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 meanfield 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.

We measure the electrical resistivity of hcp iron up to similar to 170 GPa and similar to 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 similar to 80 +/- 5 mu Omega cm and similar to 100 +/- 10 W m(-1) K-1, respectively, at conditions near the core-mantle boundary. Our results indicate an adiabatic heat flow of similar to 10 +/- 1 TW out of the core, supporting a present-day geodynamo driven by thermal and compositional convection.

Here we report on the first structural and optical high-pressure investigation of MASnBr(3) (MA = [CH3NH3](+)) and CsSnBr3 halide perovskites. A massive red shift of 0.4 eV for MASnBr(3) and 0.2 eV for CsSnBr3 is observed within 1.3 to 1.5 GPa from absorption spectroscopy, followed by a huge blue shift of 0.3 and 0.5 eV, respectively. Synchrotron powder diffraction allowed us to correlate the upturn in the optical properties trend (onset of blue shift) with structural phase transitions from cubic to orthorhombic in MASnBr(3) and from tetragonal to monoclinic for CsSnBr3. Density functional theory calculations indicate a different underlying mechanism affecting the band gap evolution with pressure, a key role of metal-halide bond lengths for CsSnBr3 and cation orientation for MASnBr(3), thus showing the impact of a different A-cation on the pressure response. Finally, the investigated phases, differently from the analogous Pb-based counterparts, are robust against amorphization showing defined diffraction up to the maximum pressure used in the experiments.

Here, multiple isotope systems are tracked simultaneously in models of mantle convection and it is show that this can provide powerful constraints on the role of oceanic crust recycling in the development of isotopic end-member compositions. The dynamical models are based on high-resolution cylindrical calculations with force-balanced plates and variable chemical density. The dynamic results span a parameter space of variable realistic excess crustal density compared to experimental estimates and convective vigor measured by plate velocities and surface heat flow. Isotope geochemistry is then modeled for the U-Th-Pb, Sm-Nd, Rb-Sr, and Re-Os isotope systems. The role of a dense crustal layer in development of a HIMU-isotope signature is confirmed. The extraction of continental crust is found to be essential for the formation of all isotope compositional end-members, including HIMU. This extraction is implemented as an ad-hoc process secondary to partial melting at mid-ocean ridges and constrained by estimated isotopic abundances in the present-day crust. Whereas previous studies generated mantle isotopic arrays that spanned DMM-HIMU, the additional isotope systems in this analysis indicate that enrichment purely from ancient oceanic crust may also generate an EM-I component without invoking the subduction of sediment. In this case, the EM-I signature may be indicative of mantle enriched by oceanic crust produced before 2.25 Byr, while the HIM signature indicates enrichment by oceanic crust extracted more recently. However, it is found to be difficult to maintain a true DMM isotopic end member in Sr-Nd isotope space when significantly enriched end-members are present. This may highlight the sensitivity of the Rb-Sr system to mass exchange between the upper and lower mantle. (C) 2008 Elsevier B.V. All rights reserved.

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 predominantly as molecular carbon dioxide and carbonate, whereas at greater depths, carbon forms complex polymerized 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.

A tradeoff exists between triplet-pair separation versus relaxation that can limit the ability to utilize singlet fission for enhancing solar cell efficiency beyond the Shockley-Queisser limit. Here, we show that this tradeoff can be avoided in crystalline environments by studying a functionalized pentacene compressed in a diamond anvil cell. We demonstrate, using ultrafast transient absorption spectroscopy, that there is a "sweet spot" where the rate of triplet pair separation can be accelerated by nearly an order of magnitude without causing fast excited state relaxation. X-ray diffraction and computational modeling allow us to quantify the corresponding increase of intermolecular coupling. Our findings suggest that increased coupling enhances excited state relaxation but that crystalline environments can suppress these relaxation processes in pentacene derivatives The combination of these effects leads to the sweet spot and informs efforts to enhance triplet-pair separation rates in amorphous systems such as polymers.

The partitioning of light elements between liquid and solid at the inner core boundary (ICB) governs compositional difference and density deficit between the outer and inner core. Observations of high S and low Fe concentration on the surface of Mercury from MESSENGER mission indicate that Mercury is formed under much more reduced conditions than other terrestrial planets, which may result in a Si and S-bearing metallic Fe core. In this study, we conducted high-pressure experiments to investigate the partitioning behavior of Si and S between liquid and solid in the Fe-Si-S system at 15 and 21GPa, relevant to Mercury's ICB conditions. Experimental results show that almost all S partitions into liquid. The partitioning coefficient of Si (D-Si) between liquid and solid is strongly correlated with the S content in liquid (X-S(liquid)) as: log(10)(D-Si) = 0.0445 + 5.9895 * log(10)(1 - X-S(liquid)). Within our experimental range, pressure has limited effect on the partitioning behavior of Si and S between liquid and solid. For Mercury with an Fe-Si- S core, compositional difference between the inner and outer core is strongly dependent on the S content of the core. The lower S content is in the core, the smaller compositional difference and density deficit between the liquid outer core and solid inner core should be observed. For a core with 1.5wt% bulkS, amodel ICB temperature would intersect with the melting curve at similar to 17GPa, corresponding to an inner core with a radius of similar to 1600km. (C) 2021 Elsevier B.V. All rights reserved.

We investigate energetically favorable structures of ABO(2)N oxynitrides as functions of pressure and strain via swarm-intelligence-based structure prediction methods, density functional theory (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 implies 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 YSiO2N (I4cm) is metastable up to at least 600 K under compressive epitaxial strain before converting to the stable wollastonitelike structures (I4/mcm). We predict that YGeO2N, LaSiO2N, and LaGeO2N are metastable as ferroelectric perovskites (P4mm) at zero pressure even without epitaxial strain.

We study the high-pressure structures of SrB6 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 (14/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 14/mmm structure of SrB6 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 SrB6, which have broad implications for further explorations of hexaboride materials.