The transport properties (electrical resistivity, thermal conductivity, and viscosity) of iron-hydrogen alloys are of great significance in the stability and evolution of planetary magnetic fields. Here, we investigate the thermal transport properties of iron doped with varying hydrogen content as functions of pressure (P) and temperature (T ) for the top and bottom of Earth's outer core and beyond, corresponding to pressures of about 130 to 380 GPa and temperatures of 4000 to 7000 K. Using first-principles density functional theory molecular dynamic simulations (FPMD), we verify that crystalline FeHx is superionic with H diffusing freely. We find a low-frequency viscosity of 10-11 mPa s for liquid Fe-H alloys at Earth's outer core conditions. We find resistivity saturation with increasing temperatures in liquid Fe-H alloy at core pressures. The effect of H on electrical and thermal transport we find is small, so that the exact H content of the core is not needed. The primary effect of H is on the equation of state, decreasing the density at constant P and T. We find the Lorenz number is smaller than the ideal value, and obtain for X(H)= 0.20 or 0.45 wt % H, thermal conductivity kappa of similar to 105 and similar to 190 Wm-1 K-1 at conditions near the core-mantle and inner-outer core boundary, respectively.

Using density functional perturbation theory, we computed the phonon frequencies and Raman and IR activities of hafnia polymorphs (P42nmc, Pca21, Pmn21, Pbca OI, brookite, and baddeleyite) for phase identification. We investigated the evolution of Raman and IR activities with respect to epitaxial strain and provide plots of frequency differences as a function of strain for experimental calibration and identification of the strain state of the sample. We found Raman signatures of different hafnia polymorphs: ω(A1g)=300 cm−1 for P42nmc, ω(A1)=343 cm−1 for Pca21, ω(B2)=693 cm−1 for Pmn21, ω(Ag)=513 cm−1 for Pbca (OI), ω(Ag)=384 cm−1 for brookite, and ω(Ag)=496 cm−1 for baddeleyite. We also identified the Raman B1g mode, an anti-phase vibration of dipole moments [ω(B1g)=758 cm−1 for OI and ω(B1g)=784 cm−1 for brookite], as the Raman signature of antipolar Pbca structures. We calculated a large splitting between the longitudinal optical and transverse optical modes [ΔωLO−TO(A1z)=255 cm−1 in Pca21 and ΔωLO−TO(A1)=263 cm−1 in Pmn21] to the same order as those observed in perovskite ferroelectrics and related them to the anomalously large Born effective charges of Hf atoms [Z*(Hf)=5.54].

Germanates are often used as structural analogs of planetary silicates. We have explored the high-pressure phase relations in Mg2GeO4 using diamond-anvil cell experiments combined with synchrotron X-ray diffraction and computations based on density functional theory. Upon room temperature compression, forsterite-type Mg2GeO4 remains stable up to 30 GPa. At higher pressures, a phase transition to a forsterite-III type (Cmc21) structure was observed, which remained stable to the peak pressure of 105 GPa. Using a third-order Birch Murnaghan fit to the experimental data, we obtained V0 = 305.1(3) & Aring;3, K0 = 124.6(14) GPa, and K0 ' $\begin{array}{} \displaystyle K_{0}<^>{\prime} \end{array}$ = 3.86 (fixed) for forsterite-type Mg2GeO4 and V0 = 263.5(15) & Aring;3, K0 = 175(7) GPa, and K0 ' $\begin{array}{} \displaystyle K_{0}<^>{\prime} \end{array}$ = 4.2 (fixed) for the forsterite-III type phase. The forsterite-III type structure was found to be metastable when compared to the stable assemblage of perovskite/post-perovskite + MgO, as observed during laser-heating experiments. Understanding the phase relations and physical properties of metastable phases is crucial for studying the mineralogy of impact sites, understanding metastable wedges in subducting slabs, and interpreting the results of shock compression experiments.

We study ferroelectricity in the classic perovskite ferroelectric PbTiO3 to high pressures with density functional theory (DFT) and experimental diamond-anvil techniques. We use second harmonic generation spectroscopy to detect lack of inversion symmetry. Consistent with early understanding and experiments, we find that ferroelectricity disappears at moderate pressures. However, DFT computations show that the disappearance arises from the overtaking of zone boundary instabilities, and not from the squeezing out of the off-centering ferroelectric displacements with pressure, as previously thought. Moreover, at high pressures the distorted perovskite phases are metastable with respect to a new dense centrosymmetric postperovskite phase with P21/m symmetry and 8-coordinated Ti, which becomes stable at about 70 GPa.

We predict theoretically a carbon-based clathrate in the bipartite sodalite structure, SrB3C3, that is thermodynamically stable at high pressure. This clathrate is predicted to be a dynamically stable superconductor with an estimated Tc of 42 K at ambient pressure. Calculated stress-strain relations for SrB3C3 clathrate demonstrate its intrinsic hard nature with Vickers hardness of 24-31 GPa. Boron substitution aids in the stabilization of SrB3C3 clathrate, and offers valuable insights into design guidelines for various carbon-based materials.

We clarify the nature of hafnia as a proper ferroelectric and show that there is a shallow double well involving a single soft polar mode as in well-known classic ferroelectrics. Using symmetry analysis, density-functional theory (DFT) structural optimizations with and without epitaxial strain, and density functional perturbation theory (DFPT), we examine several important possible hafnia structures derived ultimately from the cubic fluorite structure, including baddeleyite ($P2_{1}/c$) tetragonal antiferroelectric $P4_{2}nmc$, $Pbca$ (nonpolar and brookite), ferroelectric rhombohedral ($R3m$ and $R3$), $Pmn2_{1}$, and $Pca2_{1}$ structures. The latter is considered to be the most likely ferroelectric phase seen experimentally, and has an antiferroelectric parent with space group $Pbcn$, with a single unstable polar mode and a shallow double well with a well depth of 24 meV/atom. Strain is not required for switching or other ferroelectric properties, nor is coupling of the soft-mode with any other modes within the ferroelectric $Pca2_{1}$, $Pmn2_{1}$, $R3m$ or $R3$ phases.

(Mg, Fe)SiO$_3$ post-perovskite is the highest pressure silicate mineral phase in the Earth's interior. The extreme pressure and temperature conditions inside large extrasolar planets will likely lead to phase transitions beyond pPv. In this work we have explored the high-pressure phase relations in Mg$_2$SiO$_4$ using computations based on density functional theory. We find that a partially disordered I-42d type structure would be stable in the interiors of these super-Earth planets. The discovery of a structure where two very dissimilar cations, Mg$^{2+}$ and Si$^{4+}$ occupy the same crystallographic site opens up a domain of interesting crystal chemistry and provides a foundation for other silicates and oxides with mixed occupancy. We have explored the mechanism of the phase transition from the ordered ground state and the effect of the disordering on electronic properties of the silicate phase.

Boron substitution represents a promising approach to stabilize carbon clathrate structures, but no thermodynamically stable substitution schemes have been identified for frameworks other than the type-VII (sodalite) structure type. To investigate the possibility for additional tetrahedral carbon-based clathrate networks, more than 5000 unique boron decoration schemes were investigated computationally for type-I and type-II carbon clathrates with a range of guest elements including Li, Na, K, Rb, Cs, Mg, Ca, Sr, and Ba. Density functional theory calculations were performed at 10 and 50 GPa, and the stability and impact of boron substitution were evaluated. The results indicate that the boron-substituted carbon clathrates are stabilized under high-pressure conditions. Full cage occupancies of intermediate-sized guest atoms (e.g., Na, Ca, and Sr) are the most favorable energetically. Clathrate stability is maximized when the boron atoms are substituted within the hexagonal rings of the large [5(12)6(2)]/[5(12)6(4)] cages. Several structures with favorable formation enthalpies <-200 meV/atom were predicted, and type-I Ca8B16C30 is on the convex hull at 50 GPa. This structure represents the first thermodynamically stable type-I clathrate identified and suggests that boron-substituted carbon clathrates may represent a large family of diamond-like framework materials with a range of structure types and guest/framework substitutions.

We study ferroelectricity in the classic perovskite ferroelectric PbTiO_3 to high pressures with density functional theory (DFT) and experimental diamond-anvil techniques. We use second harmonic generation (SHG) spectroscopy to detect lack of inversion symmetry. Consistent with early understanding and experiments, we find that ferroelectricity disappears at moderate pressures. However, DFT computations show that the disappearance arises from the overtaking of zone boundary instabilities, and not from the squeezing out of the off-centering ferroelectric displacements with pressure, as previously thought. Moreover, at high pressures the distorted perovskite phases are metastable with respect to a new dense centrosymmetric post-perovskite phase with P2_1/m symmetry and 8-coordinated Ti, which becomes stable at about 70 GPa.

We performed density functional theory (DFT) calculations on epitaxially strained hafnia. We demonstrate the stabilization of the ferroelectric ($Pca2_{1}$) phase from the antiferroelectric ($Pbcn$) in bulk hafnia in the presence of electric field. We found that the polar ($Pca2_{1}$) phase can be efficiently stabilized with an adequate choice of film orientation. We show that for a (010)-oriented Pbcn, the ferroelectric $Pca2_{1}$ phase can be reached with a relatively small electric field ($\mid\varepsilon\mid\geq 150$ KV/m). We thus provide a simple explanation to the experimental observation of polarization enhancement through electric field cycling, or wake-up effect, as a ferroelectric phase transition driven by electric field. We find, in contrast, that stress free pure hafnia does not become ferroelectric for any reasonable electric field. So we explain the wake up effect and stabilization of ferroelectric pure hafnia as coming from a combination of epitaxial strain under applied electric field perpendicular to the film. We find that strain (or doping) primarily destabilizes the baddeleyite structure, so that the antiferroelectric Pbcn and ferroelectric phases can form.

The thermal equation of states for fcc iridium (Ir) is obtained from first-principles molecular dynamics up to 3000 K and 540 GPa. The equation of state (EoS) is globally fitted to a simplified free energy model and various parameters are derived. The theoretical principal Hugoniot is compared with shockwave experiments, where discrepancy suggests formation of new Ir phases. A few representative EoS parameters, such as bulk modulus $K_T$, thermal expansivity $α$, Grüneisen parameter $γ$, and constant pressure capacity $C_P$, Debye temperature, $Θ_D$ are computed to compare with experimental data

The discovery of more than 4500 extrasolar planets has created a need for modeling their interior structure and dynamics. Given the prominence of iron in planetary interiors, we require accurate and precise physical properties at extreme pressure and temperature. A first-order property of iron is its melting point, which is still debated for the conditions of Earth's interior. We used high-energy lasers at the National Ignition Facility and in situ x-ray diffraction to determine the melting point of iron up to 1000 gigapascals, three times the pressure of Earth's inner core. We used this melting curve to determine the length of dynamo action during core solidification to the hexagonal close-packed (hcp) structure. We find that terrestrial exoplanets with four to six times Earth's mass have the longest dynamos, which provide important shielding against cosmic radiation.

Subduction of oceanic lithosphere transports surface H2O into the mantle. Recent studies show that dense SiO2 in the form of stishovite, an abundant mineral in subducted oceanic crust at depths greater than ∼270 km, has the potential to host and transport a considerable amount of H2O into the lower mantle, but the H2O storage capacity of SiO2 phases at high pressure and temperature remains uncertain. We investigate the hydration of stishovite and its higher-pressure polymorphs, β-stishovite and seifertite, with in situ X-ray diffraction experiments at high pressures and temperatures. The H2O contents in SiO2 phases are quantified based on observed increases in unit cell volume relative to the anhydrous SiO2 system. Density functional theory (DFT) computations permit calibration of water content as a function of volume change based on interstitial substitution of H2O. Regression of our experimental data indicates an H2O storage capacity in stishovite of ∼3.5 wt% in the transition zone and shallow lower mantle, decreasing to about 0.8 wt% at the base of the mantle. We find that SiO2-bearing subducted oceanic crust can accommodate all the H2O in slab lithosphere that survives sub-arc dehydration. Hydration of silica phases in subducted oceanic crust and their unparalleled capacity to host significant amounts of H2O even at high mantle temperatures provides a unique mechanism for transport and storage of water in the deepest mantle.

We examine the thermodynamic and dynamic stability (i.e., phonon dispersion) of mixed silicon–carbon clathrate frameworks using first-principles calculations as a function of pressure and composition. Silicon atoms were substituted on special framework Wyckoff positions in the Type-I and Type-II empty carbon clathrate structures over a broad compositional range, and the enthalpies of the mixed clathrates were compared to pure silicon and carbon clathrates, as well as the thermodynamic ground states. While all mixed clathrates examined were found to be metastable with respect to elemental formation components and/or silicon carbide, certain empty binary host lattices are found to be lower-energy phases than the pure-component clathrate endmembers at high pressure, in particular Type-I C22Si24 and Type-II C32Si104. This enhanced energetic stability is rationalized by a decrease of energy upon doping specific crystallographic positions. When occupied by small guest ions like Li+ and Na+, these mixed C–Si clathrate structures exhibit minima in their formation enthalpies under high-pressure conditions, providing insights into potential synthetic pathways.

We investigate energetically favorable structures of ABO$_2$N oxynitrides as functions of pressure and strain via swarm-intelligence-based structure prediction methods, DFT lattice dynamics and first-principles molecular dynamics. We predict several thermodynamically stable polar oxynitride perovskites under high pressures. In addition, we find that ferroelectric polar phases of perovskite-structured oxynitrides can be thermodynamically stable and synthesized at high pressure on appropriate substrates. The dynamical stability of the ferroelectric oxynitrides under epitaxial strain at ambient pressure also imply the possibility to synthesize them using pulsed laser deposition or other atomic layer deposition methods. Our results have broad implications for further exploration of other oxynitride materials as well. We performed first-principles molecular dynamics and find that the polar perovskite of YSiO$_2$N is metastable up to at least 600 K under compressive epitaxial strain before converting to the stable wollastonite-like structures. YSiO$_2$N is stabilized under pressure with extensional epitaxial strain. We predict that LaSi$_2$N, LaGeO$_2$N, BiSiO$_2$N, and BiGeO$_2$N are metastable as ferroelectric perovskites at zero pressure even without epitaxial strain.

Ferroelectric hafnia is being explored for next generation electronics due to its robust ferroelectricity in nanoscale samples and its compatibility with silicon. However, its ferroelectricity is not understood. Other ferroelectrics usually lose their ferroelectricity for nanoscopic samples and thin films, and the hafnia ground state is non-polar baddeleyite. Here we study hafnia with density functional theory (DFT) under epitaxial strain, and find that strain not only stabilizes the ferroelectric phases, but also leads to unstable modes and a downhill path in energy from the high temperature tetragonal structure. We find that under tensile epitaxial strain η the tetragonal phase will distort to one of the two ferroelectric phases: for η > 1.5%, the Γ5 mode is unstable and leads to Pmn21 , and at η > 3.75% coupling between this mode and the zone boundary M1 mode leads to Pca21 . Furthermore, under compressive epitaxial strain η < 0.55% the ferroelectric Pca21 is most stable, even more stable than baddeleyite. Ferroelectrics are used in electronics as dielectrics and nonvolatile random access memories (NVFRAMs) [1], transistor elements and switches [2–5], but the drive towards smaller electronic devices is a challenge because of the limiting effect of depolarization field, which is inversely proportional to the material thickness [6, 7]. Surprisingly when studying thin hafnia films, Boscke et al. [8] found polarization at the nanoscale. This was surprising because polar phases of hafnia were not known to be stable, and furthermore the depolarizing field was expected to quench polarization in nanoscale ferroelectrics. The recent stabilization of ferroelecticity in bulk yttrium-doped hafnia [9, 10], opens up the possibility of mesoscopic hafnia-based FE devices, thus the ubiquity of hafnia in future electronics. Early interest in HfO2 stemmed from its refractory and dielectric properties. Its ability to withstand high temperature, combined with its large dielectric permittivity relative to SiO2, motivated several studies to focus on the use of hafnia as a replacement to SiO2 as a gate dielectric in Complementary Metal-Oxides-Semiconductors (CMOS) [11, 12]. Hafnia is isomorphous to zirconia (ZrO2) [13, 14] and is non-polar, with the monoclinic P21/c baddeleyite structure at ambient conditions in bulk. At high temperatures it transforms to tetragonal P42nmc at T > 2000K and cubic Fm3̄m fluorite at T > 2870K. Previous DFT studies include Refs. 15 and 16, who proposed Pmn21 or Pca21 as possible polar crystal structures of hafnia, and other studies focused on how to stabilize polar phases, such as the importance of dopants [17, 18], or kinetics and growth processes [9, 19, 20]. Maxima were found in the transition paths at zero stress between different polymorphs [19] suggesting that the ferroelectric phase is metastable and formed kinetically. With inplane shear-strain (111)-oriented P42nmc hafnia becomes unstable, and relaxes to the polar (Pmn21) structure [20] . From the high temperature fluorite structure a zone boundary mode at X leads to the P42nmc , similar to zirconia [21]. Coupling between the zone center with the zone boundary phonon modes leads from fluorite to the antipolar Pbca structure [22]. We will show in this work that the tetragonal P42nmc structure can transform to the polar Pca21 or Pmn21 phases (Fig. S1 in the Supplemental Material[23]). There are two different Pbca structures, with 24 atoms in the primitive cell. One is denoted “oI” and the other is brookite. These two structures have different energy when strain-free (Table I), and respond differently to epitaxial strain (Fig. 4). We find that brookite is denser than oI, and is a candidate high pressure phase. We performed first-principles calculations using Quantum Espresso [24–26], with optimized norm-conserving Vanderbilt (ONCV) pseudopotentials [27]. The atomic positions

Carbon-based frameworks composed of sp3 bonding represent a class of extremely lightweight strong materials, but only diamond and a handful of other compounds exist despite numerous predictions. Thus, there remains a large gap between the number of plausible structures predicted and those synthesized. We used a chemical design principle based on boron substitution to predict and synthesize a three-dimensional carbon-boron framework in a host/guest clathrate structure. The clathrate, with composition 2Sr@B6C6, exhibits the cubic bipartite sodalite structure (type VII clathrate) composed of sp3-bonded truncated octahedral C12B12 host cages that trap Sr2+ guest cations. The clathrate not only maintains the robust nature of diamond-like sp3 bonding but also offers potential for a broad range of compounds with tunable properties through substitution of guest atoms within the cages.

We measure the electrical resistivity of hcp iron up to ∼170  GPa and ∼3000  K using a four-probe van der Pauw method coupled with homogeneous flattop laser heating in a DAC, and compute its electrical and thermal conductivity by first-principles molecular dynamics including electron-phonon and electron-electron scattering. We find that the measured resistivity of hcp iron increases almost linearly with temperature, and is consistent with our computations. The results constrain the resistivity and thermal conductivity of hcp iron to ∼80±5  μΩ cm and ∼100±10  W m^{-1} K^{-1}, respectively, at conditions near the core-mantle boundary. Our results indicate an adiabatic heat flow of ∼10±1  TW out of the core, supporting a present-day geodynamo driven by thermal and compositional convection.