report the discovery of a new allotrope of iron by first principles calculations. This phase has $Pmn2_1$ symmetry, a six-atom unit cell (hence the name Fe$_6$), and the highest magnetization density (M$_s$) among all known crystalline phases of iron. Obtained from the structural optimizations of the Fe$_3$C-cementite crystal upon carbon removal, $Pmn2_1$ Fe$_6$ is shown to result from the stabilization of a ferromagnetic FCC phase, further strained along the Bain path. Although metastable...
Topics: Materials Science, Condensed Matter
Source: http://arxiv.org/abs/1410.5875
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41
Mar 8, 2012
03/12
Mar 8, 2012
by
Han Hsu; Peter Blaha; Renata M. Wentzcovitch
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With local density approximation + Hubbard $U$ (LDA+$U$) calculations, we show that the ferromagnetic (FM) insulating state observed in tensile-strained LaCoO$_3$ epitaxial thin films is most likely a mixture of low-spin (LS) and high-spin (HS) Co, namely, a HS/LS mixture state. Compared with other FM states, including the intermediate-spin (IS) state (\textit{metallic} within LDA+$U$), which consists of IS Co only, and the insulating IS/LS mixture state, the HS/LS state is the most favorable...
Source: http://arxiv.org/abs/1203.1861v1
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36
Aug 21, 2011
08/11
Aug 21, 2011
by
Min Ji; Koichiro Umemoto; Cai-Zhuang Wang; Kai-Ming Ho; Renata M. Wentzcovitch
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We propose three new phases of H2O under ultrahigh pressure. Our structural search was performed using an adaptive genetic algorithm which allows an extensive exploration of crystal structure. The new sequence of pressure-induced transitions beyond ice X at 0 K should be ice X - Pbcm - Pbca - Pmc21 - P21 - P21/c phases. Across the Pmc21 - P21 transition, the coordination number of oxygen increases from 4 to 5 with a significant increase of density. All stable crystalline phases have nonmetallic...
Source: http://arxiv.org/abs/1108.4164v1
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70
Jul 22, 2011
07/11
Jul 22, 2011
by
Burak Himmetoglu; Renata M. Wentzcovitch; Matteo Cococcioni
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We investigate the electronic and structural properties of CuO, which shows significant deviations from the trends obeyed by other transition-metal monoxides. Using an extended Hubbard corrective functional, we uncover an orbitally ordered insulating ground state for the cubic phase of this material, which was expected but never found before. This insulating state results from a fine balance between the tendency of Cu to complete its d-shell and Hund's rule magnetism. Starting from the ground...
Source: http://arxiv.org/abs/1107.4399v1
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39
Feb 2, 2011
02/11
Feb 2, 2011
by
Han Hsu; Peter Blaha; Matteo Cococcioni; Renata M. Wentzcovitch
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Using density functional theory plus Hubbard $U$ calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O$_3$ perovskite, a major mineral phase in the Earth's lower mantle, has high-spin ferric iron ($S=5/2$) at both the dodecahedral (A) and octahedral (B) site. As the pressure increases, the B-site iron undergoes a spin-state crossover to the low-spin state ($S=1/2$), while the A-site iron remains in the high-spin state. Our calculation shows that the B-site spin-state crossover in the...
Source: http://arxiv.org/abs/1101.3819v2
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61
Aug 25, 2008
08/08
Aug 25, 2008
by
Pierre Carrier; João F. Justo; Renata M. Wentzcovitch
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The quasiharmonic approximation (QHA), in its simplest form also called the statically constrained (SC) QHA, has been shown to be a straightforward method to compute thermoelastic properties of crystals. Recently we showed that for non-cubic solids SC-QHA calculations develop deviatoric thermal stresses at high temperatures. Relaxation of these stresses leads to a series of corrections to the free energy that may be taken to any desired order, up to self-consistency. Here we show how to correct...
Source: http://arxiv.org/abs/0808.3373v1
146
146
May 29, 2008
05/08
May 29, 2008
by
Tao Sun; Koichiro Umemoto; Zhongqing Wu; Jin-Cheng Zheng; Renata M. wentzcovitch
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Platinum is widely used as a pressure calibration standard. However, the established thermal EOS has uncertainties, especially in the high $P$-$T$ range. We use density functional theory to calculate the thermal equation of state of platinum, up to 550 GPa and 5000 K. The static lattice energy is computed by using the LAPW method, with LDA, PBE, and the recently proposed WC functional. The electronic thermal free energy is evaluated using the Mermin functional. The vibrational part is computed...
Source: http://arxiv.org/abs/0801.3448v2
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58
Jun 14, 2000
06/00
Jun 14, 2000
by
Alexander Yu. Dobin; Wenhui Duan; Renata M. Wentzcovitch
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We have successfully calculated the electronic and structural properties of chromia (Cr_2O_3) in the Local Spin Density Approximation (LSDA). We predict a transformation from the corundum to the Rh_2O_3(II) structure around 15 GPa in the anti-ferromagnetic (AFM) phase as well as in the paramagnetic (PM) insulating state which occurs above the Neel temperature (T_N). This transition is relevant to interpreting the optical anomalies observed in the absorption spectrum of ruby under pressure. We...
Source: http://arxiv.org/abs/cond-mat/0006242v1
