58
58
Sep 18, 2013
09/13
by
Alexander Yu. Dobin; Wenhui Duan; Renata M. Wentzcovitch
texts
eye 58
favorite 0
comment 0
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
70
70
Jul 20, 2013
07/13
by
Burak Himmetoglu; Renata M. Wentzcovitch; Matteo Cococcioni
texts
eye 70
favorite 0
comment 0
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
2
2.0
Jun 29, 2018
06/18
by
Gaurav Shukla; Matteo Cococcioni; Renata M. Wentzcovitch
texts
eye 2
favorite 0
comment 0
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
13
13
Jun 27, 2018
06/18
by
Gaurav Shukla; Mehmet Topsakal; Renata M. Wentzcovitch
texts
eye 13
favorite 0
comment 0
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
7
7.0
Jun 29, 2018
06/18
by
Gaurav Shukla; Renata M. Wentzcovitch
texts
eye 7
favorite 0
comment 0
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
11
11
Jun 27, 2018
06/18
by
Gaurav Shukla; Zhongqing Wu; Han Hsu; Andrea Floris; Matteo Cococcioni; Renata M. Wentzcovitch
texts
eye 11
favorite 0
comment 0
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
39
39
Sep 22, 2013
09/13
by
Han Hsu; Peter Blaha; Matteo Cococcioni; Renata M. Wentzcovitch
texts
eye 39
favorite 0
comment 0
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
41
41
Jul 20, 2013
07/13
by
Han Hsu; Peter Blaha; Renata M. Wentzcovitch
texts
eye 41
favorite 0
comment 0
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
5
5.0
Jun 30, 2018
06/18
by
Han Hsu; Renata M. Wentzcovitch
texts
eye 5
favorite 0
comment 0
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
Jun 30, 2018
06/18
by
Koichiro Umemoto; Burak Himmetoglu; Jian-Ping Wang; Renata M. Wentzcovitch; Matteo Cococcioni
texts
eye 2
favorite 0
comment 0
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
36
36
Sep 21, 2013
09/13
by
Min Ji; Koichiro Umemoto; Cai-Zhuang Wang; Kai-Ming Ho; Renata M. Wentzcovitch
texts
eye 36
favorite 0
comment 0
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
61
61
Sep 21, 2013
09/13
by
Pierre Carrier; João F. Justo; Renata M. Wentzcovitch
texts
eye 61
favorite 0
comment 0
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
Sep 23, 2013
09/13
by
Tao Sun; Koichiro Umemoto; Zhongqing Wu; Jin-Cheng Zheng; Renata M. wentzcovitch
texts
eye 146
favorite 0
comment 0
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
