Force Constant Parameters Research Articles

Structural and magnetic properties of Eu3+substituted Mg-Cd nanoferrites: A detailed study of influence of high energy γ-rays irradiation

Abstract The Mg1-xCdxEuyFe2-yO4 (0.0 ≤ x ≤ 1.0; y = 0.0, 0.1) nanoferrites were synthesized by solution combustion method. Nanoferrites were irradiated with 60Co γ–rays source with total adsorbed dose of 310 kGy at the dose rate 6.1 kGy/h. XRD pattern revealed the formation of single phase with polycrystalline spinel cubic structure. The FTIR spectra showed two strong absorption bands at tetrahedral and octahedral sites in the range of 350–800 cm−1.All the samples of M-H loops reveal the soft magnetic material behaviour. The magnetic measurements reveal that the magnetic parameters decrease with europium ion increases for all samples due to exchange of tetrahedral-octahedral interactions. After the γ-irradiation the lattice parameter, Force constant and magnetic parameters values increases which is attributed to surface states pinning of domains released and exchange of Fe3+ions into Fe2+ ions because of ionizing effect of γ-radiation.

The Hiphive Package for the Extraction of High‐Order Force Constants by Machine Learning

AbstractThe efficient extraction of force constants (FCs) is crucial for the analysis of many thermodynamic materials properties. Approaches based on the systematic enumeration of finite differences scale poorly with system size and can rarely extend beyond third order when input data is obtained from first‐principles calculations. Methods based on parameter fitting in the spirit of interatomic potentials, on the other hand, can extract FC parameters from semi‐random configurations of high information density and advanced regularized regression methods can recover physical solutions from a limited amount of data. Here, the hiphive Python package, that enables the construction of force constant models up to arbitrary order is presented. hiphive exploits crystal symmetries to reduce the number of free parameters and then employs advanced machine learning algorithms to extract the force constants. Depending on the problem at hand, both over and underdetermined systems are handled efficiently. The FCs can be subsequently analyzed directly and or be used to carry out, for example, molecular dynamics simulations. The utility of this approach is demonstrated via several examples including ideal and defective monolayers of MoS2 as well as bulk nickel.

Analytical hessian fitting schemes for efficient determination of force-constant parameters in molecular mechanics.

Building upon our recently developed partial Hessian fitting (PHF) method (Wang et al., J. Comput. Chem. 2016, 37, 2349), we formulated and implemented two other rapid force-field parameterization schemes called full Hessian fitting (FHF) and internal Hessian fitting (IHF), and comparisons were made among these three parameterization schemes to assess their performance. FHF minimizes deviation between the Hessian matrices in Cartesian coordinates computed by quantum mechanics (QM) and molecular mechanics (MM), to determine the best possible MM force-constant parameters. While PHF requires step-by-step fittings of 3 × 3 partial Hessian matrices, FHF compares the lower triangular part of the QM and MM Hessian matrices, which allows simultaneous determination of all force-constant parameters. In addition to this simple FHF scheme, IHF was developed such that it considers the Hessian matrices in redundant internal coordinates, where all possible internal coordinates that arise from the user-defined interatomic connectivity are utilized. The results show that IHF performs best overall, followed by PHF and then FHF. Python-based programing codes were developed to automate various tedious steps involved in the parameterization processes. © 2017 Wiley Periodicals, Inc.

Specific heat and thermal expansion effects in LaCo1−xAxCoO3 (A = Mg, Rh, Ti and Nb)

Rare-earth cobaltates LaCo1−xAxCoO3 (A = Mg, Rh, Ti, Nb and 0.02 ≤ x ≤ 1) have been systematically investigated to elucidate the effect of B site substitution on thermal and elastic properties. Specific heat, thermal expansion coefficient and bulk modulus of rare earth cobaltate LaCoO3 with Mg, Rh, Ti and Nb in the perovskite structure at B-site has been studied by means of a Modified Rigid Ion Model (MRIM) and AIM theory which has been found to be adequately suitable for such description for the rare earth cobaltates Ln1−xSrxCoO3 (Ln = Pr, Nd, Sm and Dy) materials [J. of Alloys Compd. 661 (2016) 257–267]. The partial replacement of rare earth cation by divalent, trivalent, tetravalent and pentavalent introduces large size and charge mismatch at B-site affecting the bulk modulus and thermal properties. The concentration dependence of the molecular force constant (f), Reststrahlen frequency (υ), cohesive energy (ϕ), Debye temperature (θD) and Gruneisen parameter (γ) are investigated for the first time. A fairly good agreement has been found between the observed and calculated values of the cohesive energy for these doped compounds. The trend of variation of specific heat, thermal expansion coefficient, Debye temperature with molar volume, temperature and concentration respectively are predicted probably for the first time for the doped rare earth cobaltates.

Partial hessian fitting for determining force constant parameters in molecular mechanics.

We present a new protocol for deriving force constant parameters that are used in molecular mechanics (MM) force fields to describe the bond-stretching, angle-bending, and dihedral terms. A 3 × 3 partial matrix is chosen from the MM Hessian matrix in Cartesian coordinates according to a simple rule and made as close as possible to the corresponding partial Hessian matrix computed using quantum mechanics (QM). This partial Hessian fitting (PHF) is done analytically and thus rapidly in a least-squares sense, yielding force constant parameters as the output. We herein apply this approach to derive force constant parameters for the AMBER-type energy expression. Test calculations on several different molecules show good performance of the PHF parameter sets in terms of how well they can reproduce QM-calculated frequencies. When soft bonds are involved in the target molecule as in the case of secondary building units of metal-organic frameworks, the MM-optimized geometry sometimes deviates significantly from the QM-optimized one. We show that this problem is rectified effectively by use of a simple procedure called Katachi that modifies the equilibrium bond distances and angles in bond-stretching and angle-bending terms. © 2016 Wiley Periodicals, Inc.

Elastic and thermal properties of Sr1−xYxCoO3

ABSTRACTThe elastic and thermal properties of Sr1−xYxCoO3 (0 ≤ x ≤ 0.5) have been investigated, probably for the first time, by using modified rigid ion model. We present the elastic constants (C11, C12, C44) and other elastic properties like bulk modulus (B), Young's modulus (Y), shear modulus (G), Poisson's ratio (σ), Lame's parameter (μ, λ), transverse, longitudinal and average wave velocity (υt, υl and υm) and anisotropy parameter (A). Besides, we have reported the thermodynamic properties molecular force constant (f), Reststrahlen frequency (υ), cohesive energy (φ), Debye temperature (θD) and Gruneisen parameter (γ). We have also computed the variation of specific heat (C) and volume thermal expansion coefficient (α). The computed results on the elastic and thermodynamic properties are the first report on them. This model is capable of explaining the Cauchy's discrepancy, elastic and thermal properties successfully.

Simulating far infrared spectra of Zn1-xMnxSe/GaAs epifilms, MnSe/ZnSe superlattices and predicting impurity modes of N, P defects in Zn1-xMnxSe

A comprehensive lattice dynamical study is reported to emphasize the vibrational behavior of perfect/imperfect zinc-blende (zb) ZnSe, MnSe and Zn1−xMnxSe alloys. Low temperature far-infrared (FIR) reflectivity measurements performed on a series of molecular beam epitaxy grown Zn1−xMnxSe/GaAs (001) epilayers have a typical ‘intermediate-phonon-mode’ behavior. Besides perceiving ZnSe- and MnSe-like TO-phonon resonances, the study also revealed a weak Mn alloy-disorder mode below MnSe band. A classical effective-medium theory of multilayer optics is used to evaluate dielectric tensors of both epilayers and substrate for simulating reflectivity and transmission spectra of ultrathin epifilms and superlattices at near normal and/or oblique incidence. In the framework of a realistic rigid-ion model and exploiting an average t-matrix Greens function (ATM-GF) theory we appraised the vibrational properties of nitrogen and phosphorous doped Zn-Mn chalcogenides. Lattice relaxations around isolated NSe (PSe) defects in ZnSe and zb MnSe are evaluated by first principles bond-orbital model that helped construct perturbation models for simulating the localized vibrational modes (LVMs). Calculated shift of impurity modes for isotopic 14NSe (15NSe) defects in ZnSe offered a strong revelation of an inflexible defect–host interaction. By retaining force constant change parameter of 14NSe (15NSe) in heavily N-doped ZnSe, the ATM-GF theory predicted (a) three non-degenerate LVMs for the photoluminescence defect center V Se-Zn-14NSe (V Se-Zn-15NSe) of C s symmetry, and (b) six impurity modes for the second nearest-neighbor NSe-Zn-NSe pair defect of C 2v symmetry. From the range of simulated defect modes, we have ruled out the possibility of N-pairs and justified the presence of VSe-Zn-NSe complex centers – likely to be responsible for the observed large absorption bandwidth in highly N-doped ZnSe. High resolution measurements of FIR absorption and/or Raman scattering spectroscopy are needed to validate the accuracy of our theoretical conjectures.

Thermal properties of perovskite RCeO3 (R = Ba, Sr)

We have investigated the bulk modulus and thermal properties of proton conducting perovskite RCeO3 (R=Ba, Sr) for the first time by incorporating the effect of lattice distortion in modified rigid ion models (MRIM). The computed bulk modulus, specific heat, thermal expansion coefficient and other thermal properties of BaCeO3 and SrCeO3 reproduce well with the available experimental data. In addition the cohesive energy (ϕ), molecular force constant (f), reststrahlen frequency (ν), Debye temperature (θD) and Gruneisen parameter (γ) are also reported and discussed. The specific heat results can further be improved by taking into account the spin and the orbital ordering contribution in the specific heat formulae.

Vibration characteristics of open- and capped-end single-walled carbon nanotubes using multi-scale analysis technique incorporating Tersoff–Brenner potential

This research work addresses questions on the vibration characteristics of single-walled carbon nanotubes (SWCNTs) using multi-scale analysis. Atomistic finite element method (AFEM) is one such multi-scale technique where sequential mode is used to transfer information between two length scales to model and simulate the nanostructures at continuum level. This method is used to investigate the vibration characteristics of SWCNTs. Open- and capped-end armchair and zigzag nanotubes are considered with clamped-free and clamped–clamped boundary conditions. The dependence of vibration characteristic of SWCNTs on their length, diameter and atomic structure is also demonstrated. The body interatomic Tersoff–Brenner (TB) potential is used to represent the energy between two carbon atoms. Based on the TB potential, a new set of force constant parameters is established for carbon nanotubes and presented in this paper. Molecular and structural mechanics analogy is used to find the equivalent geometric and elastic properties of the space frame element to represent the carbon–carbon bond. To validate the vibration results of AFEM incorporating the proposed new set of force constants, molecular dynamics simulation is also carried out on the same structure of carbon nanotube, and it is found that they are in good agreement with each other.

Vibration analysis of single-walled carbon nanocones using multiscale atomistic finite element method incorporating Tersoff–Brenner potential

This research work addresses the free and forced vibration characteristics of single-walled carbon nanocones (SWCNCs) using multiscale atomistic finite element method (AFEM) incorporating Tersoff–Brenner (TB) potential. The multibody interatomic TB potential is used to represent the energy between two carbon atoms. Based on the TB potential, new set of force constant parameters is established for carbon nanocones, and the equivalent geometric and elastic properties of the space frame element to represent carbon–carbon bond are derived which are consistent with the material constitutive relations. The eigenvalues of clamped and cantilevered SWCNCs with different disclination angles are extracted using AFEM, and the effect of these angles on the resonant frequencies is investigated. A computational sine sweep test is carried out on the atomic structure of SWCNCs within the frequency range of 0–10 THz to investigate the steady-state forced vibration response under harmonic excitation. The frequency response of the SWCNCs to the cyclic and impulse load over an applied frequency range is calculated. Based on the forced vibration response spectra, the resonant frequency components of SWCNCs are identified. The results have been validated using molecular dynamics simulation.

Thermal and elastic properties of Sr1−x A x CoO3 (A=Zn and Ho)

We have investigated the elastic and thermal properties for Sr1−x A x CoO3 (A=Zn, Ho, and 0≤x≤0.4) probably for the first time, by using the modified rigid ion model (MRIM). The computed elastic constants (C 11, C 12, C 44) are the first report on them. Using these elastic constants, we have computed other related elastic properties like the Bulk modulus (B), Young’s modulus (Y), Shear modulus (G), Poisson’s ratio (σ), Lame’s parameter (μ, λ), transverse, longitudinal and average wave velocity (υ t, υ l, υ m) and Anisotropy parameter (A). Furthermore, by the elastic stability criteria, we predict that Sr1−x A x CoO3 (A=Zn, Ho, and 0≤x≤0.4) are mechanically stable and belongs to metallically bonding materials. Moreover, the thermodynamic properties such as the thermal expansion coefficient (α), cohesive energy (ϕ), molecular force constant (f), Reststrahlen frequency (υ), Debye temperature (θ D), and Gruneisen parameter (γ) have also been predicted. Besides, the variations of specific heat with temperature are reported for a wide range. To our knowledge, some of the properties are being reported for the first report on these materials.

A Comparative Study on the Structural and Vibrational Properties of Two Potential Antimicrobial and Anticancer Cyanopyridine Derivatives

2-Hydroxy-4,6-dimethylpyridine-3-carbonitrile and 2-chloro-4,6-dimethylpyridine-3-carbonitrile compounds have been studied from a theoretical point of view in order to know their structural and vibrational properties in gas and aqueous solution phases by means of Density Functional Theory (DFT) calculations. The stable structures in both media were optimized by using the hybrid B3LYP/6-31G* method and the solvent effects in aqueous solution were studied by using the integral equation formalism of the polarizable continuum model (IEFPCM) employing the selfconsistent reaction field (SCRF) method. Detailed vibrational analyses for both compounds in the two phases were performed combining the DFT calculations with Pulay’s Scaled Quantum Mechanics Force Field (SQMFF) methodology. The different interactions for both compounds were analyzed by means of the bond orders, atomic charges, solvation energies, dipole moments, molecular electrostatic potentials and force constants parameters. The nature of the interactions was studied by using different descriptors.

Study of elastic moduli and thermal properties of RMnO3 (R = La, Nd) compounds

In this paper, we develop a theoretical model for quantitative analysis of temperature-dependent heat capacity calculation of the magnetoresistance compounds RMnO 3 ( R = La , Nd ). The results on heat capacity obtained by us are in good agreement with the measured values. An effective interionic interaction potential (EIoIP) with the long-range Coulomb, van der Waals (vdW) interaction and short-range repulsive interaction up to second neighbor ions within the Hafemeister and Flygare approach was formulated to estimate the Debye and Einstein temperature and was found to be consistent with the available experimental data. In addition, the properties studied are the cohesive energy, molecular force constant, Restrahlen frequency and Gruneisen parameter. After characterizing thermal properties, a systematic investigation of elastic behavior has been undertaken and it has been found that the elastic moduli are decreasing continuously with increasing temperature.

Thermodynamic properties of BaRuO3

We have investigated the specific heat, thermal expansion and thermodynamic properties of BaRuO3 in rhombohedral, hexagonal and cubic phase using the modified rigid ion model (MRIM) by incorporating the effect of lattice distortions. The calculated Debye temperature, specific heat and other thermal properties reproduce well with the available experimental data, implying that MRIM represents properly the nature of the perovskite-type BaRuO3. Besides, we have reported the thermal expansion (α), bulk modulus (B), cohesive energy (ϕ), molecular force constant (f), Reststrahlen frequency (υ), Debye temperature (θ D), and Gruneisen parameter (γ).

A structural and vibrational investigation on the antiviral deoxyribonucleoside thymidine agent in gas and aqueous solution phases

A structural and vibrational study on the antiviral deoxyribonucleoside thymidine in gas and aqueous solution phases was performed combining the available infrared and Raman spectra with density functional theory (DFT) calculations. Three stable conformers for the title molecule were theoretically determined in both media by using the hybrid B3LYP method together with the 6–31G* basis set. The solvent effects were studied by means of the self-consistent reaction field method employing the polarized continuum model. Complete assignments of the vibrational spectra of thymidine in both phases were performed combining the DFT calculations with Pulay's scaled quantum mechanics force field methodology. The bond orders, atomic charges, solvation energies, dipole moments, molecular electrostatic potentials, and force constants parameters were calculated for the three conformers of thymidine in gas phase and aqueous solution. Selected descriptors, such as chemical potential (µ), electronegativity (χ), global hardness (η), global softness (S), and global electrophilicity index (ω) were used to explain the exact nature of the interactions with electrophiles and/or nucleophiles and to predict the behavior of the three conformers of thymidine in gas and aqueous solution phases. © 2013 Wiley Periodicals, Inc.

Thermal and Mechanical Properties of ACoO3 (R = Sm, Tb, Dy, Ho, and Er)

The thermal and mechanical properties of orthocobaltates, ACoO3 (A = Sm, Tb, Dy, Ho, and Er), have been investigated using the modified rigid ion model (MRIM) by incorporating the effect of lattice distortions. We have computed the variations of specific heat and thermal expansion coefficient for these orthocobaltates in wide temperature range of 1 K (−272 °C) ≤ T ≤ 1000 K (727 °C). The calculated results of specific heat, thermal expansion, bulk modulus, and other thermal and mechanical properties accord very well with the available experimental data, implying that MRIM represents properly the nature of the perovskite-type rare earth cobaltates. In addition, we have also reported the results on molecular force constant (f), Reststrahlen frequency (υ), cohesive energy (ϕ), Debye temperature (θD), and Gruneisen parameter (γ).

Effect of A-site cation radius on thermodynamic properties of ARuO3 (A = Ca, Sr, and Ba)

The thermodynamic properties of alkaline earth ruthenate ARuO3 (A = Ca, Sr, and Ba) perovskites have been investigated for the first time by means of a modified rigid ion model at temperature 1 K ≤ T ≤ 300 K. As strong electron–phonon interactions are present in these compounds, the lattice contribution to the specific heat deserves proper attention. The values of specific heat calculated by us have shown remarkably good agreement with corresponding experimental data. We have found that in ARuO3 (A = Ca, Sr, and Ba) ruthenate family, Debye temperature increases inversely with the ionic radius of the alkaline earth A cations. In addition, the results on the temperature dependence of thermal expansion coefficient (α), cohesive energy (ϕ), molecular force constant (f), Reststrahlen frequency (υ), Debye temperature (θD), and Gruneisen parameter (γ) are also reported.

ELASTIC AND THERMAL PROPERTIES OF Sr1-xCaxRuO3

We have investigated the elastic and thermal properties of Sr 1-x Ca x RuO 3(0≤x ≤1) perovskite using a modified rigid ion model (MRIM). The trend of variation of our computed specific heat in the temperature range 1 K ≤ T ≤ 1000 K are in good agreement with corresponding experimental data for almost all the compositions (x). The specific heat found to increase with temperature from 1 K to 300 K, while they decrease with concentration (x) for these perovskite ruthenates. Besides, we have reported the thermal properties, like thermal expansion (α), molecular force constant (f), Reststrahlen frequency (υ), cohesive energy (ϕ), Debye temperature (θD) and Gruneisen parameter (γ).

Thermal Expansion and Specific Heat of Solid Oxide Fuel Cell Material Bi1-xSrxMnO3 in Orthorhombic Perovskite Phase

With the objective of exploring the unknown thermodynamic phase diagram of Bi1-xSrxMnO3 family, we present here an investigation of the temperature-dependent (10K < T < 900K) thermodynamic behavior of Bi1-xSrxMnO3 (x=0.25, 0.30, 0.50). We report here probably for the first time the thermal expansion and specific heat along with other thermal properties of strontium doped perovskite manganite BiMnO3. The specific heat values revealed by using RIM are in closer agreement with the available experimental data for some concentrations (x) of Bi1-xSrxMnO3. In addition, the results on the cohesive energy (f) in orthorhombic perovskite phase, molecular force constant (¦), Reststrahlen frequency (no) and Gruneisen parameter (g) are also presented.

A new experimental and theoretical investigation on the structures of aminoethyl phosphonic acid in aqueous medium based on the vibrational spectra and DFT calculations

A new study on the structural and vibrational properties of the aminoethylphosphonic acid was performed in aqueous solution phase by using the self-consistent reaction field (SCRF) method. We have studied and characterized it by infrared and Raman spectroscopies in solid and aqueous solution phases. The Density Functional Theory (DFT) method with Pople’s basis set show that three stable zwitterions for the title molecule have been theoretically determined in aqueous solution and that probably they are present in it medium. Here, the solvent effects were studied by means of the self-consistent reaction field (SCRF) method with the polarized continuum model (PCM). The harmonic vibrational frequencies for the optimized geometries of the three zwitterions were calculated at the B3LYP/6-31G∗ level of the theory. A complete assignment of the IR and Raman spectra of the compound in aqueous solution was performed combining the DFT calculations with Pulay’s Scaled Quantum Mechanics Force Field (SQMFF) methodology in order to fit the theoretical frequency values to the experimental ones. Moreover, Natural Bond Orbital (NBO) and topological properties calculations were performed to analyze the energies and geometrical parameters of its three zwitterions in aqueous medium as well as the magnitude of the intramolecular interactions. The bond orders, atomic charges, solvation energies, dipole moments, molecular electrostatic potentials and force constants parameters calculated for zwitterions in aqueous solution, may be used to gain chemical and vibrational insights into related compounds.