Structural, spin, and metal-insulator transitions of (Mg,Fe)O at ultrahigh pressure

Fe-bearing MgO [(Mg$_{1-x}$Fe$_x$)O] is considered a major constituent of terrestrial exoplanets. Crystallizing in the B1 structure in the Earth's lower mantle, (Mg$_{1-x}$Fe$_x$)O undergoes a high-spin (HS, $S=2$) to low-spin (LS, $S=0$) transition at $\sim$45 GPa, accompanied by anomalous changes of this mineral's physical properties, while the intermediate-spin (IS, $S=1$) state has not been observed. In this work, we investigate (Mg$_{1-x}$Fe$_x$)O ($x \leq 0.25$) up to $1.8$ TPa via first-principles calculations. Our calculations indicate that (Mg$_{1-x}$Fe$_x$)O undergoes a simultaneous structural and spin transition at $\sim$0.6 TPa, from the B1 phase LS state to the B2 phase IS state, with Fe's total electron spin ($S$) re-emerging from $0$ to $1$ at ultrahigh pressure. Upon further compression, an IS--LS transition occurs in the B2 phase. Depending on the Fe concentration ($x$), metal--insulator transition and rhombohedral distortions can also occur in the B2 phase. These results suggest that Fe and spin transition may affect planetary interiors over a vast pressure range

Searching for high magnetization density in bulk Fe: the new metastable Fe6_6 phase

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 from 0 to 50 GPa, the new phase is more stable, at low pressures, than the other well-known HCP and FCC allotropes and smoothly transforms into the FCC phase under compression. If stabilized to room temperature, e.g., by interstitial impurities, Fe$_{6}$ could become the basis material for high M$_s$ rare-earth-free permanent magnets and high-impact applications such as, light-weight electric engine rotors or high-density recording media. The new phase could also be key to explain the enigmatic high M$_s$ of Fe$_{16}$N$_2$, which is currently attracting an intense research activity.Comment: 7 pages, 7 figure

Phase transitions in MgSiO3 post-perovskite in super-Earth mantles

The highest pressure form of the major Earth-forming mantle silicate is MgSiO3 post-perovskite (PPv). Understanding the fate of PPv at TPa pressures is the first step for understanding the mineralogy of super-Earths-type exoplanets, arguably the most interesting for their similarities with Earth. Modeling their internal structure requires knowledge of stable mineral phases, their properties under compression, and major element abundances. Several studies of PPv under extreme pressures support the notion that a sequence of pressure induced dissociation transitions produce the elementary oxides SiO2 and MgO as the ultimate aggregation form at ~3 TPa. However, none of these studies have addressed the problem of mantle composition, particularly major element abundances usually expressed in terms of three main variables, the Mg/Si and Fe/Si ratios and the Mg#, as in the Earth. Here we show that the critical compositional parameter, the Mg/Si ratio, whose value in the Earth's mantle is still debated, is a vital ingredient for modeling phase transitions and internal structure of super-Earth mantles. Specifically, we have identified new sequences of phase transformations, including new recombination reactions that depend decisively on this ratio. This is a new level of complexity that has not been previously addressed, but proves essential for modeling the nature and number of internal layers in these rocky mantles.Comment: Submitted to Earth Planet. Sci. Lett., 28 pages, 6 figure

Elasticity of Diamond at High Pressures and Temperatures

We combine density functional theory within the local density approximation, the quasiharmonic approximation, and vibrational density of states to calculate single crystal elastic constants, and bulk and shear moduli of diamond at simultaneous high pressures and temperatures in the ranges of 0-500 GPa and 0-4800 K. Comparison with experimental values at ambient pressure and high temperature shows an excellent agreement for the first time with our first-principles results validating our method. We show that the anisotropy factor of diamond increases to 40% at high pressures and becomes temperature independent.Comment: 10 pages, 3 figures, 1 tabl

New ultrahigh pressure phases of H2O ice predicted using an adaptive genetic algorithm

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 band structures up to 7 TPa

Lattice Dynamics and Thermal Equation of State of Platinum

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 within the quasi-harmonic approximation using density functional perturbation theory and pseudopotentials. Special attention is paid to the influence of the electronic temperature to the phonon frequencies. We find that in overall LDA results agree best with the experiments. Based on the DFT calculations and the established experimental data, we develop a consistent thermal EOS of platinum as a reference for pressure calibration.Comment: 24pages, 13 giure