Dynamical Charges Research Articles

Ferroelectric and structural instability in double perovskites Me1+Bi3+Me3+Nb5+O6 (Me1+ = Na, K, Rb; Me3+ = Sc, Ga, In, Lu)

Within the Gordon-Kim generalized model with regard to the polarizabilities of ions, the lattice constants, the high-frequency permittivity, the Born dynamic charges, and the vibration constants of the crystal lattice are calculated for cation-ordered double perovskites Me1+Bi3+Me3+Nb5+O6. The vibration spectra of all the compounds exhibit two types of instabilities: instability associated with the rotation of the oxygen octahedron and ferroelectric instability. Various combinations of distortions with respect to the rotation mode yield five energetically most favorable distorted phases. The symmetry and the energy characteristics of these phases are discussed. In four of the five phases, the distortions associated with the oxygen octahedron rotation lead to polar phases, thus allowing one to speak of improper ferroelectricity in these compounds. One phase turns out to be nonpolar; however, it contains unstable polar modes such that a displacement along the eigenvectors of these modes gives rise to polarization in the crystal.

Recent applications and developments of charge equilibration force fields for modeling dynamical charges in classical molecular dynamics simulations

With the continuing advances in computational hardware and novel force fields constructed using quantum mechanics, the outlook for non-additive force fields is promising. Our work in the past several years has demonstrated the utility of polarizable force fields, in our hands those based on the charge equilibration formalism, for a broad range of physical and biophysical systems. We have constructed and applied polarizable force fields for small molecules, proteins, lipids, and lipid bilayers and recently have begun work on carbohydrate force fields. The latter area has been relatively untouched by force field developers with particular focus on polarizable, non-additive interaction potential models. In this review of our recent work, we discuss the formalism we have adopted for implementing the charge equilibration method for phase-dependent polarizable force fields, lipid molecules, and small-molecule carbohydrates. We discuss the methodology, related issues, and briefly discuss results from recent applications of such force fields.

Atomistic simulations of copper oxidation and Cu/Cu2O interfaces using charge-optimized many-body potentials

Presented is a charge-optimized many-body potential (COMB) for metallic copper and copper oxide systems based on an extended Tersoff formalism coupled with variable charge electrostatics. To faithfully reproduce interactions between molecular oxygen and the metal surface, the potential includes atomic polarizabilities via both a point dipole model and dynamic partial charges, both of which are equilibrated through an extended Lagrangian scheme. The potential is fit to a training set composed of both experimental and ab initio computational data for cohesive energies, formation enthalpies, elastic properties, and surface energies of several metallic and oxide phases as well as bond dissociation energies for molecular oxygen and several of its anions. The potential is used in molecular dynamics simulations to model the Cu(111)\ensuremath{\parallel}Cu${}_{2}$O(100) interface and the oxidation of the Cu (100) surface.

Lattice dynamics study of lead chalcogenides

First principal calculations have been performed to study the structural and lattice-dynamical properties for lead chalcogenides compounds: PbS, PbSe and PbTe. The ground-states quantities such as the lattice parameter and bulk modulus were evaluated using the plane wave pseudo-potential method within the density-functional theory. Phonon dispersion spectra were derived from linear-response to density-functional theory. We have examined particularly the effect of pressure on the dynamical charges, longitudinal optical (LO), transverse optical (TO), and the splitting of LO–TO branches. The obtained results regarding dynamical properties under pressure have been confronted to the common III–V semiconductors. Finally thermodynamics quantities were reported by means of the quasiharmonic approximation.

Wannier orbital overlap population (WOOP), Wannier orbital position population (WOPP) and the origin of anomalous dynamical charges

Most d(0) transition metal (TM) oxides exhibit anomalously large Born dynamical charges associated with off-centering or motion of atoms along the TM-O chains. To understand their chemical origin, we introduce "Wannier orbital overlap population" (WOOP) and "Wannier orbital position population" (WOPP) in terms of the Wannier function based description of electronic structure obtained within first-principles density functional theory. We apply these concepts in a precise analysis of anomalous dynamical charges in PbTiO(3), BaTiO(3) and BaZrO(3) in the cubic perovskite structure. Determining contributions of different atomic orbitals to the dynamical charge and their break-up into local polarizability, charge transfer and covalency, we find that p orbitals of oxygen perpendicular to the -TM-O- chain contribute most prominently to the anomalous charge, by facilitating a transfer of tiny electronic charge through one unit cell from one TM atom to the next. Our results explain why the corner-shared linkage of TMO(6) octahedra, as in the perovskite structure, is ideal for large dynamical charges and hence for ferroelectricity.

Dielectric Response of Ta2O5, Nb2O5, and NbTaO5 from First-Principles Investigations

High-κ dielectrics are intensively investigated as a replacement for in integrated nanoelectronics. In particular, for the next generation of dynamic random access memory metal–insulator–metal capacitors, the Ta- and Nb-based oxides are among the most interesting candidates that display a relatively large dielectric constant with a bandgap larger than 3 eV. In this paper, we show that it is possible to modulate the dielectric response of by admixing it with . The dynamical charges and dielectric constants of and for different crystal phases were calculated at the density functional theory level. The averaged dielectric constants range between 27 (38) and 42 (77) for in the hexagonal and orthorhombic varieties. A mixed orthorhombic composition exhibits a directionally averaged dielectric constant of 54, close to the arithmetic mean value of the binary species. The origins of the dielectric permeability are discussed and compared to experimental measurements.

Direct calculations of vibrational absorption and circular dichroism spectra of alanine dipeptide analog in water: Quantum mechanical/molecular mechanical molecular dynamics simulations

The vibrational absorption (IR) and vibrational circular dichroism (VCD) spectra of alanine dipeptide analog in water are directly calculated by Fourier transforming the time correlation functions of the electric and magnetic dipole moments, which are calculated using the dynamic partial charges and trajectory of the peptide generated from the quantum mechanical/molecular mechanical molecular dynamics simulations. The alanine dipeptide analog is treated at the Hartree-Fock level with 3-21G, 4-31G, 6-31G, and 6-31G(*) basis sets and the solvent H(2)O is modeled with the TIP3P water. The atomic partial charges are obtained from the Lowdin population analysis, which gives consistent IR spectral profiles irrespective of the basis sets used. The simulated VCD spectrum by a polyproline II(P(II))-dominant trajectory is compatible with the previous experimental results of the polyproline peptides, where the amide I and II VCD bands are negative couplets with a weak positive peak to the high frequency region. The sampling efficiency of the P(II) conformer is much lower than the other ones at all basis levels used. The simulated VCD spectrum of alpha-helix averaged over five trajectories has the reverse sign pattern compared to the P(II) spectrum and is found to be consistent with the previously observed spectral features of alpha-helical polypeptides. The sign patterns of the beta-strand VCD spectrum are qualitatively similar to the experimental spectra of beta-sheet rich proteins. The VCD spectra obtained from the trajectories containing several extended conformers such as beta and P(II) are not clearly distinguishable from the beta-strand-dominant spectra. It is interesting that the P(II) and the coil VCD spectra coincide in sign pattern and relative intensity for all amide modes. This demonstrates that the negative couplet structures of the amide I and II VCD spectra do not necessarily prove the dominance of either P(II) or coil conformation. We anticipate that the present method can be used to directly simulate the IR and VCD spectra of structurally heterogeneous biomolecules in condensed phases.

Dielectric Response of Ta2O5, NbTaO5 and Nb2O5 from First-Principles Investigations

High-κ dielectrics are intensively investigated as a replacement for SiO2 in integrated nanoelectronics. Ta and Nb based oxides are among the list of interesting candidates that display a relatively large dielectric constant with a band gap larger than 3 eV. In this paper, we show that it is possible to modulate the dielectric response of the Ta2O5 by admixing it with Nb2O5. The dynamical charges and dielectric constants of Ta2O5 and of Nb2O5 were calculated at the Density Functional Theory (DFT) level for different crystal phases. The averaged dielectric constants range between 27(38) and 42(77) for Ta2O5 (Nb2O5) in the hexagonal and orthorhombic varieties. Interestingly, a mixed NbTaO5 composition exhibits a directionally averaged dielectric constant of 54 and a relatively large band gap, close to the arithmetic mean value of the binary species. The origins of the dielectric permeability are discussed and confronted to experimental measurements.

Effect of Charge Redistribution on the Thermal-Expansion Behaviors in III–V Semiconductors

The thermal-expansion behaviors of zincblende GaP, GaAs, GaSb, BP, BAs, and BSb are comparatively studied by first-principles response-function calculation. In contrast to these gallium phases and the most of other III–V semiconductors, low-temperature negative thermal expansion is not found in the three borides. In order to explore the reason for the difference, the roles of the pressureinduced redistributions of the static and dynamic charges in the lattice-dynamic process of these phases are analyzed in details. Our study shows that the static charge moves towards the bond center and the dynamic effective charge decreases as a hydrostatic pressure is applied to these gallium phases, which leads to the reductions in their polarity and ionicity. However, both of the static and dynamic charges behave inversely in the three borides. As a result, these borides’ polarity and ionicity will increase under pressure. The fact suggests a correlation between polarity reduction and negative thermal expansion in III–V semiconductors.

Dipole-active optical phonons inYTiO3: Ellipsometry study and lattice-dynamics calculations

The anisotropic complex dielectric response was accurately extracted from spectroscopic ellipsometry measurements at phonon frequencies for the three principal crystallographic directions of an orthorhombic $(Pbnm)$ ${\text{YTiO}}_{3}$ single crystal. We identify all 25 infrared-active phonon modes allowed by symmetry $7{B}_{1u}$, $9{B}_{2u}$, and $9{B}_{3u}$ polarized along the $c$, $b$, and $a$ axes, respectively. From a classical dispersion analysis of the complex dielectric functions $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}(\ensuremath{\omega})$ and their inverses $\ensuremath{-}1/\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}(\ensuremath{\omega})$, we define the resonant frequencies, widths, and oscillator strengths of the transverse-optical (TO) and longitudinal-optical phonon modes. We calculate eigenfrequencies and eigenvectors of ${B}_{1u}$, ${B}_{2u}$, and ${B}_{3u}$ normal modes and suggest assignments of the TO phonon modes observed in our ellipsometry spectra by comparing their frequencies and oscillator strengths with those resulting from the present lattice-dynamics study. Based on these assignments, we estimate dynamical effective charges of the atoms in the ${\text{YTiO}}_{3}$ lattice. We find that in general, the dynamical effective charges in ${\text{YTiO}}_{3}$ lattice are typical for a family of perovskite oxides. By contrast to a ferroelectric ${\text{BaTiO}}_{3}$, the dynamical effective charge of oxygen related to a displacement along the $c$ axis does not show the anomalously large value. At the same time, the dynamical effective charges of Y and $ab$ plane oxygen exhibit anisotropy, indicating a strong hybridization along the $a$ axis.

Lattice Dynamics at Zone-Center of Sulphide and Selenide Spinels

A rigid-ion model is used to calculate the force constants and effective dynamical charges of sulphide and selenide spinels. The Raman and infrared phonon modes of normal cubic sulphide spinels MCr2S4 (M = Mn, Co, Fe, Hg, Zn, and Cd) and selenide spinels MCr2Se4 (M = Hg, Zn, and Cd) are calculated at the first Brillouin zone-centre using above model. The significant outcome of the present work is (i) the interatomic interaction between Cr-S (Se) dominates over the Cr-S(Se) and S-S(Se-Se) type of interatomic interactions, (ii) the effective dynamical charges of the bivalent metal ions are nearly zero, and (iii) the selenide spinels are less ionic than the sulphide spinels and the ionicity decreases as MnCr2S4 > FeCr2S4 > CoCr2S4 > and CdCr2C4 > ZnCr2C4 > HgCr2C4 (C = S and Se). The zone-center phonon frequencies, calculated using these parameters, are found to be in very good agreement with the observed results.

Lattice Dynamics of (001) PbTiO3 Thin Films in a Polarizable Ions Model

The results of the lattice dynamics calculation of PbTiO3 thin films in framework of Gordon-Kim model accounting for polarizability of ions are presented. Slab with two different terminations of surface is considered. The films under consideration with various thickness have ferroelectric modes in which vibrations of atoms are parallel to surface and instable polar modes were found in PbO-terminated slab with thickness of 3.95 Á. The dependence of some film's properties such as dielectric permittivities, dynamical charges, in-plane polarization, and frequencies of unstable ferroelectric modes on the film thickness is obtained from calculation.

Lattice dynamics study of bismuth III–V compounds

We present first-principles calculations of the structural and lattice-dynamical propertiesfor cubic bismuth III–V compounds: BBi, AlBi and GaBi. The ground-state properties, i.e.,the lattice constant and the bulk modulus, are calculated using a plane wavepseudopotential method within density functional theory. A linear-response approach todensity functional theory is used to derive the phonon frequencies. The effect of pressure onthe dynamical charges and the longitudinal optical–transverse optical splitting is alsoexamined.

Characterization of Water‐soluble Luminescent Quantum Dots by Fluorescence Correlation Spectroscopy

Quantum dots (QDs) are nanoscale inorganic particles generally composed of II-VI and III-V elements. Recently, QDs have received great interest in biological applications because of their unique and fascinating optical properties. In this short review, we introduce the synthesis of luminescent QDs in the aqueous phase and characterize certain fundamental parameters of luminescent QDs synthesized in the aqueous phase by fluorescence correlation spectroscopy (FCS), which include, for example, their dynamic diameters and surface charges. Meanwhile, combining with some ensemble spectroscopy techniques, we applied FCS to investigate the quenching process and mechanism of heavy metal ions (e.g., Ag(+)) on CdTe QDs, as well as the aggregation and subsequent photoactivation process of 3-mercaptopropionic acid-capping CdTe QDs induced by laser irradiation. Our preliminary results demonstrate that FCS is a sensitive and efficient tool to characterize fluorescent nanoparticles, such as QDs, at the single-molecule detection level.

Lattice dynamics of BiFeO3: The untypical behavior of the ferroelectric instability under hydrostatic pressure

Within a nonempirical model of an ionic crystal with the inclusion of the dipole and quadrupole ion polarizations, the lattice vibrational frequencies, high-frequency dielectric constant, Born dynamic charges, and the elasticity moduli of the BiFeO3 crystal have been calculated and their dependencies on the hydrostatic pressure in the cubic, rhombic, and rhombohedral phases have been determined. The results indicate the presence of the ferroelectric instability, which depends weakly on the pressure in all of the phases investigated. The dependence of the crystal lattice dynamics on the applied pressure for the cubic phases of BiAlO3, BaTiO3, and PbTiO3 has been calculated for comparison.

Dielectric constant boost in amorphous sesquioxides

High-κ dielectrics for insulating layers are a current key ingredient of microelectronics. X2O3 sesquioxide compounds are among the candidates. Here, we show for a typical material of this class, Sc2O3, that the relatively modest dielectric constant of its crystalline phase is enhanced in the amorphous phase by over 40% (from ∼15 to ∼22). This is due to the disorder-induced activation of low frequency cation-related modes which are inactive or inefficient in the crystal and by the conservation of effective dynamical charges (a measure of atomic polarizability). The analysis employs density-functional energy-force and perturbation-theory calculations of the dielectric response of amorphous samples generated by pair-potential molecular dynamics.

Lattice Dynamics of BaTiO3 (100) Thin Film in a Model of Polarizable Ions

The result of the lattice dynamics calculation for BaTiO 3 thin film in framework Gordon-Kim model, accounting for the polarizability of ions is presented. A slab with two different termination of surface is considered. Ferroelectric instability was revealed in this film concerning the vibration atoms in plane parallel to surface and this mode was found in slab with thickness in 4–8 Å for different terminations. The variation of some properties of film such as dielectric permittivities, dynamical charges and frequencies of unstable ferroelectric modes and out-of-plane low-lying polar mode with variation of slab thickness was obtained.

Lattice dynamics and ferroelectric instability in ordered and disordered PbSc1/2Ta1/2O3 and PbSc1/2Nb1/2O3 solid solutions

The dynamic Born charges and the frequency spectra of lattice oscillations in the crystals of ordered and disordered PbSc1/2Ta1/2O3 (PST) and PbSc1/2Nb1/2O3 (PSN) solid solutions have been calculated within the framework of the generalized Gordon-Kim model with allowance for the dipole and quadrupole polarizabilities. The phonon spectra of both compounds contain ferroelectric soft modes. The influence of various interactions on the magnitude of dynamic charges and ferroelectric instability in PSN and PST solid solutions has been studied and it is shown that both these charges and the ferroelectric instability are determined by the competition between long-range dipole-dipole interactions and short-range dipole-charge interactions, the determining role played by the interaction of Nb (Ta) cations and oxygen anions in the Nb-O (Ta-O) bond direction.

Conservation of dielectric constant upon amorphization in perovskite oxides

We report calculations indicating that amorphous $RA{\mathrm{O}}_{3}$ oxides, with $R$ and $A$ trivalent cations, have approximately the same static dielectric constant as their perovskite crystal phase. The effect is due to the disorder-activated polar response of nonpolar crystal modes at low frequency, which compensates a moderate but appreciable reduction of the ionic dynamical charges. The dielectric response was studied via density-functional perturbation theory. Amorphous samples were generated by molecular dynamics melt-and-quench simulations.

Structure, Vibrational Analysis, and Insights into Host−Guest Interactions in As-Synthesized Pure Silica ITQ-12 Zeolite by Periodic B3LYP Calculations

As-made and calcined ITQ-12 zeolites are structurally characterized by means of the analysis of their vibrational modes. The experimental IR spectra made on high crystalline samples are compared with accurate B3LYP periodic calculations performed with the CRYSTAL06 code. The fair agreement between both sets of data allows us to make a reliable assignment of the IR modes. Thanks to the detailed information provided by the theoretical calculations, the analysis of the IR intensities, the Born dynamic charges, and the whole set of vibrational frequencies at Gamma-point shed light on several aspects of the host-guest interaction, structure-direction issues, including the role of fluoride anions in allowing the crystallization of silica structures with strained double-four rings, and the role played by the framework flexibility.