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 methyl ammonium lead iodide (MAPbI(3)) 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 MAPbI(3). Molecular dipoles in epitaxially strained MAPbI(3) are more susceptible to an electric field. We also report various caloric effects in MAPbI(3). The adiabatic thermal change is estimated directly by introducing different driving fields in the simulations. We find that MAPbI(3) exhibits both electrocaloric and mechanocaloric effects at room temperature. Local structural analysis reveals that the rearrangement of molecular cations in response to electric and stress fields is responsible for the caloric effects. The enhancement of caloric response could be realized through strain engineering and chemical doping.

Through use of in situ Raman spectroscopy and single-crystal/powder X-ray diffraction, we resolve the "C-0" phase structure discovered recently in the H-2 + H2O system. This phase forms at similar to 400 MPa and 280 K with the nominal composition (H2O)(2)H-2 and three formula units per unit cell. The hexagonal structure is chiral, consisting of interpenetrating spiral chains of hydrogen-bonded water molecules and rotationally disordered H-2 molecules, and shows topological similarities with the mineral quartz. Like other clathrate hydrates and forms of ice, the protons of H2O molecules within C-0 are disordered. The large zeolite-like channels accommodate significant amounts of hydrogen (5.3% by weight) in a unique hydrogen-bonded lattice, which might be applicable to the thermodynamic conditions found on icy planetary bodies.

Two of the three natural quasiperiodic crystals found in the Khatyrka meteorite show a composition within the Al-Cu-Fe system. Icosahedrite, with formula Al63Cu24Fe13, coexists with the new Al62Cu31Fe2 quasicrystal plus additional Al-metallic minerals such as stolperite (AlCu), kryachkoite [(AlCu)(6)(Fe, Cu)], hollisterite (AlFe3), khatyrkite (Al2Cu) and cupalite (AlCu), associated to high-pressure phases like ringwoodite/ahrensite, coesite, and stishovite. These high-pressure minerals represent the evidence that most of the Khatyrka meteoritic fragments formed at least at 5 GPa and 1200 degrees C, if not at more extreme conditions. On the other hand, experimental studies on phase equilibria within the representative Al-Cu Fe system appear mostly limited to ambient pressure conditions, yet. This makes the interpretation of the coexisting mineral phases in the meteoritic sample quite difficult.

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 PbTiO3 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.

The high-pressure behavior of lithium dicyanamide (LiN(CN)(2)) was studied with in situ Raman and infrared (IR) spectroscopies, and synchrotron angle-dispersive powder X-ray diffraction (PXRD) in a diamond anvil cell (DAC) to 22 GPa. The fundamental vibrational modes associated with molecular units were assigned using a combination of experimental data and density functional perturbation theory. Some low-frequency modes were observed for the first time. On the basis of spectroscopic and diffraction data, we suggest a polymorphic phase transformation at similar to 8 GPa, wherein dicyanamide ions remain as discrete molecular species. Above ca. 18 GPa, dicyanamide units polymerize, forming a largely disordered network, and the extent of polymerization may be increased by annealing at elevated temperature. The polymerized product consists of tricyanomelaminate-like groups containing sp(2)-hybidized carbon nitrogen bonds and exhibits a visible absorption edge near 540 nm. The product is recoverable to ambient conditions but is not stable in air/moisture.

We carried out sound velocity and density measurements on solid hcp-Fe and an hcp-Fe-Si alloy with 9 wt.% Si at 300 K up to similar to 170 and similar to 140 GPa, respectively. The results allow us to assess the density (rho) dependence of the compressional sound velocity (V-p) and of the shear sound velocity (V-s) for pure Fe and the Fe-Si alloy. The established V-p-rho and V-s-rho relations are used to address the effect of Si on the velocities in the Fe-FeSi system in the range of Si concentrations 0 to 9 wt.% applicable to the Earth's core. Assuming an ideal linear mixing model, velocities vary with respect to those of pure Fe by similar to+80 m/s for V-p and similar to-80 m/s for Vs for each wt.% of Si at the inner core density of 13 000 kg/m(3). The possible presence of Si in the inner core and the quantification of its amount strongly depend on anharmonic effects at high temperature and on actual core temperature. (C) 2017 Elsevier B.V. All rights reserved.

Charge-neutral 180 degrees domain walls that separate domains of antiparallel polarization directions are common structural topological defects in ferroelectrics. In normal ferroelectrics, charged 180 degrees 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 degrees 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 LiNbO3 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 investigated the stability and mechanical and electronic properties of 15 metastable mixed sp(2)-sp(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 TPa-1. We find all the IGN allotropes have negative linear compressibilities with maximum values varying from -0.74 to -133 TPa-1. The maximal negative linear compressibility of Z33 (-133 TPa-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 C-60 fullerene is about 0.4 eV/atom), and energetically feasible binary compound pathways are possible.

Majoritic garnet [(Ca, Mg, Fe2+)(3)(Fe3+, Al, Si)(2)(SiO4)(3)] is one of the predominant and important constituents of upper mantle peridotite and ultra-deep subducted slabs. Majoritic substitution in garnet depends on pressure, and it has been used to estimate the formation pressure of natural majoritic garnet. Ferric iron (Fe3+) substitution occurs in natural majoritic garnets from mantle diamonds and shocked meteorites. However, available majorite geobarometers were developed without considering the effect of Fe3+ substitution in the structure. In this study, we systematically synthesized Fe3+-bearing majoritic garnets from 6.5 GPa to 15 GPa to evaluate the effect of Fe3+ on the majorite geobarometer. The Fe3+ contents of synthetic majoritic garnets were analyzed using the "Flank method" with the electron probe microanalyzer (EPMA). The results were compared with those based on the charge balance calculations. From the known synthesis pressures and measured Fe3+ contents, we developed a new majorite geobarometer for Fe3+-bearing majoritic garnets. Our results show that the existing majorite geobarometer, which does not take into account the Fe3+ substitution, could underestimate the formation pressure of majoritic garnets, especially for samples with a high majoritic component. (C) 2018 Elsevier Ltd. All rights reserved.

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. YSiO2N stood out with huge predicted polarization and nonlinear optic coefficients. In this study, we demonstrate the synthesis of perovskite-structured YSiO2N 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.