Experimental Vibrational Density Of States Research Articles

Selecting reliable interatomic potentials for classical molecular dynamics simulations of glasses: The case of amorphous SiO2

This paper presents an approach to judge the quality of classical interatomic potentials used in molecular dynamics simulations of glasses. The static structure and dynamical properties of amorphous SiO2 were simulated by classical molecular dynamics using a series of well known interatomic potentials. Theoretical X-ray and neutron structure factors and effective neutron-weighted vibrational density of states of amorphous SiO2 were computed from the obtained atomistic configurations and quantitatively compared to experimental results. The interatomic potential which best reproduced the experimental X-ray and neutron scattering data severely failed to reproduce the experimental vibrational density of states of amorphous SiO2. It is found that only the potential developed by van Beest, Kramer, and van Santen (BKS) was able to adequately reproduce both static structure and dynamical properties of amorphous SiO2. Thus, the fact that an interatomic potential is able to properly reproduce static structures of amorphous systems should not be considered as a basis to use this potential to simulate other properties of these systems.

Study of translational, librational and intra-molecular motion of adsorbed liquid water monolayers at various TiO2 interfaces

Equilibrium classical molecular dynamics (MD) simulations have been performed to investigate the vibrational motion of water in contact with rutile-(110), rutile-(100), rutile-(001), anatase-(101) and anatase-(001) surfaces at room temperature (300 K). The vibrational density of states (VDOS) of the first adsorbed monolayer of liquid water has been analysed for each surface. These have been compared with reported experimental INS values involving rutile and anatase polymorph surfaces, along with ab initio MD results. It is observed that good qualitative agreement is obtained for the rutile-(110) and the anatase-(101) surfaces with the corresponding experimental VDOS. A significant contribution from librational dynamics is found for planar rutile surfaces, but no such demarcation is seen in the experimental VDOS.

Iron isotope fractionation between pyrite (FeS 2), hematite (Fe 2O 3) and siderite (FeCO 3): A first-principles density functional theory study

In addition to equilibrium isotopic fractionation factors experimentally derived, theoretical predictions are needed for interpreting isotopic compositions measured on natural samples because they allow exploring more easily a broader range of temperature and composition. For iron isotopes, only aqueous species were studied by first-principles methods and the combination of these data with those obtained by different methods for minerals leads to discrepancies between theoretical and experimental isotopic fractionation factors. In this paper, equilibrium iron isotope fractionation factors for the common minerals pyrite, hematite, and siderite were determined as a function of temperature, using first-principles methods based on the density functional theory (DFT). In these minerals belonging to the sulfide, oxide and carbonate class, iron is present under two different oxidation states and is involved in contrasted types of interatomic bonds. Equilibrium fractionation factors calculated between hematite and siderite compare well with the one estimated from experimental data (ln α 57Fe/ 54Fe = 4.59 ± 0.30‰ and 5.46 ± 0.63‰ at 20 °C for theoretical and experimental data, respectively) while those for Fe(III) aq-hematite and Fe(II) aq-siderite are significantly higher that experimental values. This suggests that the absolute values of the reduced partition functions ( β-factors) of aqueous species are not accurate enough to be combined with those calculated for minerals. When compared to previous predictions derived from Mössbauer or INRXS data [Polyakov V. B., Clayton R. N., Horita J. and Mineev S. D. (2007) Equilibrium iron isotope fractionation factors of minerals: reevaluation from the data of nuclear inelastic resonant X-ray scattering and Mössbauer spectroscopy. Geochim. Cosmochim. Acta 71, 3833–3846], our iron β-factors are in good agreement for siderite and hematite while a discrepancy is observed for pyrite. However, the detailed investigation of the structural, electronic and vibrational properties of pyrite as well as the study of sulfur isotope fractionation between pyrite and two other sulfides (sphalerite and galena) indicate that DFT-derived β-factors of pyrite are as accurate as for hematite and siderite. We thus suggest that experimental vibrational density of states of pyrite should be re-examined.

High frequency surface vibrational modes and relaxation of MgO nanocrystals.

We present a computational proof for the puzzling experimental vibrational density of states in MgO nanocrystals measured by neutron scattering. For the first time, the experimental peak of longitudinal optical modes in the high frequency region is theoretically reproduced and traced back to surface inward relaxation. Practically perfect agreement of theory with the experiment demonstrates the importance of nanocrystal size effects. Dependence of the transversal optical model on the volume/surface ratio is also verified. Strong (up to 2%) buckling of nanocrystal faces is found. Our calculations show varieties of relaxation shapes for clusters, shedding light on contradictory data on nanocrystals and infinite surfaces.

Partial vibrational spectra for Ni and Nb atoms in the NiNb amorphous system

The partial frequency distributions of the individual chemical constituents and the total vibrational density of states of the amorphous alloy Ni62Nb38 have been measured using neutron inelastic scattering. Samples with different Ni isotope composition were involved. The average interaction constant of the Nb atoms with all neighbours is estimated to be about 40% higher than that of the Ni atoms. The low-temperature heat capacity of the amorphous alloys, Ni62Nb38 and Ni44Nb56, has been investigated by the adiabatic method. An increase of the Nb concentration decreases the Debye temperature (∼ 3%) and increases the density of the electron states at the Fermi level (∼ 26%). The temperature dependence of the heat capacity calculated from the experimental vibrational density of states of amorphous Ni62Nb38 and a model vibrational density of state of Ni44Nb56 were compared with the experimental ones. The former gives a better agreement than the latter.

Atomistic model of orthorhombic YBa2Cu

We have developed a rigid-ion model of the orthorhombic Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ phase. The structure of the model lattice is in good agreement with available experimental structural data. The lattice is found to be unstable at 700 K where the initiation of disordering of the oxygen ions in the copper-oxygen linear chains is observed. Vibrational properties of the lattice have been calculated and compared with the experimental vibrational density of states.