SHINERS Study of Chloride Order-Disorder Phase Transition and Solvation of Cu(100).

Shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) and density functional theory (DFT) are used to probe Cl adsorption and the order-disorder phase transition associated with the c(2 × 2) Cl adlayer on Cu(100) in acid media. A two-component ν(Cu-Cl) vibrational band centered near 260 ± 1 cm is used to track the potential dependence of Cl adsorption. The potential dependence of the dominant 260 cm component tracks the coverage of the fluctional c(2 × 2) Cl phase on terraces in good agreement with the normalized intensity of the c(2 × 2) superstructure rods in prior surface X-ray diffraction (SXRD) studies.

2D honeycomb transformation into dodecagonal quasicrystals driven by electrostatic forces.

Dodecagonal oxide quasicrystals are well established as examples of long-range aperiodic order in two dimensions. However, despite investigations by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), photoemission spectroscopy as well as density functional theory (DFT), their structure is still controversial. Furthermore, the principles that guide the formation of quasicrystals (QCs) in oxides are elusive since the principles that are known to drive metallic QCs are expected to fail for oxides.

Density Functional Theory Study of the Structure of the Pillared Hofmann compound Ni(3-Methy-4,4'-bipyridine)[Ni(CN)] (Ni-BpyMe or PICNIC-21).

We use dispersion-corrected density functional theory (DFT) to investigate the structure of the pillared Hofmann compound Ni(3-Methy-4,4'-bipyridine)[Ni(CN)] (Ni-BpyMe for short, or PICNIC-21). We model the disorder found in experimental X-ray structure refinement via an ensemble of supercells with ordered ligand orientation configurations. The ensemble-averaged structure agrees very well with experiment, except for the positions of the methyl group hydrogen atoms.

Structural Basis of CO₂ Adsorption in a Flexible Metal-Organic Framework Material.

This paper reports on the structural basis of CO₂ adsorption in a representative model of flexible metal-organic framework (MOF) material, Ni(1,2-bis(4-pyridyl)ethylene)[Ni(CN)₄] (NiBpene or PICNIC-60). NiBpene exhibits a CO₂ sorption isotherm with characteristic hysteresis and features on the desorption branch that can be associated with discrete structural changes. Various gas adsorption effects on the structure are demonstrated for CO₂ with respect to N₂, CH₄ and H₂ under static and flowing gas pressure conditions.

Density Functional Theory Meta GGA Study of Water Adsorption in MIL-53(Cr).

We use density functional theory meta-GGA TPSS+D3(BJ)+U+J calculations to investigate the energetics and geometry of water molecules in the flexible metal-organic framework material MIL-53(Cr) as a function of cell volume. The critical concentration of water to cause the transition from the large pore () to the narrow pore () structure is estimated to be about 0.13 water molecule per Cr.

The PNR to Relaxor Transition in PSN with nearest neighbor Pb-O divacancies.

In previous work, molecular dynamics simulations based on a first-principles-derived effective Hamiltonian for (PSN), with nearest-neighbor Pb-O divacancy pairs, was used to calculate vs. T, phase diagrams for PSN with: ideal rock-salt type chemical order; nanoscale chemical short-range order; and random chemical disorder. Here, we show that the phase diagrams should include additional regions in which a glassy relaxor-phase (or state) is predicted.

Local atomic geometry and Ti 1s near-edge spectra in PbTiO and SrTiO.

We study Ti 1s near-edge spectroscopy in PbTiO at various temperatures above and below its tetragonal-to-cubic phase transition, and in SrTiO at room temperature. molecular dynamics (AIMD) runs on 80-atom supercells are used to determine the average internal coordinates and their fluctuations. We determine that one vector local order parameter is the dominant contributor to changes in spectral features: the displacement of the Ti ion with respect to its axial O neighbors in each Cartesian direction, as these displacements enhance the cross section for transitions to E-derived core-hole exciton levels.

Thermodynamics of the Flexible Metal-Organic Framework Material MIL-53(Cr) From First Principles.

We use first-principles density functional theory total energy and linear response phonon calculations to compute the Helmholtz and Gibbs free energy as a function of temperature, pressure, and cell volume in the flexible metal-organic framework material MIL-53(Cr) within the quasiharmonic approximation. GGA and metaGGA calculations were performed, each including empirical van der Waals (vdW) forces under the D2, D3, or D3(BJ) parameterizations. At all temperatures up to 500 K and pressures from -30 MPa to 30 MPa, two minima in the free energy versus volume are found, corresponding to the narrow pore () and large pore () structures.

Structure of Periodic Crystals and Quasicrystals in Ultrathin Films of Ba-Ti-O.

We model the remarkable thin-film Ba-Ti-O structures formed by heat treatment of an initial perovskite BaTiO thin film on a Pt(111) surface. All structures contain a rumpled Ti-O network with all Ti threefold coordinated with O, and with Ba occupying the larger. mainly TiO, pores.

Density functional theory meta-GGA + U study of water incorporation in the metal-organic framework material Cu-BTC.

Water absorption in the metal-organic framework (MOF) material Cu-BTC, up to a concentration of 3.5 H2O per Cu ion, is studied via density functional theory at the meta-GGA + U level. The stable arrangements of water molecules show chains of hydrogen-bonded water molecules and a tendency to form closed cages at high concentration.

Time-dependent CO2 sorption hysteresis in a one-dimensional microporous octahedral molecular sieve.

The development of sorbents for next-generation CO(2) mitigation technologies will require better understanding of CO(2)/sorbent interactions. Among the sorbents under consideration are shape-selective microporous molecular sieves with hierarchical pore morphologies of reduced dimensionality. We have characterized the non-equilibrium CO(2) sorption of OMS-2, a well-known one-dimensional microporous octahedral molecular sieve with manganese oxide framework.

The Pole Term in Linear Response Theory: An Example From the Transverse Response of the Electron Gas.

In linear response theory, the dielectric response at zero frequency sometimes appears to violate the f-sum rule, which has apparent implications for causality. Here, we study the origin of this apparent discrepancy, focusing on Lindhard's formula for the transverse response of the electron gas. At non-zero frequency, first-order poles contribute to the imaginary part of the dielectric function in the usual way.

Origin of the relaxor state in Pb(B{x}B{1-x}{'})O(3) perovskites.

Molecular dynamics simulations of first-principles-based effective Hamiltonians for Pb(Sc{1/2}Nb{1/2})O(3) under hydrostatic pressure and for Pb(Mg{1/3}Nb{2/3})O(3) at ambient pressure show clear evidence of a relaxor state in both systems. The Burns temperature is identified as the temperature below which dynamic nanoscale polar clusters form, pinned to regions of quenched chemical short-range order. The effect of pressure in Pb(Sc{1/2}Nb{1/2})O(3) demonstrates that the stability of the relaxor state depends on a delicate balance between the energetics that stabilize normal ferroelectricity and the average strength of random local fields which promote the relaxor state.