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Quantum Monte Carlo (QMC) methods are useful for studies of strongly correlated materials because they are many body in nature and use the physical Hamiltonian. Typical calculations assume as a starting point a wave function constructed from single-particle orbitals obtained from one-body methods, e.g., density functional theory. However, mean-field-derived wave functions can sometimes lead to systematic QMC biases if the mean-field result poorly describes the true ground state. Here, we study the accuracy and flexibility of QMC trial wave functions using variational and fixed-node diffusion QMC estimates of the total spin density and lattice distortion of antiferromagnetic iron oxide (FeO) in the ground state $B1$ crystal structure. We found that for relatively simple wave functions the predicted lattice distortion was controlled by the choice of single-particle orbitals used to construct the wave function, rather than by subsequent wave function optimization techniques within QMC. By optimizing the orbitals with QMC, we then demonstrate starting-point independence of the trial wave function with respect to the method by which the orbitals were constructed by demonstrating convergence of the energy, spin density, and predicted lattice distortion for two qualitatively different sets of orbitals. The results suggest that orbital optimization is a promising method for accurate many-body calculations of strongly correlated condensed phases.

The amount of heat transport from the core, which constrains the dynamics and thermal evolution of the region, depends on the transport properties of iron. Ohta et al.(2016) and Konopkova et al.(2016) measured electrical resistivity and thermal conductivity of iron, respectively, in laser-heated diamond anvil cells (DACs) at relevant Earth's core pressure-temperature (P-T) conditions, and obtained dramatically contradictory results. Here we measure the electrical resistivity of hcp-iron up to ~170 GPa and ~3,000 K using a four-probe van der Pauw method coupled with homogeneous flat-top laser-heating in a DAC. We also compute its electrical and thermal conductivity by first-principles methods including electron-phonon and electron-electron scattering. We find that the measured resistivity of hcp-iron increases almost linearly with increasing temperature, and is consistent with current first-principles computations. The proportionality coefficient between resistivity and thermal conductivity (the Lorenz number) in hcp-iron differs from the ideal value (2.44*10^-8 W Omega K^-2), so a non-ideal Lorenz number of ~(2.0-2.1)*10^-8 W Omega K^-2 is used to convert the experimental resistivity to the thermal conductivity of hcp-Fe at high P-T. The results constrain the resistivity and thermal conductivity of hcp-iron to ~80(5) u Omega cm and ~100(10) W/mK, respectively, at conditions near core-mantle boundary. Our results indicate an adiabatic heat flow of ~10(1) TW through the core-mantle boundary for a liquid Fe alloy outer core, supporting a present-day geodynamo driven by thermal convection through the core's secular cooling and by compositional convection through the latent heat and gravitational energy during the inner core's solidification., Comment: 39 pages, 14 figures

Using first-principles calculations, we predict a lightweight room-temperature ferroelectric carbon-boron framework in a host/guest clathrate structure. This ferroelectric clathrate, with composition ScB$_3$C$_3$, exhibits high polarization density and low mass density compared with widely used commercial ferroelectrics. Molecular dynamics simulations show spontaneous polarization with a moderate above-room-temperature T$_c$ of $\sim$370 K, which implies large susceptibility and possibly large electrocaloric and piezoelectric constants at room temperature. Our findings open the possibility for a new class of ferroelectric materials with potential across a broad range of applications., 5 pages 4 figures

We study the aging and Mn doping effect on third generation lead based relaxor single crystals. We measured the polarization (PE) and strain with applied field on two perpendicular orientations of ...

Pressure-induced phase transitions in single-crystal PbZr0.54Ti0.46O3 are investigated with high-pressure Raman scattering and x-ray single crystal and powder diffraction. The appearance of a Raman peak near 380cm(-1) indicates a structural transition at 3 GPa. A second transition, driven by an soft optical phonon, occurs at 9 GPa. A third transition occurs above 27 GPa, accompanied by a large changes in the Raman spectrum and splitting of the (pseudo-cubic) (111) and (220) diffraction lines. We identify the transitions as a monoclinic (Cm) to rhombohedral (R3m) transition at 3 GPa, followed by a rhombohedral (R3m) to rhombohedral (R-3c) transition at 9 GPa, and a further symmetry-lowering transition at 27 GPa.

Varying the substrate temperature changes structural and magnetic properties of spinel ferrite Ni0.65Zn0.35Fe2O4 (NZFO) thin films. X-ray diffraction of films grown at different temperature display only (004) reflections, without any secondary peaks, showing growth orientation along the c-axis. We find an increase in crystalline quality of these thin films with the rise of substrate temperature. The surface topography of thin films grown at different growth temperatures reveal that these films are smooth with low roughness; however, the thin films grown at 800 °C exhibit lowest average and root mean square (rms) roughness among all thin films. We find iron and nickel to be more oxidized (greater Fe3+ and Ni3+ content) in films grown and annealed at 700 °C and 800 °C, compared to those films grown at lower temperatures. The magnetic moment is observed to increase with an increase of substrate temperature and all thin films possess high saturation magnetization and low coercive field at room temperature. Films grown at 800 °C exhibit a ferrimagnetic–paramagnetic phase transition well above room temperature. The observed large magnetizations with soft magnetic behavior in NZFO thin films above room temperature suggest potential applications in memory, spintronics, and multifunctional devices.

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.

International audience; To improve our understanding of the Earth's global carbon cycle, it is critical to characterize the distribution and storage mechanisms of carbon in silicate melts. Presently, the carbon budget of the deep Earth is not well constrained and is highly model-dependent. In silicate melts of the uppermost mantle, carbon exists predomi nantly as molecular carbon dioxide and carbonate, whereas at greater depths, carbon forms complex polymer ized species. The concentration and speciation of carbon in silicate melts is intimately linked to the melt's composition and affects its physical and dynamic properties. Here we review the results of experiments and calculations on the solubility and speciation of carbon in silicate melts as a function of pressure, temperature, composition, polymerization, water concentration, and oxygen fugacity. 16 16.1. INTRODUCTION Evidence of carbon-bearing phases in the Earth's mantle includes the release of CO 2 in volcanic eruptions, dissolved CO 2 in magmatic glasses and glass inclusions (Mörner & Etiope, 2002), diamonds and carbonate minerals in mantle xenoliths (Eggler, 1987; Sobolev & Shatsky, 1990), and the existence of carbonatite and kimberlite magmas (Wyllie et al., 1990). There are two possible sources of carbon: primordial carbon and the carbon delivered by later come tary and asteroid bombardment. Primordial carbon existed in the proto-Earth and subsequently survived the moon-forming impact, and its amount is currently unknown. From all existent carbon, part of it might be locked in the core, part in some deep mantle reservoir, another fraction lies at the surface, and the remaining is resurfaced after surviving subduction. To determine how much carbon may have remained in the Earth after the giant impact, the chemistry and thermodynamics of carbon in silicate (particularly, with the bulk silicate Earth composition) must be determined as a function of pressure and temper ature. Estimates of the amount of carbon exchanged between the surface and the mantle range between 30 and 130 megatons per year, and estimates of the carbon concentration stored within the core range between 0.2 and 4 wt.% (McDonough, 2003; Mookherjee et al., 2011; Wood, 1993). Carbon is mainly subducted into the Earth in the form of carbonates within metasomatically calcium-enriched basaltic rock, calcified serpentinites, and sedi mentary carbonaceous ooze of the seafloor (Brenker et al.

We study the high-pressure structures of ${\mathrm{SrB}}_{6}$ up to 200 GPa using first-principles structure prediction calculations and high-pressure x-ray diffraction experiments. The computations show that the ambient-pressure cubic phase transforms to an orthorhombic structure $(Cmmm)$ at 48 GPa, and then to a tetragonal structure $(I4/mmm)$ at 60 GPa. The high-pressure experiments are consistent with the theoretically predicted tetragonal structure, which was quenched successfully to ambient conditions. Pressure induces simple boron octahedra to form complex networks in which the electrons are delocalized, leading to metallic ground states with large density of states at the Fermi level. Calculated stress-strain relations for the $I4/mmm$ structure of ${\mathrm{SrB}}_{6}$ demonstrate its intrinsic hard nature with an estimated Vickers hardness of 15 GPa, and reveal a novel deformation mechanism with transient multicenter bonding that results in the combination of high strength and high ductility. Our findings offer valuable insights for understanding the rich and complex crystal structures of ${\mathrm{SrB}}_{6}$, which have broad implications for further explorations of hexaboride materials.

The realization of a novel guest/host carbon-boron sp3 framework opens the door for a new class of 3D carbon-based materials., 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.

The prediction of reaction pathways for solid-solid transformations remains a key challenge. Here, we develop a pathway sampling method via swarm intelligence and graph theory, and demonstrate that our PALLAS method is an effective tool to help understand phase transformations in solid-state systems. The method is capable of finding low-energy transition pathways between two minima without having to specify any details of transition mechanism a priori. We benchmarked our PALLAS method against known phase transitions in cadmium selenide (CdSe) and silicon (Si). PALLAS readily identifies previously-reported, low-energy phase transition pathways for the wurtzite to rock-salt transition in CdSe and reveals a novel lower-energy pathway that has not yet been observed. In addition, PALLAS provides detailed information that explains the complex phase transition sequence observed during the decompression of Si from high pressure. Given the efficiency to identify low-barrier-energy reaction pathways, the PALLAS methodology represents a promising tool for materials by design with valuable insights for novel synthesis., 8 pages, 5 figures

The power conversion efficiency for solar cells fabricated using organometal halide perovskites (OMHPs) has risen to more than 20% in a short span of time, making OMHPs promising solar materials for harnessing energy from sunlight. The hybrid perovskite architecture that consists of organic molecular cations and an inorganic lattice could also potentially serve as a robust platform for materials design to realize functionalities beyond photovoltaic applications. Taking methylammonium lead iodide (MAPbI3) as an example, we explore the response of organometal halide perovskites to various stimuli, using all-atom molecular dynamics simulations with a first-principles-based interatomic potential. We find that a large electric field is necessary to introduce a sizable molecular ordering at room temperature in unstrained MAPbI3. Molecular dipoles in epitaxially strained MAPbI3 are more susceptible to an electric field. We also report various caloric effects in MAPbI3. The adiabatic thermal change is estimated d...

We compute the thermal conductivity and electrical resistivity of solid hcp Fe to pressures and temperatures of Earth's core. We find significant contributions from electron-electron scattering, usually neglected at high temperatures in transition metals. Our calculations show a quasi-linear relation between electrical resistivity and temperature for hcp Fe at extreme high pressures. We obtain thermal and electrical conductivities that are consistent with experiments considering reasonable error. The predicted thermal conductivity is reduced from previous estimates that neglect electron-electron scattering. Our estimated thermal conductivity for the outer core is 77$\pm$10 W/m/K, and is consistent with a geodynamo driven by thermal convection., Comment: 6 pages, 6 figures (Supplemental material: 7 pages, 4 figures, 6 tables)

We study Mn substitution for Ti in BaTiO3 with and without compensating oxygen vacancies using density functional theory (DFT) in combination with dynamical mean field theory (DMFT). We find strong charge and spin fluctuations. Without compensating oxygen vacancies, the ground state is found to be a quantum superposition of two distinct atomic valences, 3d4 and 3d5. Introducing a compensating oxygen vacancy at a neighboring site reduces both charge and spin fluctuations due to the reduction of electron hopping from Mn to its ligands. As a consequence, valence fluctuations are reduced, and is closely fixed to the high spin 3d5 state. Here we show that inclusion of charge and spin fluctuations is necessary to obtain an accurate ground state of transition metal doped ferroelectrics., accepted in Phys. Rev. B

We study the electrocaloric effect in the classic ferroelectric BaTiO3 through a series of phase transitions driven by applied electric field and temperature. We find both negative and positive electrocaloric effects, with the negative electrocaloric effect, where temperature decreases with applied field, in monoclinic phases. Macroscopic polarization rotation is evident through the monoclinic and orthorhombic phases under applied field, and is responsible for the negative electrocaloric effect.

Piezoelectrics with negative longitudinal piezoelectric coefficients will contract in the direction of an applied electric field. Such piezoelectrics are thought to be rare, but there is no fundamental physics preventing the realization of negative longitudinal piezoelectric effect in a single-phase material. Using first-principles calculations, we demonstrate that several hexagonal ABC ferroelectrics possess significant negative longitudinal piezoelectric effects. The data mining of a first-principles-based database of piezoelectrics reveals that this effect is a general phenomenon. The origin of this unusual piezoelectric response relies on the strong ionic bonds associated with small effective charges and rigid potential energy surfaces. Moreover, ferroelectrics with negative longitudinal piezoelectric coefficients show anomalous pressure-enhanced ferroelectricity. Our results offer design principles to aid the search for new piezoelectrics for novel electromechanical device applications.

Ferroelectrics usually adopt a multidomain state with domain walls separating domains with polarization axes oriented differently. It has long been recognized that domain walls can dramatically impact the properties of ferroelectric materials. The enhancement of low-field susceptibility/permittivity under subswitching conditions is usually attributed to reversible domain wall vibration. Recent experiments highlight the stationary domain wall contribution to the dielectric susceptibility irrespective of any lateral displacements or deformations of the wall. We study the effects of domain walls on the low-field permittivity of ${\mathrm{PbTiO}}_{3}$ with density functional theory and molecular dynamics simulations. The static dielectric constant is calculated as a function of increasing domain wall density and temperature. We find an increase of dielectric permittivity with increasing domain wall density, which is expected to occur at a low driving field where the lateral motion of domain walls is forbidden. Real-space decomposition of the dielectric response reveals that frustrated dipoles within the finite width of the domain walls are responsible for the enhanced low-field permittivity. We explain the $100%$ enhancement of the dielectric susceptibility form domain walls, which arises from the softer potential wells within them.

Raman spectroscopy of dense hydrogen and deuterium performed to 325 GPa at 300 K reveals previously unidentified transitions. Detailed analysis of the spectra from multiple experimental runs, together with comparison with previous infrared and Raman measurements, provides information on structural modifications of hydrogen as a function of density through the I-III-IV transition sequence, beginning near 200 GPa at 300 K. The data suggest that the transition sequence at these temperatures proceeds by formation of disordered stacking of molecular and distorted layers. Weaker spectral changes are observed at 250, 285, and 300 GPa, that are characterized by discontinuities in pressure shifts of Raman frequencies, and changes in intensities and linewidths. The results indicate changes in structure and bonding, molecular orientational order, and electronic structure of dense hydrogen at these conditions. The data suggest the existence of new phases, either variations of phase IV, or altogether new structures.

An organic polymer exhibits a phase transition that is associated with improved electromechanical properties. This feature links organic polymers with widely used perovskite materials, and could have many applications. Behaviour akin to a morphotropic phase boundary in a ferroelectric polymer.

Molecular dynamics simulations predict a giant electrocaloric effect at the ferroelectric-antiferroelectric phase boundary in PZT (PbTiO3-PbZrO3). These large-scale simulations also give insights into the atomistic mechanisms of the electrocaloric effect in Pb(ZrxTi1–x)O3. We predict a positive electrocaloric effect in ferroelectric PZT, but antiferroelectric PZT exhibits a negative-to-positive crossover with the increasing temperature or electric field. At the antiferroelectric-to-ferroelectric phase boundary, we find complex domain patterns. We demonstrate that the origin of the giant electrocaloric change of temperature is related to the easy structural response of the dipolar system to external stimuli in the transition region.

Theoretical calculations and an assessment of recent experimental results for dense solid hydrogen lead to a unique scenario for the metallization of hydrogen under pressure. The existence of layered structures based on graphene sheets gives rise to an electronic structure related to unique features found in graphene that are well studied in the carbon phase. The honeycombed layered structure for hydrogen at high density, first predicted in molecular calculations, produces a complex optical response. The metallization of hydrogen is very different from that originally proposed via a phase transition to a close-packed monoatomic structure, and different from simple metallization recently used to interpret recent experimental data. These different mechanisms for metallization have very different experimental signatures. We show that the shift of the main visible absorption edge does not constrain the point of band gap closure, in contrast with recent claims. This conclusion is confirmed by measured optical spectra, including spectra obtained to low photon energies in the infrared region for phases III and IV of hydrogen.

We provide a fundamental insight into the microscopic mechanisms of the ageing processes. Using large scale molecular dynamics simulations of the prototypical ferroelectric material PbTiO3, we demonstrate that the experimentally observed ageing phenomena can be reproduced from intrinsic interactions of defect-dipoles related to dopant-vacancy associates, even in the absence of extrinsic effects. We show that variation of the dopant concentration modifies the material's hysteretic response. We identify a universal method to reduce loss and tune the electromechanical properties of inexpensive ceramics for efficient technologies., Comment: 6 pages, 3 figures

The role of defects in solids of mixed ionic-covalent bonds such as ferroelectric oxides is complex. Current understanding of defects on ferroelectric properties at the single-defect level remains mostly at the empirical level, and the detailed atomistic mechanisms for many defect-mediated polarization-switching processes have not been convincingly revealed quantum mechanically. We simulate the polarization-electric field (P-E) and strain-electric field ({$\epsilon$}-E) hysteresis loops for BaTiO3 in the presence of generic defect dipoles with large-scale molecular dynamics and provide a detailed atomistic picture of the defect dipole-enhanced electromechanical coupling. We develop a general first-principles-based atomistic model, enabling a quantitative understanding of the relationship between macroscopic ferroelectric properties and dipolar impurities of different orientations, concentrations, and dipole moments. We find that the collective orientation of dipolar defects relative to the external field is the key microscopic structure feature that strongly affects materials hardening/softening and electromechanical coupling. We show that a small concentration (0.1 at.%) of defect dipoles dramatically improves electromechanical response. This offers the opportunity to improve the performance of inexpensive polycrystalline ferroelectric ceramics through defect dipole engineering for a range of applications including piezoelectric sensors, actuators, and transducers.

Ferroelectric perovskite oxides possess a large electrocaloric (EC) effect, but usually at high temperatures near the ferroelectric/paraelectric phase transition temperature, which limits their potential application as next-generation solid-state cooling devices. We use classical molecular dynamics to study the electric field induced phase transitions and EC effect in PMN-PT (PbMg1/3Nb2/3O3-PbTiO3). We find that the maximum EC strength of PMN-PT occurs within the morphotropic phase boundary (MPB) region at 300 K. The large adiabatic temperature change is caused by easy rotation of polarization within the MPB region., Comment: 17 pages, 7 figures

We investigated the stability and mechanical and electronic properties of 15 metastable mixed $s{p}^{2}\text{\ensuremath{-}}s{p}^{3}$ carbon allotropes in the family of interpenetrating graphene networks (IGNs) using density functional theory (DFT). IGN allotropes exhibit nonmonotonic bulk and linear compressibilities before their structures irreversibly transform into new configurations under large hydrostatic compression. The maximum bulk compressibilities vary widely between structures and range from 3.6 to 306 ${\mathrm{TPa}}^{\ensuremath{-}1}$. We find all the IGN allotropes have negative linear compressibilities with maximum values varying from --0.74 to $--133{\mathrm{TPa}}^{\ensuremath{-}1}$. The maximal negative linear compressibility of Z33 ($--133{\mathrm{TPa}}^{\ensuremath{-}1}$ at 3.4 GPa) exceeds previously reported values at pressures higher than 1.0 GPa. IGN allotropes can be classified as either armchair or zigzag type, and these two types of IGNs exhibit different electronic properties. Zigzag-type IGNs are node-line semimetals, while armchair-type IGNs are either semiconductors or node-loop or node-line semimetals. Experimental synthesis of these IGN allotropes might be realized since their formation enthalpies relative to graphite are only 0.1--0.5 eV/atom (that of ${\mathrm{C}}_{60}$ fullerene is about 0.4 eV/atom), and energetically feasible binary compound pathways are possible.

We found an error of a factor of two that is due to our neglect of spin degeneracy (two electrons per band), which would halve the electron–electron resistivity and probably make the electron–electron scattering insignificant for the geodynamo, at least for pure iron. We therefore wish to retract this Letter.

Silica (SiO 2 ) is an abundant component of the Earth whose crystalline polymorphs play key roles in its structure and dynamics. First principle density functional theory (DFT) methods have often been used to accurately predict properties of silicates, but fundamental failures occur. Such failures occur even in silica, the simplest silicate, and understanding pure silica is a prerequisite to understanding the rocky part of the Earth. Here, we study silica with quantum Monte Carlo (QMC), which until now was not computationally possible for such complex materials, and find that QMC overcomes the failures of DFT. QMC is a benchmark method that does not rely on density functionals but rather explicitly treats the electrons and their interactions via a stochastic solution of Schrödinger’s equation. Using ground-state QMC plus phonons within the quasiharmonic approximation of density functional perturbation theory, we obtain the thermal pressure and equations of state of silica phases up to Earth’s core–mantle boundary. Our results provide the best constrained equations of state and phase boundaries available for silica. QMC indicates a transition to the dense α -PbO 2 structure above the core-insulating D” layer, but the absence of a seismic signature suggests the transition does not contribute significantly to global seismic discontinuities in the lower mantle. However, the transition could still provide seismic signals from deeply subducted oceanic crust. We also find an accurate shear elastic constant for stishovite and its geophysically important softening with pressure.

Synchrotron infrared spectroscopy on sodium shows a transition from a high reflectivity, nearly free-electron metal to a low-reflectivity, poor metal in an orthorhombic phase at 118 GPa. Optical spectra calculated within density functional theory (DFT) agree with the experimental measurements and predict a gap opening in the orthorhombic phase at compression beyond its stability field, a state that would be experimentally attainable by appropriate choice of pressure-temperature path. We show that a transition to an incommensurate phase at 125 GPa results in a partial recovery of good metallic character up to 180 GPa, demonstrating the strong relationship between structure and electronic properties in sodium.

We investigate by first-principles calculations the effect of ferrous iron, Fe 2+ , on the structure and the equation of state of MgSiO 3 post-perovskite. We find that ferrous iron is high-spin over the pressure range of the mantle assuming a ferromagnetic structure. The bulk modulus and the specific volume increase with the addition of ferrous iron to MgSiO 3 . We find that Fe partitions preferentially to post-perovskite and broadens the two-phase pressure range.

A piezoelectric material is one that generates a voltage in response to a mechanical strain (and vice versa). The most useful piezoelectric materials display a transition region in their composition phase diagrams, known as a morphotropic phase boundary, where the crystal structure changes abruptly and the electromechanical properties are maximal. As a result, modern piezoelectric materials for technological applications are usually complex, engineered, solid solutions, which complicates their manufacture as well as introducing complexity in the study of the microscopic origins of their properties. Here we show that even a pure compound, in this case lead titanate, can display a morphotropic phase boundary under pressure. The results are consistent with first-principles theoretical predictions, but show a richer phase diagram than anticipated; moreover, the predicted electromechanical coupling at the transition is larger than any known. Our results show that the high electromechanical coupling in solid solutions with lead titanate is due to tuning of the high-pressure morphotropic phase boundary in pure lead titanate to ambient pressure. We also find that complex microstructures or compositions are not necessary to obtain strong piezoelectricity. This opens the door to the possible discovery of high-performance, pure-compound electromechanical materials, which could greatly decrease costs and expand the utility of piezoelectric materials.

The lithium-carbon binary system possesses a broad range of chemical compounds, which exhibit fascinating chemical bonding characteristics, which give rise to diverse and technologically important properties. While lithium carbides with various compositions have been studied or suggested previously, the crystal structures of these compounds are far from well understood. In this work, we present the first comprehensive survey of all ground state (GS) structures of lithium carbides over a broad range of thermodynamic conditions, using ab initio density functional theory (DFT) crystal structure searching methods. Thorough searches were performed for 29 stoichiometries ranging from ${\mathrm{Li}}_{12}\mathrm{C}$ to ${\mathrm{LiC}}_{12}$ at 0 and 40 GPa. Based on formation enthalpies from optimized van der Waals density functional calculations, three thermodynamically stable phases $({\mathrm{Li}}_{4}{\mathrm{C}}_{3}$, ${\mathrm{Li}}_{2}{\mathrm{C}}_{2}$, and ${\mathrm{LiC}}_{12})$ were identified at 0 GPa, and seven thermodynamically stable phases $({\mathrm{Li}}_{8}\mathrm{C}$, ${\mathrm{Li}}_{6}\mathrm{C}$, ${\mathrm{Li}}_{4}\mathrm{C}$, ${\mathrm{Li}}_{8}{\mathrm{C}}_{3}$, ${\mathrm{Li}}_{2}\mathrm{C}$, ${\mathrm{Li}}_{3}{\mathrm{C}}_{4}$, and ${\mathrm{Li}}_{2}{\mathrm{C}}_{3})$ were predicted at 40 GPa. A rich diversity of carbon bonding, including monomers, dimers, trimers, nanoribbons, sheets, and frameworks, was found within these structures, and the dimensionality of carbon connectivity existing within each phase increases with increasing carbon concentration. We find that the well-known composition ${\mathrm{LiC}}_{6}$ is actually a metastable one. We also find a unique coexistence of carbon monomers and dimers within the predicted thermodynamically stable phase ${\mathrm{Li}}_{8}{\mathrm{C}}_{3}$, and different widths of carbon nanoribbons coexist in a metastable phase of ${\mathrm{Li}}_{2}{\mathrm{C}}_{2}$ (Imm2). Interesting mixed ${\mathit{sp}}^{2}\text{\ensuremath{-}}{\mathit{sp}}^{3}$ carbon frameworks are predicted in metastable phases with composition ${\mathrm{LiC}}_{6}$.

Physical review / B 95(21), 214120 (2017). doi:10.1103/PhysRevB.95.214120, For decades, numerous attempts have been made to produce polar oxynitride perovskites, where some of the oxygen is replaced by nitrogen, but a polar ordered oxynitride has never been demonstrated. Caracas and Cohen [Appl. Phys. Lett. 91, 092902 (2007)] studied possible ordered polar oxynitrides within density-functional theory (DFT) and found a few candidates that were predicted to be insulating and at least metastable. $YSiO_{2}N$ stood out with huge predicted polarization and nonlinear optic coefficients. In this study, we demonstrate the synthesis of perovskite-structured $YSiO_{2}N$ by using a combination of a diamond-anvil cell and in situ laser-heating techniques. Subsequent in situ x-ray diffraction, second-harmonic generation, and Raman-scattering measurements confirm that it is polar and a strong nonlinear optical material, with structure and properties similar to those predicted by DFT., Published by APS, Woodbury, NY

A complete set of elastic and piezoelectric constants for single-domain rhombohedral Pb(Zn1/3Nb2/3)O3 4.5%PbTiO3 is obtained using Brillouin scattering. The bulk modulus and elastic constants agree with the values obtained from ultrasonic methods, but the piezoelectric constants are smaller. Differences in piezoelectric constants from different techniques are due to frequency dispersion and the contributions of domain boundaries. The pressure dependence of the Brillouin shifts of amorphous BeH2 was measured from ambient pressure to 17 GPa. The equation of state is deduced from the pressure dependence of the sound velocity; the bulk modulus is 14.2 (±3.0) GPa and its pressure derivative is 5.3 (±0.5).

We report first principles density functional calculations of the Born effective charges and electronic dielectric tensors for the relaxor PMN (PbMg1/3Nb2/3O3). Visualization of the Born charge tensors as charge ellipsoids have provided microscopic insights on the factors governing piezoelectric enhancements with polarization rotation. Several 15 and 30-atom ferroelectric and antiferroelectric supercells of PMN involving 1:2 and 1:1 chemical ordering have been studied. A cascading set of ferroelectric phonon instabilities lead to several low symmetry monoclinic structures. We find a ground state with a 15-atom unit cell with 1:2 chemical ordering along [111] with a monoclinic C2 structure., 12 pages, 4 figures and 5 animations. Animations available from Elsevier Gateway, Computational Materials Science Site online publications. Computational Materials Science (In Press). Computational Materials Science (In Press)

We have used first-principles linear response calculations and molecular dynamics to study the relaxor ferroelectric Pb(Mg 1/3 Nb 2/3 )O 3 -xPbTiO 3 (PMN-x PT). First-principles calculations for ordered PMN show a low-symmetry monoclinic ground state. A new set of phase transitions to lower symmetry rhombohedral and monoclinic structures is predicted for PT at high pressures, showing predicted piezoelectric response as high as is seen in the giant coupling relaxor ferroelectrics. We have developed a transferable shell-model potential for PMN-x PT by fitting to first-principles data. The potential qualitatively reproduces the compositional phase diagram for PMN-x PT. A new exchange-correlation potential, which gives excellent predictions for ferroelectric materials, is discussed.

We develop a many-body force field (FF) for tantalum based on extensive ab initio quantum mechanical (QM) calculations and illustrate its application with molecular dynamics (MD). As input data to the FF we use ab initio methods (LAPW-GGA) to calculate: (i) the zero temperature equation of state (EOS) of Ta for bcc, fcc, and hcp crystal structures for pressures up to ~500 GPa, and (ii) elastic constants. We use a mixed-basis pseudopotential code to calculate: (iii) volume-relaxed vacancy formation energy also as a function of pressure. In developing the Ta FF we also use previous QM calculations of: (iv) the EOS for the A15 structure; (v) the surface energy bcc (100); (vi) energetics for shear twinning of the bcc crystal. We find that, with appropriate parameters, an embedded atom model FF (denoted as qEAM FF) is able to reproduce all this QM data. We illustrate the use of the qEAM FF with MD to calculate such finite temperature properties as the melting curve up to 300 GPa and thermal expansivity in a wide temperature range. Both our predictions agree well with experimental values.

Magnetism in iron plays a central role in understanding the physical properties of its polymorphs, including the close-packed high pressure phases. We explore the rich and complex magnetic structures of these phases in two ways. We use a first-principles based, magnetic tight-binding total energy model to study non-collinear magnetic structures, and an all-electron method to study the collinear state in hcp iron that we predict in the hcp iron stability range. For the non-collinear study we compute the magnetization energy and moments for various non-collinear ordered spin configurations. For fcc iron we find non-collinear structures with a wavevector (0,0,q) with q close to 0.5 to be energetically stable, in agreement with previous first-principles calculations. In the high pressure stability field of hcp iron we find a stable collinear antiferromagnetic structure (afmII), previously predicted with an all-electron method. We further investigate the afmII structure, computing physical properties from first principles that support the notion of antiferromagnetic correlations in hcp iron. We show that a recently observed anomalous splitting in Raman spectra of hcp iron under compression can be quantitatively explained by spin?phonon interactions. To address the absence of M?ssbauer splitting in experiments on hcp iron we have also calculated the hyperfine field of afmII iron and find it to be so small that the predicted splitting would be smaller than the resolution limit of experiments.

We have developed a shell model potential to describe PbTiO3 and PbMg1/3Nb2/3O3 (PMN) by fitting to first-principles results. At zero pressure, the model reproduces the temperature behavior of PbTiO3, but with a smaller transition temperature than experimentally observed. We then fit a shell model potential for the complex PMN based on the transferability of the interatomic potentials. We find that even for ordered PMN, quenching the structure gives a non-polar state, but with local polarization (off-center ions) indicative of relaxor behavior.

The magnetic state of hexagonal close-packed iron has been the subject of debate for more than three decades. Although Mössbauer measurements find no evidence of the hyperfine splitting that can signal the presence of magnetic moments, density functional theory predicts an antiferromagnetic (afm) ground state. This discrepancy between theory and experiment is now particularly important because of recent experimental findings of anomalous splitting in the Raman spectra and the presence of superconductivity in hexagonal close-packed iron, which may be caused by magnetic correlations. Here, we report results from first principles calculations on the previously predicted theoretical collinear afm ground state that strongly support the presence of afm correlations in hexagonal close-packed iron. We show that anomalous splitting of the Raman mode can be explained by spin–phonon interactions. Moreover, we find that the calculated hyperfine field is very weak and would lead to hyperfine splitting below the resolution of Mössbauer experiments.

Using a mixed basis pseudopotential method, total energy calculations were performed to obtain the enthalpy of vacancy formation in Ta as a function of pressure, which is important for understanding the effects of pressure on mechanical properties. The vacancy formation enthalpy is found to increase from 2.95 eV at ambient pressures to 12.86 eV at 300 GPa, and the vacancy formation volume decreases from being 53 ± 5% of the bulk volume per atom at ambient pressure to 20 ± 2% at 300 GPa, for a 54-atom supercell. We also show that there is a strong correspondence between the vacancy formation enthalpy and the melting temperature in Ta.

The high-pressure behavior of rocksalt-structured FeO has been investigated using the LDA + U method, a first-principles computational technique that allows treatment of correlated electrons with strong localized repulsions. Within the local density approximation (LDA) FeO is predicted to be a metal, but with LDA + U , an insulating state is obtained at zero pressure. Electronic and magnetic behavior, the equation of state, and lattice strain are determined for three values of the Coulomb repulsion U . We find two self-consistent solutions, one with rhombohedral and one with monoclinic electronic symmetry. For U = 4.6 eV, the monoclinic solution becomes more stable than the rhombohedral solution at 110 GPa, leading to an insulator-metal transition; with increasing U , metallization occurs at higher pressures. Results from the LDA + U calculation suggest that the high-spin magnetic state should persist to pressures greater than 300 GPa. The method gives improved agreement with experiments for ground state properties as compared to LDA and GGA methods that do not explicitly include a local Coulomb repulsion.

We report an accurate study of interactions between Benzene molecules using variational quantum Monte Carlo (VMC) and diffusion quantum Monte Carlo (DMC) methods. We compare these results with density functional theory (DFT) using different van der Waals (vdW) functionals. In our QMC calculations, we use accurate correlated trial wave functions including three-body Jastrow factors, and backflow transformations. We consider two benzene molecules in the parallel displaced (PD) geometry, and find that by highly optimizing the wave function and introducing more dynamical correlation into the wave function, we compute the weak chemical binding energy between aromatic rings accurately. We find optimal VMC and DMC binding energies of -2.3(4) and -2.7(3) kcal/mol, respectively. The best estimate of the CCSD(T)/CBS limit is -2.65(2) kcal/mol [E. Miliordos et al, J. Phys. Chem. A 118, 7568 (2014)]. Our results indicate that QMC methods give chemical accuracy for weakly bound van der Waals molecular interactions, comparable to results from the best quantum chemistry methods., Comment: Accepted for publication in the Journal of Chemical Physics, Vol. 143, Issue 11, 2015

We have carried out molecular dynamics simulations of MgSiO 3 perovskite (pv) in order to determine its equation of state and elastic parameters under pressure and temperature conditions of the Earth’s lower mantle. Combining this information with density and elastic constants for (Mg,Fe)O magnesiowustite (mw) and including the effects of iron on the density of perovskite, we investigated possible compositional models for the lower mantle by calculating seismic velocities and densities as functions of depth and comparing them to the globally averaged seismological models ak135-f and PREM. The calculations were done along several possible geothermal gradients, both adiabatic and non-adiabatic, and for a range of Mg–Fe partitioning coeffcients, K Fe–Mg pv–mw =0.10–0.35. Our results indicate that the most probable compositional models for the lower mantle are pyrolitic, with X Mg ≈0.87 and X pv ≈0.54. For increasing temperatures at the top of the lower mantle, the best fitting models increase slightly in both iron and silica content.

Charge-neutral 180$^\circ$ domain walls that separate domains of antiparallel polarization directions are common structural topological defects in ferroelectrics. In normal ferroelectrics, charged 180$^\circ$ domain walls running perpendicular to the polarization directions are highly energetically unfavorable because of the depolarization field and are difficult to stabilize. We explore both neutral and charged 180$^\circ$ domain walls in hyperferroelectrics, a class of proper ferroelectrics with persistent polarization in the presence of a depolarization field, using density functional theory. We obtain zero temperature equilibrium structures of head-to-head and tail-to-tail walls in recently discovered $ABC$-type hexagonal hyperferroelectrics. Charged domain walls can also be stabilized in canonical ferroelectrics represented by LiNbO$_3$ without any dopants, defects or mechanical clamping. First-principles electronic structure calculations show that charged domain walls can reduce and even close the band gap of host materials and support quasi-two-dimensional electron(hole) gas with enhanced electrical conductivity.

We have performed quantum Monte Carlo (QMC) simulations and density functional theory calculations to study the equations of state of MgSiO3 perovskite (Pv, bridgmanite) and post-perovskite (PPv) up to the pressure and temperature conditions of the base of Earth's lower mantle. The ground-state energies were derived using QMC simulations and the temperature-dependent Helmholtz free energies were calculated within the quasiharmonic approximation and density functional perturbation theory. The equations of state for both phases of MgSiO3 agree well with experiments, and better than those from generalized gradient approximation calculations. The Pv-PPv phase boundary calculated from our QMC equations of state is also consistent with experiments, and better than previous local density approximation calculations. We discuss the implications for double crossing of the Pv-PPv boundary in the Earth.

Recent studies reveal a pressure induced transition from a paramagnetic tetragonal phase (T) to a collapsed tetragonal phase (CT) in ${\text{CaFe}}_{2}{\text{As}}_{2}$, which was found to be superconducting with pressure at low temperature. We have investigated the effects of electron correlation and a local fluctuating moment in both tetragonal and collapsed tetragonal phases of the paramagnetic ${\text{CaFe}}_{2}{\text{As}}_{2}$ using self-consistent DFT-DMFT with continuous time quantum Monte Carlo as the impurity solver. From the computed optical conductivity, we find a gain in the optical kinetic energy due to the loss in Hund's rule coupling energy in the CT phase. We find that the transition from T to CT turns ${\text{CaFe}}_{2}{\text{As}}_{2}$ from a bad metal into a good metal. Computed mass enhancement and local moments also show a significant decrease in the CT phase, which confirms the suppression of the electron correlation in the CT phase of ${\text{CaFe}}_{2}{\text{As}}_{2}$.

We predict a candidate high-temperature, high-pressure structure of ${\mathrm{FeSiO}}_{3}$ with space-group symmetry Cmmm by applying an evolutionary algorithm within density functional theory (DFT)+U that we call post-perovskite II (PPv-II). An exhaustive search found no other competitive candidate structures with ${AB\text{O}}_{3}$ composition. We compared the x-ray diffraction pattern of ${\mathrm{FeSiO}}_{3}$ PPv-II with experimental results of the recently reported ``H phase'' of (Fe,Mg)${\mathrm{SiO}}_{3}$. The intensities and positions of two main x-ray diffraction peaks of PPv-II ${\mathrm{FeSiO}}_{3}$ compare well with those of the H phase. We also calculated the static equation of state, the enthalpy, and the bulk modulus of the PPv-II phase and compared it with those of the perovskite (Pv) and post-perovskite (PPv) phases of ${\mathrm{FeSiO}}_{3}$. According to the static DFT+U computations, the PPv-II phase of ${\mathrm{FeSiO}}_{3}$ is less stable than the Pv and PPv phases under lower mantle pressure conditions at $T=0$ K and has a higher volume. PPv-II may be entropically stabilized, and may be a stable phase in Earth's lower mantle, coexisting with -${\mathrm{PbO}}_{2}$ (columbite-structure) silica and perovskite, or with magnesiowustite and/or ferropericlase, depending on the bulk composition.