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71
Jul 20, 2013
07/13
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
3
3.0
Jun 29, 2018
06/18
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
14
14
Jun 27, 2018
06/18
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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13
Jun 27, 2018
06/18
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
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42
Sep 22, 2013
09/13
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
3
3.0
Jun 30, 2018
06/18
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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37
Sep 21, 2013
09/13
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