Dielectric-polarization phenomena in glasses of the system Ge-S

The experimental study of electrical transport in GeS_x glasses (1.3 ? x ? 1.5) was performed by means of a thermo-stimulated depolarization technique (TSD), charging- and discharging-current and dc-conductivity measurements. It is shown that the results obtained point to non-electronic dielectric polarization phenomena. The nature of the mobile ions is briefly discussed.     

Synthesis and characterization of Bi3.15Nd0.85Ti3O12 nanotube arrays

Nd-substituted bismuth titanate (Bi_3.15Nd_0.85Ti₃O₁₂, BNT) nanotube arrays are fabricated by means of a sol–gel method utilizing porous anodic aluminum oxide (AAO) template. The morphologies and structures have been determined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The diameter and length of these nanotubes are about 200 nm and 60 μm, respectively, and their wall thickness is about 30 nm. The average grain size is around 40 nm. XRD data show that the BNT nanotubes possess bismuth-layered perovskite structure. High-resolution electron microscopy (HRTEM) image demonstrates that the BNT nanotubes are polycrystalline. Polarization–electric field (P–E) response curves of BNT nanotube arrays were measured, and a size induced polarization reduction phenomenon is observed.     

Potentiodynamic polarization studies on amorphous Zr46.75Ti8.25Cu7.5Ni10Be27.5, Zr65Cu17.5Ni10Al7.5, Zr67Ni33 and Ti60Ni40 in aqueous HNO3 solutions

Potentiodynamic polarization studies were carried out on virgin specimens of Zr-based bulk amorphous alloys Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 and Zr₆₅Cu_17.5Ni₁₀Al_7.5, and conventional-type binary amorphous alloys Zr₆₇Ni₃₃ and Ti₆₀Ni₄₀ in solutions of 0.2 M, 0.5 M and 1.0 M HNO₃ at room temperature. The values of the corrosion current density (I_corr) for the bulk amorphous alloy Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 were found to be comparable with those of Zr₆₅Cu_17.5Ni₁₀Al_7.5 in 0.2 M and 0.5 M HNO₃, but the value of I_corr for the former was almost three times more than that of the latter in 1.0 M HNO₃. In the case of conventional binary amorphous alloys, Ti₆₀Ni₄₀ showed lower value of I_corr as compared to Zr₆₇Ni₃₃ in 0.5 M and 1.0 M HNO₃ and a comparable value of I_corr in 0.2 M HNO₃. In general, the binary Ti₆₀Ni₄₀ displayed the best corrosion resistance among all the alloys in all the cases and the corrosion current density (I_corr) for all the alloys was found to increase with the increasing concentration of nitric acid. It is noticed that the bulk amorphous alloys do not possess superior corrosion resistance as compared to conventional binary amorphous alloys in aqueous HNO₃ solutions. The observed differences in their corrosion behavior are attributed to different alloy constituents and composition of the alloys investigated.     

Ionic transports and optical spectra of 50Li2O–50B2O3 glass

The ionic and optical transports of 50Li₂O–50B₂O₃ glass were investigated. Time-resolved current spectra (TRCS) showed repetitions of typical charge and discharge processes. The model circuit and equation for the TRCS and the equation for the relation between the turning times of polarization (TTP) and configuration energy (CE) have also been suggested. Equations were satisfied by the experimental results, and the potential and temperature dependence of the configuration energy (CE) and the dipole length from the TTP were investigated in detail. Kink phenomena showed that three or more types of charges were involved in the polarizations. The bi-exponential conductivity equation was applied to obtain the temperature dependence of the ac conductivity (TDAC). Complex impedance spectra (CIS) consisted of a semi-arc and a spur due to the electrode-specimen polarization. The shift of inflection points and peak frequencies in complex modulus spectra (CMS) showed non-Debye type relaxations, and the activation energy for relaxation was consistent with the relaxation for the temperature dependence of the dc conductivity (TDC). The migration concept (MC) applied to the CMS was satisfied at the dispersion and peak region, but slightly swerved at the tail. The fractal dimensionality displayed a three-dimensional character. The band gap, Urbach energy and optical indexes were obtained from optical spectra using Kramers–Kronig analysis (KKA).     

Effect of ion beam irradiation on the corrosion behavior of the melt-spun ribbon Ti60Ni40

Potentiodynamic polarization studies were carried out on unirradiated and irradiated Ti₆₀Ni₄₀ specimens in 1 M HNO₃ aqueous medium at room temperature. The irradiation was carried out using N⁺ 150 keV ions at a fluence of 1 × 10¹⁶ ions/cm². Polarization results revealed that the irradiated specimen exhibits less corrosion current density as compared to the unirradiated Ti₆₀Ni₄₀ specimen. X-ray photoelectron spectroscopy (XPS) studies were also carried out on the irradiated and unirradiated specimens after corrosion test. It was observed that the absence of Ti³⁺ species in the oxide film of the irradiated specimen as compared to the unirradiated specimen results in the improvement of the corrosion resistance. Polarization results are also corroborated by weight loss data.     

Preparation and properties of ferroelastic/ferroelectric polytypes of antimony(III) oxide iodine,Sb5O7I

Antimony oxide iodide, Sb₅O₇I (?7Sb₂O₃.SbI₃), is monoclinic and exists in at least 8 polytypic modifications. Crystals of these (transparent, pseudohexagonal prisms up to 15 × 10 × 5 mm³) were grown by vacuum sublimation and identified by optical and X-ray methods. Depending on the stacking sequence of characteristic slabs of antimony and oxygen in the (monoclinic) b-direction, centric and acentric polytypes result. The former show pure ferroelastic, the latter combined ferroelastic + ferroelectric behaviour. All polytypes transform reversibly into hexagonal, nonferroic phases at characteristic Curie temperatures between 165 and 208°C. The crystals can assume one of the three stable orientation states which may be mutually switched into each other by successive application of comprehensive stress (of the order of 10⁶ N/m²) on the three pairs of prism faces. In the ferroelectric polytypes the spontaneous polarization (about 5 × 10^-3 C/m²) is not reversible but only reorientable by ± 120° by coercive fields in the order of 2–5 kV/cm. Ferroelectricity and ferroelasticity are fully coupled.     

A photoelectric study on the interface between metallic silver and vitreous As2S3

The electrostatic potential at the AgAs₂S₃ interface was investigated. In the dark, cells of a structure, Ag/As₂S₃/Al behaved like and an electrochemical battery. When light fell onto the cells, short-circuit currents were observed, but their appearancesvaried much, depending on the excitation wavelength and the material of the illuminated electrode. At wavelengths longer than the absorption edge of As₂S₃ glass, photocurrents were characteristic of the polarization current and little influenced by the external field. A model for explaining these findings was proposed on an assumption of an interface reaation between silver and vitreous As₂S₃ in the dark. The interfacial reaction was supposed to accompany a charge separation leading to formation of a potential barrier at the interface.     

Crystallization of amorphous Ni35Zr65 and Fe40Ni40P14B6 under proton irradiation

Two amorphous alloys, Ni₃₅Zr₆₅ and Fe₄₀Ni₄₀P₁₄B₆, were irradiated using 400 keV protons at several temperatures below the crystallization temperature, T_x, to peak doses in the neighborhood of 3.5 to 4.5 dpa. Irradiation at 250°C resulted in the crystallization of both alloys, which were examined by transmission electron microscopy of samples electrolytically polished to various distances from the irradiated surface to study the effect of dose. Samples masked from the proton beam remained amorphous during irradiation. In the Ni₃₅Zr₆₅ alloy crystallization of the equilibrium phases propagated throughout the entire sample, while the in the Fe₄₀Ni₄₀P₁₄B₆ alloy crystallization was observed only in those parts of the samples lying within the proton range. Neither alloy crystallized during irradiation at 100°C. In both these alloys the amorphous phase is therefore evidently stable at irradiation temperatures below approximately 0.6 T_x. An examination of the literature on irradiation damage of binary alloys and intermetallic compounds suggests that there is a tendency for initially amorphous alloys to remain amorphous at irradiation temperatures, T_irr < 0.3 T_L, where T_L (≈T_x) is the “melting” temperature (either a eutectic, peritectic or congruent melting temperature). Also, these same alloys, even when they are initially crystalline, transform to the amorphous state during irradiation at T < 0.3 T_L. Some other crystalline alloys have also been shown to transform to the amorphous state at T_irr < 0.3 T_L even though they have never been prepared in this condition by rapid quenching techniques. The temperature 0.3 T_L appears to be a lower limit, however, since the crystalline to amorphous transformation occurs in many of these alloys at temperatures greater than 0.3 T_L. It is suggested, by analogy with results on void formation in irradiated metals, that this low temperature limit is related to the low mobility of vacancies in these materials, although the mechanism of crystallization, or conversely amorphization, is not fully understood.     

Raman scattering of the mixed chalcogenide glass system As2SxSe3-x

Polarized room temperature Raman spectra of glassy As₂S_xSe_3-x for 0≦×≦3 have been measured. Spectra for crystalline As₂S₁Se₂ are reported. The polarization and intensity dependence upon composition are consistent with mixed pyramids of composition As₂S_nSe_3-n and preclude phase separation in the glassy system.     

AC impedance analysis of CsNO3:Al2O3 composite solid electrolyte systems

Complex impedance spectroscopic studies were carried out on CsNO₃ and CsNO₃-Al₂O₃ dispersed solid electrolyte systems (DSES) in the temperature range of 100-350 °C and the frequency 50 kHz-1 MHz. Dielectric constant, loss tangent, ac conductivity and dc ionic conductivity (obtained from CIS) in these systems are presented. DC ionic conductivity is noticed to increase with temperature in the extrinsic region in pure and dispersed systems. The enhancement of conductivity in DSES was observed to be about two orders of magnitude over its pure form in the extrinsic region. This enhancement of conductivity was attributed to the formation of space charge layer between the host material and the dispersoid. Enhancement in conductivity is found to increase with m/o up to 40 m/o where as it decreased for 80 m/o. Dielectric constant, dielectric loss and ac conductivity are also found to increase with temperature, and with mole percent it maintained the similar behavior as that of dc conductivity. These dielectric properties are interpreted in terms of space charge polarization and increased concentration of defects in the interfacial layer formed between the host and the dispersoid.     

Optical Properties of Bi2O3-TeO2-B2O3 Glasses

Glasses of the Bi₂O₃-TeO₂-B₂O₃ ternary system were developed and their linear and nonlinear optical properties were investigated. The absorption edges of these glasses were found to be 367-384 nm with a good transmittance in visible wavelength, although they exhibit the refractive indices as high as 1.98-2.12 at 633 nm. The absorption edges are quite steep and they are analyzed by the Urbach theory. The obtained Urbach energies of these glasses are 73-79 meV which are comparable to silica glasses. The high refractive index and its glass composition dependency are discussed according to the basics of the electronic polarizability and optical basicity. The high third order nonlinear susceptibility χ⁽³⁾ = 2.0 × 10^− 12 esu at 800 nm was also obtained in the 36Bi₂O₃-18TeO₂-46B₂O₃ glass.     

AC conductivity of (As2Te3)95Ge5

The elctrical conductivity of amorphous chalcogenide (As₂Te₃)₉₅Ge₅ is investigated at variable frequency, from 1 kHz up to 35 GHz, and a variable temperature, from 77 to 300 K. The low-temperature conductivity is constant with temperature at a fixed frequency. At a fixed temperature, it obeys a ω^0.8 law only at low frequencies. The results are analysed with respect to the polaron problem, but also with a simple model of several dilute localized levels having a wide energy distribution in the forbidden band.     

Crystal growth and characterization of two-leg spin ladder compounds: Sr14Cu24O41 and Sr2Ca12Cu24O41

Single crystals of Sr_14−xCa_xCu₂₄O₄₁ (x=0 and 12) are grown by the travelling solvent floating zone technique using an image furnace. The grown crystals are characterized for their single crystallinity by the X-ray and Neutron Laue method. The magnetic susceptibility measurements in Sr₁₄Cu₂₄O₄₁ show considerable anisotropy along the main crystallographic axes. Low-temperature specific heat measurement and DC susceptibility measurement in Ca-doped crystal showed antiferromagnetic ordering at 2.8 K at ambient pressure. High-pressure AC susceptibility measurement on Ca-doped crystal showed a sharp superconducting transition at 2 K under 40 kbars. T_c onset reached a maximum value of 9.9 K at 54 kbars. The bulk superconductivity of the sample is confirmed by the high-pressure AC calorimetry with T_c max=9.4 K and T_N=5 K at 56 kbars.     

EPR study of RLi2O.V2O5, RNa2O.V2O5, RCaO.V2O5, and RBaO.V2O5 modified vanadate glass systems

Experimental EPR spectra in several modified vanadate glass systems reveal hyperfine structure (hfs) lines whose widths vary with the molar ratio of modifier to vanadium pentoxide, R. In the RNa₂O.V₂O₅ system, for example, hfs lines show no resolution at low R values (near 0.1); by contrast, these lines exhibit dramatic narrowing as R approaches 0.5. In the model proposed here, this narrowing is due to an increase in hopping time for small polarons associated with V⁴⁺ ions in these systems. Increases in polaron hopping times are accompanied by increases in electron spin-spin relaxation times T₂'s, and, an associated narrowing of EPR linewidths. Experiments confirm that spectral widths are limited by electron T₂'s due to the fact that EPR linewidths do not vary with temperature down to 4.2 K. Resolved spectra in RNa₂O.V₂O₅ at R = 0.5 reveal a hyperfine coupling parameter of 0.0177 ± 0.0008 T, corresponding to an upper-limit polaron hopping frequency of 487 ± 20 MHz. By similar analyses, the systems of RCaO.V₂O₅, RBaO.V₂O₅, and RLi₂O.V₂O₅ exhibit comparable polaron hopping frequencies limits of 480 ± 20 MHz, 469 ± 20 MHz, and 468 ± 20 MHz, respectively, when R is near 1.0. In addition to the relaxation effects discussed here, results of modeling of resolved spectra to obtain hyperfine coupling constants A_|| and A_┴, and g_┴ values g_|| and g_┴ are presented and discussed.     

Effect of hydrostatic pressure on the elastic moduli of As2S3 and As2Se3 glasses at 195 and 296 K

Velocities of 30 MHz longitudinal and shear ultrasonic waves have been measured in As₂S₃ and As₂Se₃ glasses as a function of hydrostatic pressure up to 1.5 kbar at 195 K and 3 kbar at 296 K. The elastic stiffness moduli are found to have relatively large, positive, pressure dependences which are about the same at both temperatures for both glasses. This behavior is attributed to the weakness of bonding between layers comprised of AsS₃ and AsS₃ pyramids.Inspection of data for a variety of glasses reveals a correlation between the value of C_L/3C_T and whether the elastic moduli are increased or decreased by pressure. (C_L is the longitudinal modulus and C_T the shear modulus.)Using the pressure dependences of the elastic moduli obtained in the present work, it is found that volume change is responsible for most of the temperature dependences of the moduli. In addition elastic gammas are obtained which are consistent with thermal Grüneisen gammas at 12 K. The pressure dependence of the volume of As₂S₃ glass at 296 K is calculated using the present results in the Murnagham equation. Agreement with volumetric data of Weir is obtained.     

Phase separation and crystallization in the system SiO2-Al2O3-P2O5-B2O3-Na2O glasses

The phase separation and crystallization behavior in the system (80 − X)SiO₂ · X(Al₂O₃ + P₂O₅) · 5B₂O₃ · 15Na₂O (mol%) glasses was investigated. Glasses with X = 20 and 30 phase separated into two phases, one of which is rich in Al₂O₃-P₂O₅-SiO₂ and forms a continuous phase. Glasses containing a larger amount of Al₂O₃-P₂O₅ (X = 40 and 50) readily crystallize and precipitates tridymite type AlPO₄ crystals. It is estimated that the phase separation occurs forming continuous Al₂O₃-P₂O₅-SiO₂ phase at first, and then tridymite type AlPO₄ crystals precipitate and grow in this phase. Highly transparent glass-ceramics comparable to glass can be successfully obtained by controlling heat treatment precisely. The crystal size and percent crystallinity of these transparent glass-ceramics are 20-30 nm and about 50%, respectively.     

The growth and structure of Pb3Ga2Ge4O14 and Ba3Ga2Ge4O14 single crystals

Conditions for the flux synthesis of Pb₃Ga₂Ge₄O₁₄ and Ba₃Ga₂Ge₄O₁₄ single crystals and their solid solutions Pb_{3 − x} Ba_xGa₂Ge₄O₁₄ are studied. Structural analysis showed that the Ga³⁺-and Ge⁴⁺-cation positions in flux-grown Pb₃Ga₂Ge₄O₁₄ and Ba₃Ga₂Ge₄O₁₄ single crystals are not mixed. __________ Translated from Kristallografiya, Vol. 49, No. 2, 2004, pp. 325–328. Original Russian Text Copyright ? 2004 by Bezmaternykh, Vasil’ev, Gudim, Temerov. This work was presented at the National Conference on Crystal Growth (NCCG-2002, Moscow).     

X-ray diffraction study of phase transitions in Na3Sc2(PO4)3 and Ag3Sc2(PO4)3 single crystals in temperature range from 160 to 500 K

Two phase transitions are revealed for the first time in Ag₃Sc₂(PO₄)₃ single crystals in the vicinity of the temperatures 303 and 165–180 K. It is established that the phase transition at 303 K corresponds to the well-known phase transition to the superionic state in Na₃Sc₂(PO₄)₃ single crystals in the temperature range 423–433 K, whereas the phase transition observed in the temperature range 170–180 K corresponds to the phase transition from the rhombohedral to monoclinic phase at about 320 K in the monoclinic Na₃Sc₂(PO₄)₃ single crystals. It is also established that rhombohedral Na₃Sc₂(PO₄)₃ single crystals undergo the second phase transition. __________ Translated from Kristallografiya, Vol. 50, No. 1, 2005, pp. 122–126. Original Russian Text Copyright ? 2005 by Shilov, Atovmyan, Kovalenko.     

Synthesis and crystal structure of rare earth selenates Nd(HSeO4)3, Sm(HSeO4)3, and Nd2(SeO4)3 · 5H2O

Single crystals of rare earth (RE) selenates of the compositions Nd(HSeO₄)₃, Sm(HSeO₄)₃, and Nd₂(SeO₄)₃ · 5H₂O are synthesized and studied by X-ray diffraction analysis at T = 297 and 180 K. It is established that Nd and Sm hydrogen selenates are isostructural to one another and to the corresponding hydrogen sulfates. Neodymium selenate pentahydrate is not isostructural to the analogous RE sulfates, although their structural motifs are similar. __________ Translated from Kristallografiya, Vol. 49, No. 5, 2004, pp. 835–840. Original Russian Text Copyright ? 2004 by Zakharov, Troyanov, Kemnitz.     

Electric conductivity of glasses in the system Na2O/CaO/SiO2/Fe2O3

The addition of polyvalent transition metal ions to the usually insulating traditional soda-lime-silica glasses can lead to semiconducting properties. We report on synthesis of glasses and glass-ceramics in a soda-lime-silicate based system containing Fe₂O₃ in the concentration range from 5 to 30 mol%. Two sub-systems were considered, in one of them the ratio [Na₂O]/[Fe₂O₃] was varied while in the other one, the ratio [SiO₂]/[Fe₂O₃] was changed. The phase composition of the synthesized products was characterized by X-ray diffraction and energy dispersive X-ray analysis, while the electrical properties were studied by impedance spectroscopy. Partially crystallized non-reduced samples are semiconducting even at room temperature while the glassy samples (both reduced and non-reduced) exhibit semiconducting properties at temperatures equal or larger than 100 °C. An attempt is done to predict the physical approximation explaining the conduction process in the glasses.