Intense Relaxation Process Research Articles

Competition between Polar and Antiferrodistortive Modes and Correlated Dynamics of the Methylammonium Molecules in MAPbI3 from Anelastic and Dielectric Measurements.

The mechanisms behind the exceptional photovoltaic properties of the metallorganic perovskites are still debated and include a ferroelectric (FE) state from the ordering of the electric dipoles of the organic molecules. We present the first anelastic (complex Young's modulus) and new dielectric measurements on CH3NH3PbI3, which provide new insight into the reorientation dynamics of the organic molecules and the reason why they do not form a FE state. The permittivity is fitted within the tetragonal phase with an expression that includes the coupling between FE and octahedral tilt modes, indicating that the coupling is competitive and prevents FE ordering. The onset of the orthorhombic phase is accompanied by sharp stiffening, analogous to the drop of permittivity, due to the hindered molecular dynamics. On further cooling, an intense anelastic relaxation process without a dielectric counterpart suggests the reorientation of clusters of molecules with strong antiferroelectric correlations.

Internal friction in chalcogenide glasses Ge-As-Se system in glass transition region

Purpose. The purpose of this work is detecting and determine of mechanisms of relaxation transitions in chalcogenide glasses Ge-As-Se system based on research of temperature dependences of internal friction, shear modulus over a wide temperature range on infralow frequencies. Methods. The temperature dependences of the internal friction and the shear modulus for glasses in the Ge–As–Se system at infralow frequencies of forced torsional vibrations were measured by torsional pendulum method. Results. For all the glasses under investigation, the mechanical losses rapidly increase and, simultaneously, the shear modulus decreases in the range of the glass transition temperature T g . With an increase in the strain frequency, these effects shift toward the high_temperature range, which indicate that the intensive relaxation process proceeds in this temperature range. The parameters U α and τ 0 of this relaxation process were determined from the frequency shift in the hightemperature branch Q –1 ( T ). Conclusions. An increase in the internal friction and the mechanical compliance of glasses in the Ge–As–Se system in the range of the glass transition temperature T g is caused by the activation of the α relaxation process. At Z =2.6, the activation energy of this process increases considerably as a result of the change in the kinetic structural unit responsible for the relaxation process under consideration. α relaxation process occurs as a result of the full defreezing of the mobility of glass structure in T g region.

Electrical properties of chalcogenide glass of composition Ge8As32S60

The results of the studies of the dielectrical and impedance spectra of chalcogenide glass samples of the Ge8As32S60 composition in an AC field of a frequency range of 103–106 Hz are presented. Intense relaxation processes are revealed in the samples under study and a dispersion of the electrical parameters corresponding to a relaxation time distribution by the contribution of dipole compositions, which arise with the participation of charged defect centers caused by the presence of arsenic atoms’ dangling bonds in the structure of chalcogenide semiconductor, is established. It is established that a low-frequency region of the complex impedance’s components of the parameter mentioned is associated with the electrical properties of the contact metal–vitreous semiconductor, while a high-frequency one is associated directly with the properties of the material under study.

Ferroelectricity and molecular dynamics of poly(vinylidenefluoride-trifluoroethylene) nanoparticles

Here we show that ferroelectric polymeric nanoparticles of a poly(vinylidene fluoride-trifluoroethylene) random copolymer, can be prepared by dialysis nanoprecipitation. By this method semicrystalline particles with mean diameter of 280 nm, as revealed by atomic force microscopy, can be obtained. The ferroelectric nature at room temperature of the nanoparticles has been proven by piezoresponse force microscopy, differential scanning calorimetry, X-ray diffraction at wide angles and broad band dielectric spectroscopy. The poly(vinylidene fluoride-trifluoroethylene) nanoparticles exhibit a ferroelectric-to-paraelectric transition at lower temperature compared with the bulk. In the paraelectric phase the nanoparticles show an intense dielectric relaxation process associated to the rotational motion of the dipoles within the paraelectric crystalline phase. The nature of this dielectric process is similar to that of the bulk.

Polymorphism and disorder in caffeine: Dielectric investigation of molecular mobilities

Using dielectric relaxation data we have characterized the molecular mobilities of caffeine both in phase I (stable and metastable) and in phase II. In phase I effects of sublimation and phase transformation kinetics were carefully considered. In plane rotational motions were followed on a wide temperature range. A noticeable antiferroelectric short range order developing at the approach of the glass-like transition is characterized. Condition for occurrence of a critical-like behaviour is discussed. At high temperature the emergence of an additional ultra slow relaxation process is highlighted. Possible molecular mechanisms are proposed for both processes. In phase II the existence of a less intense relaxation process is confirmed. Close similarity with the main process developing in phase I hints at a common origin of the dipolar motions. Careful consideration of recent structure determinations leads to suggest that this process is associated to similar molecular in plane rotations but developing at the surface of crystalline samples. Lower cooperativity at the surface is reflected in the smaller activation entropy of the relaxation.

Local dielectric spectroscopy of polyvinylpyrrolidone–Mo6S2I8 nanowire composite

A polymeric nanocomposite of polyvinylpyrrolidone and Mo6S2I8 inorganic nanowires was obtained, and its electric properties were characterized on a local scale in the vicinity of the interface between polymer and nanoinclusions. Local dielectric spectroscopy was applied to investigate relaxation dynamics of the polymer close to such interface, at variance of temperature and moisture content, within a thin film supported on a conductive substrate. An intense relaxation process is observed at the polymer/nanoinclusions interface, which could be a secondary relaxation process of the polymer enhanced by the proximity to the conductive nanowires. Furthermore, its relaxation dynamics becomes slower when moisture content is increased which could be explained by the hindering of pyrrolidone ring motions due to hydrogen bonding.

Relaxation processes in the polymer structure of an azomethine base

The results of studying the isothermal polarization of thin polymer films of an azomethine compound (Schiff base) with varying polarization field intensity are described. Intense relaxation processes induced by space-charge accumulation in the metal-polymer near-contact zone are revealed in the studied samples. Experimental data are analyzed in terms of the relay mechanism of charge transfer in the conduction band involving deep local levels. Values of the microparameters characterizing the observed relaxation phenomena are determined.

Principles of generalized spin-echo spectroscopy

The concept is proposed for determining the total dynamic scattering function of an object under study, representing a sum of odd and even parts measured by the generalized neutron spin-echo method in the form of the signals Sodd(q, t) ∼ ΣS(ω, q)sin(ωt)dω and Seven(q, t) ∼ ΣS(ω,q)cos(ωt)dω as functions of the momentum q transferred to the neutron and the time t corresponding to the frequency ω and the transferred energy ℏω. The principle of the generalized spin echo and the results of mathematical modeling are confirmed in experiments on inelastic scattering on magnetic fluids and polymer solutions. The developed method makes it possible to study the features of the dynamics of atomic and molecular systems, e.g., to analyze soft vibrational spectra of nanoparticle ensembles against the background of intense relaxation processes, which is inaccessible for classical spin-echo spectrometry.

Isothermal polarization in thin-film Al-As2Se3-Al metal-dielectric-metal structures

Results of the study of the kinetics of isothermal polarization current in As2Se3 films are presented. In the samples under study, intense relaxation processes related to the accumulation of bulk charge in the metal-semiconductor contact area occur. The experimental results are interpreted using the model of the relay conductivity mechanism.

Peculiarities of dielectric relaxation in poly(vinylidene fluoride) with different thermal history

A method of broad-band dielectric relaxation spectroscopy (10−1–107Hz) has been used to study the relaxation processes in the region of melting in two isotropic poly(vinylidene fluoride) films prepared by way of crystallization from melts under different conditions. The intensive relaxation process observed is attributed to manifestation of a space charge formed by extrinsic carriers. The relaxation time and the intensity of the process reduce when the crystallinity increases. This is related to the increase of extrinsic carriers’ mobility in the amorphous areas. It is supposed that at isothermal crystallization there are more chemical defects (end groups, branches, HHTT-type defects) in amorphous areas. Such defects may impede compact chain packing in the amorphous domains and provide conditions for a higher level of mobility of free carriers.

Dielectric relaxation in vinylidene fluoride–hexafluoropropylene copolymers

AbstractThe molecular mobility in copolymers of vinylidene fluoride–hexafluoropropylene VDF/HFP of 93/7 and 86/14 ratios has been investigated by means of broadband dielectric relaxation spectroscopy (10−1–107 Hz), differential scanning calorimetry DSC (−100 to 150°C), and of wide angle X‐ray diffraction WAXS. Four relaxation processes and one ferroelectric‐paraelectric phase transition have been detected. The process of the local mobility β‐ (at temperatures below glass transition point) is not affected by chemical composition of the copolymer and the formed structure. Parameters of segmental mobility in the region of glass transition (αa‐relaxation) depend on the ratio of comonomer with lower kinetic flexibility. αc‐relaxation is clearly observed only in VDF/HFP 93/7 copolymer, which is characterized by a higher crystallinity and a higher perfection of crystals of α‐ (αp‐) phase. Diffuse order–disorder relaxor type ferroelectric transition connected with the destruction of the domains in low‐perfect ferroelectric phase in the amorphous regions has been detected for both copolymers. An intensive relaxation process (α‐process) was observed for both copolymers in high‐temperature region. DSC data shows that it falls on the broad temperature region of α‐phase crystals melting. It is considered to be connected with the space charge relaxation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Investigation into the dielectric relaxation of vinylidene fluoride copolymers with hexafluoropropylene

Four relaxation processes and one ferroelectric-paraelectric phase transition are revealed in vinylidene fluoride-hexafluoropropylene copolymers with different ratios of the components in the temperature range from −100 to 150°C. The relaxation process occurring at the lowest temperature is associated with the local mobility of the chains, whereas the relaxation process at a higher temperature is due to micro-Brownian motion of segments in the amorphous phase in the glass transition range. A smeared relaxor phase transition from the polar modification of the α phase of vinylidene fluoride units to the paraelectric phase is observed in the temperature range 50–70°C. At higher temperatures, there occurs an intensive relaxation process that can be attributed to space-charge relaxation or manifestation of the normal relaxation mode.

Molecular mobility and fragility in n-ethylene glycol dimethacrylate monomers

Di-, tri- and tetra-ethylene glycol dimethacrylate monomers were investigated by dielectric relaxation spectroscopy in the frequency range from 20 Hz to 1 MHz and in the temperature region comprised between −120 and +25 °C. A rather intense relaxation process associated with the glass transition is found for all the monomers in the temperature region between −80 and −50 °C. The mobility of the cooperative process decreases with the molecular weight increase. The temperature dependence of the relaxation times reveals some curvature, which increases with the size of the ethylene glycol moiety (di≅tri<tetra). In spite of this behavior the di- and tri-ethylene monomers could be treated as Arrhenius type, presenting similar activation energies. The fragility indexes estimated by both dielectric relaxation spectroscopy and differential scanning calorimetry techniques agree for the di- and tri-ethylene monomers only when Arrhenius behavior is assumed. Despite the different values estimated, the fragility is comparable to a quite strong glass former as glycerol.

Search for incipient lattice instabilities inMgB2by anelastic spectroscopy

We measured the complex dynamic elastic modulus of MgB2 between 1.3 and 650 K at frequencies included between 5 and 70 kHz. The main purpose of this work is to search for possible lattice instabilities, which could contribute to enhance the electron-phonon coupling. The Young's modulus indeed presents an anomalous softening on cooling below 400 K, and hysteresis between cooling and heating. Such anomalies, however, can be accounted for by intense relaxation processes with maxima between 50 and 150 K, whose nature is not yet clarified, and by geometric defects generated by the anisotropic thermal expansion between the grains during thermal cycling. Additional intense relaxation processes are observed above room temperature, and their possible origin is discussed. The influence of the highly anharmonic in-plane vibration modes of the B atoms on the elastic and anelastic properties of MgB2 is discussed in detail.

Dilational rheology of Langmuir polymer monolayers: Poor-solvent conditions

The viscoelastic moduli (elasticity and dilational viscosity) of monolayers of P4HS have been studied over a broad frequency range (0.1 mHz–200 kHz) using a combination of relaxation and capillary waves techniques. The analysis of the surface pressure, the elasticity and the viscosity in the semidilute regime show that the air–water interface is a poor (near-Θ) solvent for the monolayer. The results of viscoelastic moduli show that there is a broad relaxation process in the low-frequency range (ω&amp;lt;1 Hz), and another very intense relaxation process centered in the kHz region. This behavior contrasts with the one previously found for PVAc, a polymer for which the interface is a good solvent. For PVAc the relaxation found at low frequencies is much narrower, and two processes are clearly distinguished at higher frequencies: one centered at 500 Hz and another one at 40 kHz.

Dislocation pattern formation in the course of plastic deformation

The self-consistent evolution of a two-dimensional screw dislocation ensemble is investigated in plastically deformed crystal. A generalized isotropical multicomponent model for dislocation dynamics is proposed. It is found that the intense relaxation process of the excess Burgers vector of the two-dimensional screw dislocat ensemble results in the establishment of an effective diffusion regime for average density ρ,(r,t) evolution. In a two-component system this gives rise to mutual component interdiffusion, playing an important role in the nonlinear evolution stage. Two types of spatial structures are found to emerge in the course of plastic deformation: i) a one-dimensional structure, which is established due to rivalry between unstable modes and results in the emergence of reorientation bands in the crystal; ii) a two-dimensional structure, which is established due tocooperation of the modes and is responsible for the hexagonal cell formation in the crystal. It is shown that for a sufficiently low strain level a hexagonal cellular structure is preferable.

Thermally activated dynamics of the tilts of the CuO6octahedra, hopping of interstitial O, and possible instability towards the LTT phase in La2CuO4+δ

The anelastic spectrum (dynamic Young's modulus and elastic energy absorption) of La${}_{2}$CuO${}_{4+\ensuremath{\delta}}$ has been measured between 1 and 700 K with $0&lt;\ensuremath{\delta}&lt;0.02$. The spectrum of stoichiometric La${}_{2}$CuO${}_{4}$ in the low-temperature orthorhombic (LTO) phase is dominated by two intense relaxation processes which cause softenings of 16% around 150 K and 9% below 30 K at $f\ensuremath{\sim}1$ kHz. The relaxation at 150 K is attributed to the presence of a fraction of the CuO${}_{6}$ octahedra which are able to change their tilted configuration by thermal activation between orientations which are nearly energetically equivalent, possibly within the twin boundaries. The relaxation below 30 K is governed by tunneling, and involves a considerable fraction of the lattice atoms. It is proposed that the double-well potentials for the low-temperature relaxation are created by the tendency of the LTO phase to form low-temperature tetragonal (LTT) domains, which however are not stabilized like when La is partially substituted with Ba. On doping with excess O, the relaxation rates of these processes are initially enhanced by hole doping, while their intensities are depressed by lattice disorder; an explanation of this behavior is provided. Excess O also causes two additional relaxation processes. The one appearing at lower values of $\ensuremath{\delta}$ is attributed to the hopping of single interstitial O${}^{2\ensuremath{-}}$ ions, with a hopping rate equal to ${\ensuremath{\tau}}^{\ensuremath{-}1}=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\mathrm{exp}(\ensuremath{-}5600/T)$ s. The second process is slower and can be due to O pairs or other complexes containing excess O.

Microwave dielectric relaxation in barium lanthanide tetratitanates (BLT)

Abstract Polycrystalline monophase mixed oxides BaLn2Ti4O12 where Ln = La, Pr, Nd, Sm and Eu have been studied at the frequency interval 10–1013 Hz in temperature range 4–1000 K in order to established a reasons of unusual combination its properties: low microwave losses, high dielectric permittivity and its thermal stability. In the BLT we have first detected an intensive electronic relaxation process with very low activation energy. The last suggesting that large polarizable and strongly coupled with the lattice 4f/5d hybridizated bonds viscously suppress paraelectric-like soft lattice mode.

The measurement of early metastable states in amorphous strips

Evidence suggests that a freshly formed ultra-rapidly quenched sample possesses a considerable excess of free volume in comparison with a sample at room temperature. Taking this issue into account, it is assumed that a sample is significantly relaxed at room temperature, i.e. an intensive relaxation process takes place. The authors describe the experimental techniques used for the investigation of a relaxation process which takes place immediately after sample formation and ends at room temperature. Two elements which enabled this investigation are shown. The procedure for producing amorphous strips with a 'frozen-in' structure formed immediately after quenching is described first. The detailed measurement technique of changes in electrical resistance is then explained, which enables research into the relaxation processes which start at liquid nitrogen temperature where the samples are kept after formation and ends at room temperature.

Temperature dependence of the high-speed stretching diagrams of synthetic yarns

Judging from values of activation energy, the deformation transition which is displayed on high-speed stretching diagrams may be interpreted as the start of appearance of intense relaxation processes which are caused by molecular mobility. The kinetic constants which are determined by the proposed procedure make it possible to predict the change in the indicated transition over a wide temperature range.