Point defects in GaP single crystals investigated by mechanical damping

The dependence of mechanical damping of LEC (liquid encapsulated Czochralski) crystals of GaP: S was measured in the temperature range 300… 600 K and at frequencies near 70 and 140 kHz, respectively. In samples of different stoichiometry two peaks could be observed: Peak A at 350 K and peak B at 450 K. Peak A is attributed to the relaxation of point defects near the surface, peak B is probably due to Ga vacancies.     

Sub-Tg mechanical relaxation of a La55Al25Ni20 amorphous alloy

Dynamic mechanical properties of an amorphous La₅₅Al₂₅Ni₂₀ alloy were measured by a forced oscillation method in the temperature range from room temperature to 453 K, just below the glass transition temperature (T_g = 481 K). The internal friction at a constant frequency 62.8 rad/s of this alloy showed a peak at about 400 K and the peak position shifts with frequency. Structural relaxation reduces the magnitude of the relaxation peak but has little affect on the peak position. Using the time-temperature superposition process, master curves for storage E′ (ln ω) and loss E″ (ln ω) moduli are constructed. Activation energy of the relaxation obtained from shift factors is low, 100 kJ/mol, which is close to that for diffusion of the Al in Al and Ni in Al. The relaxation spectra are very broad with a half width of 2 3 decades.     

Impedance Measurements on Sintered Discs of Sodium Metavanadate

Impedance measurements of sintered discs of sodium metavanadate (NaVO₃) are investigated in the frequency range 10 KHz to 1 MHz and in the temperature region from room temperature (≈ 30 °C) to 450 °C. A.C. conductivity is calculated from the data. D.C. Conductivity in measured in the temperature range 150 to 450 °C. Debye type of relaxation effects are observed in the loss measurements. The activation energy of the dipoles involved in the relaxation is estimated to be 0.76 eV. The activation energy for A.C. conduction in the temperature range 380 to 450 °C is about 0.78 eV. The relaxation phenomenon observed is explained in terms of space charge polarisation due to defects.     

Magnetic Order in Organic Superconductors

We have undertaken an electron spin resonance study at both low and high magnetic field on the superconducting phase of the ambient pressure superconductor (BEDT-TTF>₂I₃, (ET)₂I₃, over the temperature range 1-300 K and hydrostatic pressure range from 0 to 2 kbar. At ambient pressure the ESR results are consistent with the picture of (ET)₂I₃ as a metal. Superconductivity is observed at 1.6 K via low field ESR. Application of modest pressures strongly suppresses the superconducting transition temperature. At pressures above about 0.3 kbar the superconductivity is suppressed in favor of an as-yet-unidentified magnetic state whose onset is at 7K. Through an analysis of the microwave ESR lineshape we find that the microwave conductivity over the temperature range 5-50 K is in agreement with dc measurements.     

Structural relaxation of SiO2 at elevated temperatures monitored by in situ Raman scattering

Raman scattering spectra of high-purity synthetic silica glass were measured in situ in the temperature range from 950 to 1200 °C by means of an experimental approach which gives very low thermal background. The temperature dependence of the scattering permits an analysis of the spectra into the first-order and overtone contributions in a consistent manner. For low-OH content the dynamics of relaxation of the D₂ defect follow a single exponential decay, but more complex relaxation is found when OH content is high. In the latter case a double-exponential fit describes the observed relaxation well. The activation energies found are: for creation of D₂ defects 0.53 ± 0.07 eV; for single exponential relaxation in low-OH material, 6.0 ± 0.3 eV; for high-OH material, primary relaxation 5.1–5.2 ± 0.3 eV, secondary relaxation 2.4–2.5 ± 0.5 eV.     

Acoustic behaviour of amorphous selenium

The attenuation of longitudinal sound waves in amorphous selenium has been measured at frequencies of 15 and 25 MHz and in the range of temperature between 10 and 310 K. It is shown that absorption exhibits a minimum around Tτ150 K, while a pronounced maximum takes place around 30 K. Such a behaviour is briefly discussed in the frame of acoustic properties of amorphous material below the glass temperature. It is shown that the experimental results can be described, almost qualitatively, in terms of a recently proposed theoretical model involving a stochastic resonance that seems to be characteristic of the disordered systems.     

The structural relaxation and glass transition of La---Al---Ni and Zr---Al---Cu amorphous alloys with a significant supercooled liquid region

An amorphous phase with a wide supercooled liquid region, > 50 K, was found to form over wide composition ranges in the La---Al---Ni and Zr---Al---Cu systems. The largest values for the temperature span between the crystallization temperature, T_x, and the glass transition temperature, T_g, ΔT_x(-T_x−T_g), are 69 K for La₅₅ Al₂₅Ni₂₀ and 88 K for Zr₆₅Al_7.5Cu_27.5. The structural relaxation behavior on annealing was examined for the two amorphous alloys with the largest ΔT_x values. The magnitude of the structural relaxation increases gradually with increasing annealing temperature, T_a, and then rapidly in the T_a range slightly below T_g and decreases significantly on annealing T_g. The rapid increase in the magnitude of the structural relaxation on annealing near T_g is due to the glass transition. The single-stage structural relaxation indicates that there is no distinct difference in relaxation times (atomic bonding forces) between the constituent atoms in the two metal-metal-type amorphous alloys. The existence of an optimum bonding state is thought to cause the wide supercooled liquid region for the two amorphous alloys.     

Nuclear spin lattice relaxation in the amorphous state: Towards an understanding

Full reports concerning the nuclear relaxation of ¹¹B, ²³Na, ²⁹Si and ⁷⁷Se in the following glasses: B₂O₃, Na₂B₄O₇, (Na₂O)_0.3(SiO₂)_0.7, Se are presented. Extended measurements confirm the existence of an efficient quadrupolar mechanism typical of glasses and show that the two-phonon process, responsible for the quadropolar relaxation in insulating crystals, is always negligible in our whole range of investigation. The relaxation rate T₁^?1, field independent in almost all cases, varies like T^(1+γ) with 0 < γ < 1 between 1.2 K and 100 K but exhibits different behaviour at higher T depending upon the analysed material. Whereas T₁ goes through a minimum at 300 K in B₂O₃, its T-dependence for ²³Na in Na₂B₄O₇ and (Na₂O)_0.3(SiO₂)_0.7 becomes faster than in the low-T regime. Several attempts made to interpret the data within the framework of the tunneling model, commonly invoked to explain the anomalous thermal properties of glasses, are reviewed in detail. It is concluded that the T₁ results are best accounted for by a process consisting of the modulation of the electric field gradient (EFG) at the nuclear site by its nearest neighbour tunneling defect. This EFG fluctuation is shown to be dominated by interactions between defects and an average correlation time of about 10^?9 sec is assigned to it. A complete T₁ calculation requires the previous knowledge of the physical nature of the defects, so far not available. Each defect is then tentatively identified with a bridging oxygen atom tunneling between two potential wells, which gives the correct energy dependence for the EEG matrix element. A defect number of 10²⁷ m^?3 is shown to be consistent with the results.     

Ultrasonic and brillouin scattering study of the elastic properties of vitreous silica between 10 and 300 K

The velocity and attenuation of longitudinal and transverse waves have been measured for frequencies ranging between 5 MHz and 35 GHz on the same sample of vitreous silica. The main features of the ultrasonic results are described on the basis of the usual model assuming a coupling of phonons with a thermally activated relaxation. In contrast, the frequency dependence of the attenuation peak and velocity minimum are in quantitative disagreement with the calculations based on this model. Above 100 K, at least half of the attenuation at high frequencies has to be assigned to another physical origin, most likely anharmonic three-phonon interactions. Furthermore, the minimum in the hypersonic velocity seems to occur as the result of a combination of dispersion processes more than from the frequency-temperature shift of a relaxational dispersion.     

Possible Ultrasonic Evidence for the Existence of Vortex-Antivortex Pairs

The attenuation of surface acoustic waves in the 700 MHz frequency range passing through 1000 Å NbN films has been measured on several NbN films. The films have a columnar structure where the columns are about 200 Å in diameter separated by 20 Å voids. In the superconducting state the attenuation does not follow the usual BCS curve. It appears to be composed of the sum of a BCS curve plus another curve which has a maximum below the superconducting transition temperature. The attenuation data may also be analyzed to yield an effective energy gap which is quenched at about one fifth of the BCS zero temperature energy gap. Tentatively, the Kosterlitz-Thouless vortex-antivortex model is used to determine the temperature dependence of an effective order parameter that yields reduced attenuation data which agree qualitatively with the experimental results.     

Dielectric behaviour in a vanadium phosphate glass

The dielectric constant and conductivity of 80% V₂O₅: 20% P₂O₅ glass has been measured in the frequency range 10² to 10⁹Hz and in the temperature range 80 to 350°K. It is shown that the dielectric behaviour over these ranges is described by a Debye type relaxation process with distribution of relaxation times. A method is proposed to determine the width of distribution from the data at fixed frequencies and different temperatures. The width of distribution increases at frequencies ω > 10/τ, which leads to an a.c. conductivity at these frequencies almost linearly proportional to frequency and independent of temperature. The estimated value of the static dielectric constant of about 30 was found to decrease with temperature while the infinite frequency dielectric constant of 10 was independent of temperature. The carrier concentration calculated from the dielectric relaxation time and the d.c. conductivity through a thermal diffusion model shows reasonable agreement with direct measurement using electron paramagnetic resonance.     

Electronic relaxation in the PbOSiO2Fe2O3 glass system

The a.c. and d.c. conductivities of PbOSiO₂ glass with 0–10 mole% Fe₂O₃ additions were measured in the temperature range 77–700 K. Two relaxation processes were noted. The relaxation mechanism at lower temperatures is due to polaron hopping between ferric-ferrous ion pairs with a distribution of relaxation times which is energy independent. The second relaxation phenomenon, at higher temperatures is polaron hopping along iron ion chains in the glass of which the pairs responsible for the first process are a part. A theoreticalmodel is proposed and additional evidence for this model is presented in the form of magnetic susceptibility and electron spin resonance (esr) data.     

The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition

During electric polarization charge is injected into the material. The structure is decorated with space charge and during the subsequent heating an apparent peak and the genuine peaks that are related to dipole randomization and charge detrapping are observed. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 623 K. Two weak relaxations have been observed around 337 K and around 402 K. The electrical conductivity changes with temperature in agreement with the Arrhenius law only below (W = (0.84 ± 0.03) eV ) and above ( W = (0.82 ± 0.03) eV) the temperature range where the β relaxation is observed. The variation of the electrical conductivity with temperature, in the range of the β relaxation, is controlled by the variation of the charge currier mobility with temperature and it shows a non-Arrhenius behavior. We suggest that the β₁ sub-glass relaxation is related to the rotation or oscillation of phenyl groups and the β₂ sub-glass relaxation is related to the rotation or oscillation of the imidic ring. At higher temperatures an apparent peak was observed. The relaxation time of the trapped charge, at 573 K, is high than 8895 s.     

Ultrasound propagation in glasses in the metastable immiscibility region of the sodium borosilicate system

Measurements of ultrasound wave velocity and attenuation have been made between 1.3 K and 400 K in a series of both quenched and heat-treated Na₂OB₂O₃SiO₂ glasses. As in many other inorganic glasses, the ultrasound attenuation of both longitudinal and shear waves below room temperature is dominated by a broad and intense loss peak; the height and temperature of the peak maximum are frequency sensitive. The loss peak characteristics are consistent with a structural relaxation mechanism with a distribution of activation energies and this model is used to analyse the data. The features of the acoustic loss peak and also the absolute value and temperature coefficient of ultrasound velocity are strongly dependent on the total Na₂O network modifier content of the glasses. The ultrasound wave propagation is also affected by phase-separation inducing heat treatment: a steady rise in the height of the acoustic loss peak and an upward shift in the peak temperature takes place with increasing time of heat treatment at 550°C, a finding which suggests that structural rearrangements are still occurring in the individual glassy phases even after long periods of heat treatment. It is proposed that heat treatment causes migration of Na⁺ ions away from BOB bonds in the B₂O₃ rich phase.     

DC current transport in rf sputtered SiO2 films in the temperature range 77–357 K

The dc current conduction in rf sputtered SiO₂ films has been studied in the temperature range 77–357 K with electric fields up to 9 × 10⁷V m^?1. Nonlinear current voltage characteristics have been observed. Two activation energies of about 0.3 eV and 0.03 eV appear in the high and low temperature ranges respectively. Different transport mechanisms are discussed. The Poole-Frenkel effect would not be applicable. The most probable mechanism is electron hopping between defect states but with deformation of the surroundings.     

Characterization of low temperature dielectric processes in Poly(dicyclohexyl-itaconate)

Thermally stimulated depolarization current experiments (TSDC) have been carried out on poly(dicyclohexyl itaconate) (PDCHI) in the glassy state. The polymer exhibits three relaxation zones in the interval of temperature studied: a δ-relaxation at ∼130 K, a complex γ-process, in the temperature interval of 140 K to 200 K, and the β-process in the range of 210-270 K. The loss factor of PDCHI in the glassy zone has been reproduced from the partial depolarization data by using the elementary relaxation times and activation energies. The results show the existence of at least two relaxation processes in γ- and β-zones. A tentative explanation of the molecular origin of the observed secondary relaxations has been done by means of molecular mechanics (MM). At this respect, we perform a comparison of the relaxational data of PDCHI and those corresponding to PCHMA. This comparison allows us to confirm the relevant role played by the cyclohexyl ring in the δ- and γ-relaxations.     

The Vogel-Fulcher-Tammann law in the elastic theory of glass transition

We propose that the origin of the Vogel-Fulcher-Tammann law is the increase of the range of elastic interaction between local relaxation events in a liquid. In this picture, we discuss the origin of cooperativity of relaxation, the absence of divergence of relaxation time at a finite temperature and the crossover to a more Arrhenius behavior at low temperature.     

Electronic relaxation in zinc iron phosphate glasses

The electrical and dielectric properties of 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses, melted at different temperatures were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe(II)/[Fe(II) + Fe(III)] ratio. With increasing Fe(II) ion content from 17% to 37% in these glasses, the dc conductivity increases. Procedure of scaling conductivity data measured at various temperatures into a single master curve is given. The conductivity of the present glasses is made of conduction and conduction-related polarization of the polaron hopping between Fe(II) and Fe(III), both governed by the same relaxation time, τ. The high frequency dispersion in electrical conductivity arises from the distribution in τ caused by the disordered glass structure. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions.     

Non-stoichiometry of apatites at high temperatures

Schottky defects in apatites are formed by a decomposition reaction at temperatures in excess of 1000°K. In this investigation thermogravimetrie experiments on fluorapatite single crystals and powders have been carried out in the temperature range 1000–1600°K. Relative weightlosses of 1.2 × 10⁻³ per hour corresponding to Ca/P decreases of 0.0002 per hour have been observed at 1675°K. In the temperature range studied the number of vacancy triplets formed per second is in the order of 10¹⁸. The activation energy (1.15 eV) involved indicates that dislocations might play an important role in the decomposition process.     

Dielectric relaxation in ferroelectric TlInS2 layered crystals within metastable chaotic state

The results of investigations low frequency dielectric relaxation in layered ferroelectric TlInS₂ crystals are presented. The measurements were performed in the temperature range of 180‐230 K and in the frequency range of 5 kHz–1 MHz. Two different relaxation processes were observed in mentioned temperature interval. The crystal has “slow” and “fast” relaxation mechanisms in low and high frequency region, respectively. The presence of two different relaxation mechanisms in TlInS₂ is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)