Dielectric permittivity and phase transitions in the SrTiO3-KTaO3 system

The dielectric permittivity and losses of the (SrTiO₃)_0.85(KTaO₃)_0.15 solid solution have revealed an unusual behavior at temperatures of 5–300 K and frequencies ranging from 100 Hz to 1 MHz. The dielectric permittivity exhibits a broad maximum at ≈40 K, which obeys the Curie-Weiss law in the high-temperature wing, and the onset of low-temperature relaxation contributions. Besides, while for T>40 K it is a fully ergodic, uniformly ordering system, the uniform polar ordering is replaced with decreasing temperature by two low-temperature phase transitions to a glass-like state. The proposed mechanism for the dielectric relaxation and the observed phase transformations involves reorientation and ordering in the system of electron and hole polarons, which form in the course of charge compensation of the heterovalent ions Sr²⁺, K⁺ and Ti⁴⁺, Ta⁵⁺ distributed randomly in the sublattices. Fiz. Tverd. Tela (St. Petersburg) 39, 2040–2045 (November 1997)     

Charge carrier concentration in glasses. dependence on composition

Admittance spectra of the ion conducting glasses xNa₂O(1−x)SiO₂ and xK₂O(1−x)SiO₂,(x=0,1–0,3) have been studied on small signal conditions from room temperature to 713 K. Conductivity relaxation in the bulk and space charge relaxation due to drift and diffusion near the electrodes were found in separated parts of the frequency range 10⁻⁴–10⁶ Hz. The data show Arrhenius behaviour for dc conductivity and conductivity relaxation. The determination of the charge carrier concentration is based on the analysis of the beginning of space charge relaxation. The free carrier concentration, n₀, were evaluated to be of the order of 10²³ m⁻³ at temperatures 400 – 700 K and show a weak dependence on composition. The dominant factor determining conductivity was mobility in these glasses. The strong temperature dependence of n₀ below 400 K indicates changed conditions for the development of the space charge relaxation which are discussed. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11–18 Sept. 1994.     

Synthesis and dielectric characteristics of La0.5Bi0.5MnO3 ceramics

La_0.5Bi_0.5MnO₃ ceramics with a single phase were prepared by a solid-state reaction method, and their dielectric properties were characterized. Two dielectric relaxations with a giant dielectric constant were identified in the temperature range from 125 to 350 K. The electron hopping between Mn³⁺ and Mn⁴⁺ was found to be the origin of the dielectric relaxation at low temperatures (125–200 K) with an activation energy of 0.18 eV. The high temperature (200–350 K) dielectric relaxation can be attributed to the conduction.     

Giant dielectric relaxation in SrTiO3-SrMg1/3Nb2/3O3 and SrTiO3-SrSc1/2Ta1/2O3 solid solutions

Ceramic samples of (1−x)SrTiO₃-xSrMg_1/3Nb_2/3O₃ and (1−x)SrTiO₃-xSrSc_1/2Ta_1/2O₃ were prepared, and their dielectric properties were studied at x=0.005–0.15 and 0.01–0.1, respectively, at frequencies 10 Hz–1 MHz and at temperatures 4.2–350 K. A giant dielectric relaxation was observed in the temperature range 150–300 K, and not so strong but well-developed relaxation was found in the temperature range 20–90 K. The activation energy U and the relaxation time τ₀ were determined to be 0.21–0.3 eV and from 10⁻¹¹ to 10⁻¹² s for the high-temperature relaxation and 0.01–0.02 eV and 10⁻⁸–10⁻¹⁰ s for the low-temperature relaxation, respectively. The additional local charge compensation of the heterovalent impurities Mg²⁺ and Nb⁵⁺ (or Sc³⁺ and Ta⁵⁺) by free charge carriers or the host ion vacancies is suggested to be the underlying physical mechanism of the relaxation phenomena. On the basis of this mechanism, the Maxwell-Wagner model and the model of reorienting dipole centers Mg²⁺ (or Sc³⁺) associated with the oxygen vacancy are proposed to explain the high-temperature relaxation with some arguments in favor of the latter model. The polaron-like model with the Nb⁵⁺-Ti³⁺ center is suggested as the origin of the low-temperature relaxation. The reasons for the absence of ferroelectric phase transitions in the solid solutions under study are also discussed. From Fizika Tverdogo Tela, Vol. 44, No. 11, 2002, pp. 1948–1957. Original English Text Copyright ? 2002 by Lemanov, Sotnikov, Smirnova, Weihnacht. This article was submitted by the authors in English.     

Interaction of muonium with oxygen on silica powder surfaces

Results of the first μSR studies using Merck FO Optipur silica powder, which contains paramagnetic impurities at the ppb level and has a surface area of 610±20 m²/g. are reported. Above 20 K, the transverse field muonium relaxation rate is roughly constant at 0.5 μs⁻¹. Upon the addition of oxygen at ppm levels, the relaxation rate increases linearly with O₂ concentration in the temperature range from 40–100 K yielding two-dimensional depolarization rate constants on the order of 10⁻⁴ cm² molecule⁻¹ s⁻¹. As the temperature is increased further, both oxygen and muonium desorb from the surface yielding a three-dimensional rate constants at 300 K of 3.1(3)×10–10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, in agreement with the gas phase value. Longitudinal field measurements suggest that MuO₂ is formed and is able to spin exchange with other oxygen molecules.     

Evidence of large-angle reorientation in supercooledo-terphenyl

Summary We study the reorientation process of a paramagnetic radical dissolved in supercooledo-terphenyl around and belowT _g via the Electron Spin Resonance spectroscopy. The experimental results are at variance with the hypothesis of diffusive reorientation of the radical. Instead, the ESR lineshapes are fairly well reproduced by assuming that the radical reorientates via large angular jumps with a width Δϕ=80° _−5° ^+10° . The results are compared with previous²H NMR studies. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Paramagnetic and spin-glass properties of pyrochlore-like oxides Ln2Mn2/3Mo4/3O7 (Ln=Sm, Gd, Tb, or Y)

The magnetic properties of complex oxides Ln ₂Mn_2/3Mo_4/3O₇ (Ln=Sm, Gd, Tb, or Y) with a pyrochlore-type structure are studied in the temperature range 2–300 K. For all compounds in the paramagnetic state, the temperature dependence of the magnetic susceptibility is described by a generalized Curie-Weiss law with a temperature-independent component of ∼10⁻⁶ cm³/g and with a Weiss constant Θ<0 and |Θ|<16 K. At low temperatures (T<10–12 K), the compounds have spin-glass properties; they exhibit magnetic and temperature hysteresis and the typical dependences of the imaginary and real parts of the dynamic magnetic susceptibility on temperature and the frequency of an ac magnetic field in a wide range of magnetization relaxation times. The data obtained suggest that d electrons are responsible for the formation of frustrated exchange interactions in the compounds and that 4f electrons in the compounds with Sm or Tb provide strong magnetic-anisotropy effects. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 2, 2004, pp. 287–295. Original Russian Text Copyright ? 2004 by Korolev, Bazuev.     

2-Nitroimidazole derivative studied by means of35Cl-NQR spectroscopy and molecular dynamics simulations

³⁵Cl nuclear quadrupole resonance as well as quantum chemical calculations were applied to study electronic structure and molecular dynamics of the imidazole derivative 1-(2-hydroxy-3-chloroprophyl)-2, 3-dinitro-5-methyl-imidazole. The spectra of this compound taken at 77 and 293 K show only one line, which means that there are no physically inequivalent positions of the chlorine atoms in the elementary cell. The line width was of about 28 kHz at 77 K and the relaxation time was about 320 ms at liquid-nitrogen temperature and about 8 ms at room temperature. As follows from the spin-lattice relaxation time measurements performed for the compound studied, in the range 77–200 K the libration mechanism of molecular vibrations is active, while above 200 K the activation process related to reorientation of the CH₂Cl group begins. The activation energy is 18.82 kJ/ mol. The activated process of the CH₂Cl group reorientations exerts rather insignificant effect on the averaging of the electric field gradient in the vicinity of the resonant³⁵Cl nucleus.     

Dielectric properties of (1−x)[0.7PbZrO3·0.3K0.5Bi0.5TiO3]·xSrTiO3 solid solutions in the vicinity of phase transitions

The dielectric properties of polycrystalline (1−x)[0.7PbZrO₃·0.3K_0.5Bi_0.5TiO₃]·xSrTiO₃ solid solutions, where x=0–0.7, are studied in the temperature range 150–600 K. Systematic spreading of the ferroelectric phase transition with increasing strontium titanate content is discovered. The dispersion of the dielectric constant at frequencies from 10⁻¹ to 10⁶ Hz is investigated for a composition with x⋍0.7. The existence of two relaxation processes characterized by diffuse relaxation time spectra, which broaden with decreasing temperature, is established. It is postulated that a transition to a glasslike state takes place in the material with x⋍0.7. Zh. Tekh. Fiz. 69, 35–38 (March 1999)     

Anomalies of internal friction in CsI single crystal at liquid helium temperatures caused by plastic deformation

Acoustic relaxation in undeformed and plastically deformed CsI single crystal has been studied using the composite oscillator technique at frequencies (1–7) × 10⁵ Hz in the temperature range 2–15 K. Plastic deformation leads to appearance of an internal friction peak localized in the temperature interval 4–5 K. It is shown that the peak shifts towards higher temperatures when increasing the vibration frequency and corresponds to a thermally activated relaxation process with very low values of the activation energyU ≈ 1.9×10⁻³ eV and the attack frequencyν ₀≈6.7 × 10³ s⁻¹. Interaction of sound with dislocation kinks migrating in the second order Peierls relief is considered as a possible mechanism of the peak. Research was made possible in part by Grants U9T000 and U9T200 from the International Science Foundation and supported in part by the Fundamental Research Foundation of Ukraine (Project 2.4/156 “Bion”).     

Field dependence of spin-lattice relaxation of Nd3+ ions in Y3Al5O12 crystals

Our studies involve measuring spin-lattice relaxation times for Nd³⁺ ions in yttrium-aluminum garnets over the temperature range 4–50 K at 9.25 and 36.4 GHz for different orientations of the external magnetic field in relation to the crystallographic axes. The temperature dependence of the relaxation rate is described by T ₁ ⁻¹ =AT ⁿ+b exp(−Δ/kT), where n varies from sample to sample, with n=1 for “perfect” samples (i.e., with the longest relaxation times). Here Δ is approximately 130 cm⁻¹, which is the energy of the excited Kramers doublet of the neodymium ion closest to the ground state, and this makes it possible to interpret the second term in T ₁ ⁻¹ as the contribution of two-stage relaxation proceeding through the intermediate level Δ. A strong field dependence of these processes has been discovered: when the frequency was increased fourfold, the relaxation rate increased by a factor of 10. The effect is a specific manifestation of the degeneracy of the excited level, breaking of the symmetry of the crystalline field due to lattice defects, and the prevalence of deformations of a certain type in the spin-lattice interaction. Zh. éksp. Teor. Fiz. 111, 332–343 (January 1997)     

<Superscript>1</Superscript>H Spin–Lattice Relaxation Study of Dynamical Inequivalence of Methyl Groups in Solid 1,2-<Emphasis Type="Italic">O</Emphasis>-(1-Ethylpropylidene)-α-<Emphasis Type="SmallCaps">d</Emphasis>-Glucofuranose

We investigated the dynamics of methyl groups in organic polycrystalline 1,2-O-(1-ethylpropylidene)-α-d-glucofuranose by the proton spin–lattice relaxation method. The temperature and nuclear magnetic resonance Larmor frequency dependence of relaxation time is presented and interpreted in terms of simple possible dynamical model for the reorientation of methyl groups: the random hopping for methyl groups, which are in a, b, and c sites in the crystal. The energy E _a of 13.5 kJ mol⁻¹ for the a-type methyl groups is typical for methyl groups in ethyl groups. In contrast, the b- and c-methyl groups characterized by the lower E _a values of 9.5 and 6.5 kJ mol⁻¹ are located in the crystal structure where the intermolecular interactions significantly influence the potential, leading to a decrease in the total energy.     

Electronic superconductor Pr2CuO4−x Fx: Magnetic correlations at high temperatures (150<T<600 K) according to 19F NMR data

The longitudinal and transverse relaxation rates of ¹⁹F nuclei in Pr₂CuO_4−x F_x (x=0.20) samples are measured at high temperatures (150 K<T<600 K). A feature is found in the temperature dependence of the relaxation rates at temperature T′≅300 K. The magnetic properties of the electronic superconductor Pr₂CuO_4−x F_x as a possible system with a stripe ordering of carriers and spins are discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 4, 328–332 (25 February 1997)     

β-Relaxation in polymethylacrylate studied by light scattering

Summary The relaxation processes of glass-forming polymethylacrylate have been studied in a wide temperature range above the glass transition temperatureT _g by means of light scattering measurements. Experimental spectra give evidence of a self-similar behaviourI(ω) ∝ ω^−(1−a) over a spectral range of 2–3 decades, consistent with the predictions of the mode-coupling theory. Different from the prediction of the theory, the value ofa is strongly temperature dependent. This behaviour has been attributed to the presence of a slow secondary relaxation located in the GHz frequency region, whose strength progressively increases with temperature. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Dielectric breakdown of rubber materials by femtosecond irradiation

We report the reversible micro-structuring of a synthetic rubber polymer (cis1,4-polybutadiene (PB)) by femtosecond laser illumination. Visco-elastic relaxation of the optically damaged region was observed. The recovery time, typically 10²–10⁴ ms, can be varied by changing the irradiation pulse energy. Multi-shot-induced damage recovers on the much longer scale of 10¹–10² s. It was found that the doping of PB by 4 wt. % of pentazadiene ([4-NO₂]–phenyl–N=N–N(C₃H₇)–N=N–phenyl–[4-NO₂]) reduces the threshold of light-induced photo-modification by 20%. This is explained by photo-induced (homolytic) cleavage of the pentazadiene bonds and formation of gaseous N₂, which facilitates material failure at the irradiated spot. The recovery of optical transmission can be applied to optical memory, optical and micro-mechanical applications. The underlying mechanism of the phenomenon is discussed in terms of anelastic α- and β-relaxation (polymer backbone and chains/coils relaxation, respectively). Received: 11 October 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +81-88/656-7598, E-mail: misawa@eco.tokushima-u.ac.jp     

Nonisothermal relaxation and electromigration in single crystals of Ca3Ga2Ge4O14

The photoconductivity and electrical conductivity, the thermally stimulated polarization and depolarization currents, the permittivity, and the dielectric losses of noncentrosymmetric Ca₃Ga₂Ge₄O₁₄ crystals of a structural type of trigonal calcium gallogermanate (CGG) have been studied for the first time in the temperature interval 100–700 K in air and in vacuum. The features of the temperature behavior of CGG are associated, in particular, with the effect of electrically active defects and partial disordering of the cation sublattice of the crystal structure, which generates polar formations (dipoles) of several kinds. Certain parameters of the electromigration and of the electrically active defects that cause charge-relaxation and polarization effects are determined. It is concluded that dipole relaxation in CGG makes it a new representative of a class of materials in which reorientation of the dipoles as a system of mobile local distortions of the structure has a substantial effect on the physical properties. Fiz. Tverd. Tela (St. Petersburg) 39, 871–876 (May 1997)     

Influence of x irradiation on internal friction in silicon

An investigation was made of the influence of γ irradiation on the temperature dependences of internal friction in disk-shaped silicon substrates in the kilohertz frequency range. After exposure to doses of 10⁴ and 10⁵ R, two dominant internal friction peaks were observed at ∼330 and ∼450 K with activation energies H ₁=0.6 eV and H ₂=0.9 eV, respectively. These peaks were evidently caused by reorientation of interstitial silicon atoms in dumbbell configurations. Fiz. Tverd. Tela (St. Petersburg) 40, 1257–1258 (July 1998)     

Temperature and frequency dependence of the spin-lattice relaxation times of E′1 centers in neutron-irradiated quartz glass

Results are reported for measurements of the spin-lattice relaxation times of E′₁ centers in quartz glass, produced by neutron irradiation, with the measurements made at two frequencies 9.25 and 24.0 GHz over a wide temperature interval 1.5–300 K. The experimental data are interpreted on the basis of interaction mechanisms of the spins with two-level systems with excitation energies ∼6, ∼26, and ∼420 cm⁻¹. A small modification of the existing theory allows us to explain a number of features of the observed temperature and frequency dependence of the relaxation rate. The results are compared with the data available in the literature on spin-lattice relaxation of irradiation centers in crystalline quartz and quartz glass. Fiz. Tverd. Tela (St. Petersburg) 39, 1335–1337 (August 1997)     

Dielectric spectra of LiTFSI-doped chitosan/PEO blends

Solid-polymer-blend electrolyte consisting of chitosan and polyethylene oxide (PEO) in a 1:1 weight ratio and doped with lithium trifluoromethanesulfonimide (LiTFSI) salt was prepared by solution cast technique. The highest conducting film with conductivity value of 1.40 × 10^-6 S cm⁻¹ at room temperature consists of 30 wt% LiTFSI. The temperature dependence for the highest conducting film obeyed Arrhenius relationship. From loss tangent–frequency plots at different temperatures, the frequency f _max at which the plot is a maximum was obtained. From this, ln f _max vs 10³/T was plotted. The activation energy value obtained from the log σ vs 10³/T plot and ln f _max vs 10³/T plot is about the same, suggesting that the processes of conductivity and relaxation for the charge carriers are the same. This paper was presented at the International Conference on Solid State Science and Technology 2006, Kuala Terengganu, Malaysia, Sept. 4–6, 2006.     

Dielectric relaxation in Bi12SiO20 crystals

Anomalies of the dielectric properties of undoped and aluminum-and gallium-doped crystals of Bi₁₂SiO₂₀ are investigated in the frequency and temperature range ν=10²–10⁸ Hz and T=300–800 K. They are shown to be due to Debye relaxation processes and determined by the relaxor parameters. The mechanism of electron thermal polarization is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1223–1229 (July 1997)