Effect Of Structural Relaxation Research Articles

Fluorescence spectra and energy transfer in amorphous naphthalene

The temperature evolution of fluorescence spectra was measured to study the behavior of electronic excitation in amorphous naphthalene prepared by vacuum deposition on cold substrates. Monomer- and excimer-type fluorescence was observed in the amorphous state. To study the effect of structural relaxation, spectra were measured by raising the temperature stepwise and also measured at a constant temperature each time after the annealing at different temperatures. No serious structural relaxation was observed if we keep the annealing temperature below the crystallization temperature. From the behavior of the intensities of the monomer and excimer fluorescence, it was found that thermally activated energy transfer is efficient even in the amorphous state.

Structural and Elastic Properties of Nanocrystalline Iron and Nickel Prepared by Ball Milling in Controlled Thermodynamic Environment

We present preliminary results about nanocrystalline iron and nickel produced by ball milling under different thermodynamic conditions. The average grain size and the lattice strain were determined by means of XRD profile broadening analysis. The elastic modulus and internal friction were measured for the two systems as a function of temperature. A marked structural relaxation effect starting at 600 K is experienced.

EPR study of the mixed-valent diiron sites in mouse and herpes simplex virus ribonucleotide reductases. Effect of the tyrosyl radical on structure and reactivity of the diferric center.

Reduction of ribonucleotide reductase (EC 1.17.4.1) R2 proteins in a frozen glycerol-buffer solution at 77 K by mobile electrons generated by gamma-irradiation produces EPR-detectable iron sites in mixed-valent Fe(II)/Fe(III) states. The primary EPR signals give information about the ligand arrangement of the diferric form of the iron site, whereas secondary signals observed after annealing of the sample show the effects of structural relaxation. In recombinant metR2 proteins (without free radical) from mouse and herpes virus type 1, the mixed-valent sites trapped at 77 K give rise to axial S = 1/2 EPR spectra with g values in the range 1.79-1.94, observable at temperatures up to 110 K. The spectra are assigned to mu-oxo-bridged dinuclear iron sites. In mouse metR2, the primary EPR spectrum is a mixture of two components. Annealing the R2 samples to 160-170 K transforms the primary EPR signals into rhombic spectra, characterized by gav < 1.8, and observable only below 25 K. These spectra are assigned to partially relaxed forms with a mu-hydroxo bridge, formed by protonation of the oxo bridge. Further annealing at 220 K produces new rhombic EPR spectra, which are closely similar with those observed and found to be stable after chemical reduction at room temperature. The EPR signal of the primary mixed-valent iron site in active mouse R2 protein with a tyrosyl radical also has two components. Both are different from those observed in metR2. In herpes simplex virus type 1 protein R2, one primary mixed-valent component was observed for the met protein. The dose-yield curve for the mixed-valent state in active mouse R2 is sigmoidal in shape, indicating that the tyrosyl radical is reduced by mobile electrons before the iron site. Kinetic experiments on the reduction by dithionite on mouse R2 without and with radical show a significantly enhanced rate for reduction of the iron site in the protein without radical. The results suggest that in active mouse R2 only complete diferric sites with neighboring radicals give rise to the mixed-valent spectra, and that these sites may exist in two structurally distinct forms. The results on the mouse R2 proteins confirm and extend previous results obtained on the Escherichia coli protein R2 showing that the presence of the tyrosyl radical significantly affects not only the structure but also the reactivity of the iron site.

High‐pressure behavior of anorthite: Compression and amorphization

We present the results of a detailed study of the high‐pressure behavior of anorthite (CaAl2Si2O8) between ambient pressure and 30 GPa under static conditions, using in situ Raman spectroscopy and energy‐dispersive X ray diffraction. On increasing pressure under hydrostatic conditions, the transition occurs first at 2.6 GPa. At 10 GPa, a reversible polymorphic transition is observed which transforms the polymorph into a phase of higher symmetry. With further pressure increase, large changes occur in the Raman and X ray spectra between 14 and 16 GPa, premonitory to the onset of pressure‐induced amorphization. Above 16 GPa, anorthite is fully amorphous. However, only samples pressurized to above 22 GPa remain amorphous on recovery to ambient conditions. The high‐pressure behavior of anorthite is highly sensitive to deviatoric stresses. Under less hydrostatic conditions, the 10‐GPa transition described above occurs below 9 GPa, amorphization begins below 11.2 GPa, and samples decompressed from peak pressures as low as 17 GPa are fully amorphous. We have also carried out a thermodynamic analysis of the pressure‐induced amorphization of anorthite, using a two state model for the equation of state (EOS) of CaAl2Si2O8 glass in order to account for the increase in Al coordination. With this EOS, we calculate that the free energies of the crystalline and amorphous materials are equal at about 30 GPa, or slightly lower when effects of structural relaxation in the glass are taken into account, well above the observed crystalline‐amorphous transition at 16 GPa.

Effect of structure relaxation on the holographic recording in amorphous As 2S 3 films

Results of the experimental studies of holographic recording in a-As 2S 3 films, such as exposure and intensity dependences, spatial frequency dependence, are reported. Several peculiarities are found including recording intensity threshold, nonmonotonical intensity dependences, nonlinear exposure curves, the change of the spatial frequency response due to film aging. These peculiarities are explained in terms of the phenomenological model which is based on the simultaneous action of the photostructural changes and the relaxational structural changes.

Effect of structural relaxation on electrochemical corrosion behaviour of amorphous alloys

Effect of structural relaxation on electrochemical corrosion behaviour of amorphous alloys

Nonlinear stress‐strain behavior of glassy polymers

AbstractWe have attempted to provide a unified account of the structural relaxation and mechanical properties of glassy polymers by reviewing recent results of a predictive model. One would like to understand how the effects of structural relaxation influence the performance of amorphous solids. The physics of glassy polymers is still evolving and the functional relationships between relaxation and deformation have not been firmly established. However, significant progress has already been made that can be utilized in solving problems in the large deformation of polymers. Equations have been derived that can be applied to provide quantitative prediction of the nonlinear stress‐strain relationships as a function of physical aging, strain rate, temperature, external stress field, and the filler concentration in composites.

Enhancement of superconductivity in relaxed Fe-Zr metallic glasses

The effects of irreversible structural relaxation on the superconducting transition temperature, T c, for Fe x Zr 100− x metallic glasses in the range 20 <- x <- 33 are studied. The samples were characterized by X-ray diffractometry and differential scanning calorimetry. Magnetic susceptibility, upper critical field, and superconducting transition temperature were measured for as-made and relaxed Zr-rich compositions. The results show an unusual increase in superconducting transition temperature upon thermal annealing which was not observed previously in any other metallic glass system. This phenomenon is due to the presence of spin fluctuations in Fe-Zr glasses. A theoretical basis for this T c enhancement is provided by analyzing the experimental data using the modified form of the McMillan equation together with the expression for the Stoner enhanced magnetic susceptibility.

The TiO 2(100)(1 × 3) reconstruction: insights from ab initio calculations

First-principles plane-wave pseudopotential calculations based on density functional theory within the local-density approximation have been performed on the unreconstructed TiO 2(100) surface and its (1 × 2) and (1 × 3) reconstructions, which consist of alternate (110) and (11̄0) microfacets. The calculations, which include full relaxation of all ionic coordinates, show that there is no driving force for reconstruction in the stoichiometric material. The pattern of displacements on the (110) microfacets formed by the (1 × 3) reconstruction is qualitatively similar to those on the unreconstructed (110) surface. However, the energy of the reconstructed surface is considerably lower than that predicted by multiplying the (110) surface energy by the appropriate geometrical factor. Surprisingly, this difference is due to additional electronic relaxation in the reconstructed system, and not to the effects of structural relaxation.

The effects of covalent bonds on the localized relaxations in the glassy states of linear chain and network macromolecules

Dipolar relaxations in the glassy states of five molecular liquids and of their partially and completely polymerized structures, both linear chain and network types, were studied by dielectric measurements for a fixed ac frequency of 1 kHz. Calorimetric measurements were made to determine the number of covalent bonds formed at different instants of polymerization, and the results were combined with the dielectric data to investigate how the height of the relaxation peak indicating localized dipole diffusion changed as the number of covalent bonds in the spontaneously polymerizing material increased. The height, or strength, of the peak due to the localized relaxation process in the molecular liquid gradually decreased and the process approached extinction as the number of covalent bonds approached its limiting value. Concurrently, a new localized glassy state relaxation process evolved at a temperature with a peak 70–90 K above that of the one observed for the molecular state and its height increased and approached a constant value as the number of covalent bonds formed approached its limiting value. These changes were quantitatively similar for the five materials, irrespective of the structure of the macromolecular product. The extinction of the sub-Tg relaxation peak observed originally for the unreacted state has features in common with structural relaxation effects.

Probing DNA's Dynamics and Conformational Substates by Enthalpy Relaxation and Its Recovery

The A and B form of NaDNA with hydration level of between 0.15 and 0.64 (g of water)/(g of NaDNA) have been vitrified by cooling at rates of ≈80 K min-1, and their thermal behavior on reheating at 30 K min-1 was studied from ≈120 to 300 K by differential scanning calorimetry. The calorimetric effects of annealing time and temperature are characteristic of a glass, and they are attributed to enthalpy relaxation of conformational substates of B-DNA. Structural relaxation becomes observable in form of endothermic enthalpy recovery for Γ > 3−4 (water molecules per nucleotide), and the heat effects increase with hydration level linearly up to Γ ≈ 12. Molecular motions caused by the presence of water hydrogen bonded to DNA's functional groups become unfrozen from ≈153 to ≈263 K which is attributed to a very broad distribution of relaxation times. The data suggest that structural relaxation effects as reported here can be linked to the conformational flexibility of B-DNA necessary for protein−DNA interaction.

Obtaining further information from calorimetry

This paper describes how calorimetry may be used to obtain two sets of information, namely, (i) the internal energy of a material's polymorphs by measuring their heat of solution as well as the effect of structural relaxation on this energy, and (ii) the amount of intergranular liquid (or a second solid phase) in polycrystalline solids. Experiments have confirmed the usefulness of the two methods, and have shown that a substantial amount of liquid at thermodynamic equilibrium is present at the grain junctions in a polycrystalline solid, or that such solids premelt significantly. The formalism and concepts developed here are expected to apply equally well to polycrystalline solids, which undergo order-disorder phase transformation, and where the more disordered phase remains at the grain junctions of the ordered phase.

Ab initio study of tritium defects in lithium oxide

Lithium oxide has been suggested as a suitable breeder blanket material for fusion reactors. Tritium ions and lithium vacancies are created by neutron irradiation, forming bulk defect complexes whose exact character is experimentally unclear. We have used ab initio total energy pseudopotential methods to study the structure and relative energies of tritium as a substitutional defect, and of the separate tritium interstitial and lithium vacancy. For all stable defect geometries, the formation of an OT- complex with an O-T bond length of about 1 AA is found to be energetically favoured. In the case of the substitutional defect this bond is found to point towards the vacant Li site, but the direction is fairly free for the interstitial case. The binding energy of tritium to a lithium vacancy is found to be 1.3 eV. Structural relaxation effects are included throughout, and are found to significantly affect the relative energies of different defect geometries. The effects of zero-point fluctuations are estimated and found not to be very significant. The most probable migration path of interstitial tritium is identified as a jump between nearest-neighbour oxygen ions, with an activation energy of 0.45 eV, in agreement with experimental evidence. The results suggest a picture of thermally assisted diffusion of tritium interstitials and lithium vacancies along the anion and cation sublattices respectively, with the preferential trapping of the two defects into substitutional complexes.

Relativistic band structure of ternary II-VI semiconductor alloys containing Cd, Zn, Se and Te

The relativistic band structure and band-gap bowing factors of ternary II-VI semiconductor alloys containing Cd, Zn, Se and Te are calculated by the ab initio self-consistent pseudopotential method with spin-orbit correction. The disorder in these alloys is modelled by the virtual crystal approximation (VCA). The numerical results thus obtained are compared with experimental data from photoluminescence spectroscopy. The experimental band-gap bowings for this family of alloys are always concave upwards. However, CdSexTe1-x and ZnSexTe1-x in general show much larger bowing factors than Cd1-xZnxSe and Cd1-xZnxTe. The former two alloys show a minimum in the band gap versus composition curve near x=0.35 while the latter two have it at x=0. Most of these qualitative trends are well explained by VCA. However, in order to explain the large bowing in the first two alloys, some kind of local order has to be incorporated in the calculation. It was found that a local cluster with AuCu-I-like structure, when relaxed, has quite a significant contribution to the bowing factors. The splittings in the band structure due to both spin-orbit and structural relaxation effects are also discussed.

Tight-binding calculation of ionization potentials of small silicon clusters

The size dependence of ionization potentials (IPs) of silicon clusters is studied using a localized orbital theory on the basis of the tight-binding approximation. The geometric structures of the silicon clusters are from the theoretical results of the force field model, and the hopping integral and overlap integral in the tight-binding Hamiltonian matrix are obtained from the best fitted formula for the Slater-Koster parameters. It has been found that our results conform to experiments much better than the previous LDA calculation and the estimation of the spherical droplet model. The local maximum of the IPs for Si 10 is obtained, corresponding to the magic number behaviour observed by other experimental means. The structural isomer effect and relaxation effect on the IPs of the silicon clusters are also discussed.

X-ray-diffraction study of structural relaxation in metallic glasses

The effect of structural relaxation of amorphous ${\mathrm{Zr}}_{67}$${\mathrm{Ni}}_{10}$${\mathrm{Cu}}_{23}$ was studied by measuring the structure factor with x-ray diffraction. The changes in the radial distribution of the atoms upon thermal relaxation are described quantitatively by a model initially proposed for amorphous ${\mathrm{Pd}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{P}}_{20}$. The model describes the structural changes by two orthogonal atomic processes. Radial atomic motion changes the density of the glass, while the short-range order of the glass is enhanced or reduced by local atomic motion which conserves the density of the sample. This local shear motion of the atoms is correlated over 2.5 \AA{} in ${\mathrm{Zr}}_{67}$${\mathrm{Ni}}_{10}$${\mathrm{Cu}}_{23}$, while for ${\mathrm{Pd}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{P}}_{20}$ the correlation length is larger than 7 \AA{}. Structure and chemical bonding in the purely metallic and the metal-metalloid system most likely determine these length scales.

Computational investigation of surface structural relaxation in crystalline urea

The effects of surface structural relaxation on the energetic and morphological properties of crystalline urea have been investigated. Specifically, the {001}, {110}, {200}, {101}, {111} and {[graphic ommitted]} surfaces have been considered. Upon relaxation of the cleaved surfaces, significant perturbations in the positions and internal geometries of the molecules (compared with the corresponding positions in the bulk crystal structure) are observed. Interestingly, the {[graphic ommitted]} surface has a lower surface energy than the {111} surface, and thus a polar morphology is predicted. The predicted crystal shape is compared with that observed experimentally (for crystals grown by deposition from the vapour phase), and reasons for discrepancies between the calculated and experimental morphologies are discussed.

Combined Measurements of Length and Modulus Change and Calculation of Effective Pair Potentials for the Amorphous Alloy Cu64Ti36

AbstractSimultaneous measurements of the length change and of the change of the eigenfrequency at the same specimen are performed for the metallic glass Cu64Ti36. The length change is preferably connected with a long‐range topological relaxation, while the change of the eigenfrequency preferably gives information on the change of the chemical short‐range order. The correlation between the relaxation effects of the two quantities shows a certain change of slope at a characteristic annealing temperature indicating a transition from preferably chemical to topological rearrangements. Assuming a Lennard‐Jones type potential the powers of the effective pair potential can be calculated from the temperature dependence of the measured quantities. It is also possible to get information on the variation of the powers due to the structural relaxation effect.

Magnetic behavior of an amorphous ferromagnet for temperatures close to, and above, the Curie point: Structural relaxation effects.

Detailed bulk magnetization (ac susceptibility) measurements on the amorphous ${\mathrm{Fe}}_{16}$${\mathrm{Ni}}_{64}$${\mathrm{B}}_{19}$Si alloy have been performed in the temperature region 0.95${\mathit{T}}_{\mathit{C}}$--1.5${\mathit{T}}_{\mathit{C}}$ (0.95${\mathit{T}}_{\mathit{C}}$\ensuremath{\le}T\ensuremath{\le}1.05${\mathit{T}}_{\mathit{C}}$) before and after it had undergone isothermal annealing at 400 K for 60 and 120 min. An elaborate analysis of the data so obtained, besides yielding accurate values for the asymptotic and leading ``correction-to-scaling'' (CTS) critical exponents and amplitudes, reveals that the structural relaxation consequent upon isothermal annealing does not have any influence on the asymptotic and CTS critical exponents and on the universal amplitude ratios ${\mathit{m}}_{0}$/${\mathit{M}}_{\mathit{s}}$(0) and ${\mathit{Dm}}_{0}^{\mathrm{\ensuremath{\delta}}}$/${\mathit{h}}_{0}$, whose values are close to those theoretically predicted for a three-dimensional Heisenberg ferromagnet, but has profound effect on the ratio ${\mathrm{\ensuremath{\mu}}}_{0}$${\mathit{h}}_{0}$/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{C}}$ and the Curie temperature, ${\mathit{T}}_{\mathit{C}}$. The fraction of spins actually involved in the transition at ${\mathit{T}}_{\mathit{C}}$ is small and reduces slowly with increasing annealing time. Consistent with the theoretical expectations, nonanalytic corrections (originating from the nonlinear irrelevant scaling fields) to the singular behavior at ${\mathit{T}}_{\mathit{C}}$ dominate over the analytic ones (arising on account of the nonlinear relevant scaling fields) in the critical region but the reverse is true for T\ensuremath{\gg}${\mathit{T}}_{\mathit{C}}$. Regardless of whether the sample is annealed or not, the initial susceptibility follows the generalized Curie-Weiss law for as wide a temperature range as ${\mathit{T}}_{\mathit{C}}$\ensuremath{\le}T\ensuremath{\le}1.5${\mathit{T}}_{\mathit{C}}$. The present results provide a strong evidence for weak itinerant ferromagnetism in the glassy alloy in question.

The relaxation of internal stresses during annealing of amorphous Fe 40Ni 40B 20

Because of the magneto-elastic coupling, the magnetic domain structure of amorphous materials is mainly determined by the local stress state. The stress dependence of the domain structure therefore contains important information on the internal stress distribution in the material. The three-dimensional neutron depolarization technique was used to study the stress dependence of the bulk domain structures in as-quenced and annealed Fe 40Ni 40B 20. A three-layer domain structure model corresponding to compressive and tensile internal stresses is presented to explain the measured data. Using this model, the average magnitude of the internal stresses could be followed as a function of the annealing treatment. A comparison is made between the kinetics of stress relaxation and the structural relaxation effects.