Impedance and electron diffraction studies on single crystals of BINIVOX

Single crystals of the nominal composition Bi₂V_0.9Ni_0.1O_5.35 were prepared. They were studied by TGA/DTA, impedance spectroscopy and electron diffraction. The impedance of the crystals with Pt electrodes was measured in two directions: parallel and perpendicular to the layers of the intergrowth structure in the frequency range 10⁻²−10⁷ Hz at temperatures between 300 and 940 K. Arrhenius type plots for both directions show two linear regions with different activation energies. In the low temperature region the conductivity in the direction parallel to the layers was by a factor of about 1000 larger than in the perpendicular direction. In the high temperature region the ratio of conductivities was about 100. A time dependence of the conductivity in the perpendicular direction was observed during heating and cooling in limited temperature ranges. The electron diffraction experiments gave evidence that the superlattice spots present in the diffraction patterns at low temperature, disappear reversibly at temperatures higher than 650 K. The transition temperatures observed in the DTA and in the electron diffraction experiments correspond to the temperature of changes of the activation energy. An order-disorder transition of oxygen ions is considered to be responsible for this phenomenon. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Thermal conductivity of crystalline fullerite C60 in the simple cubic phase

The behavior of the thermal conductivity k(T) of bulk faceted fullerite C₆₀ crystals is investigated at temperatures T=8–220 K. The samples are prepared by the gas-transport method from pure C₆₀, containing less than 0.01% impurities. It is found that as the temperature decreases, the thermal conductivity of the crystal increases, reaches a maximum at T=15–20 K, and drops by a factor of ∼2, proportional to the change in the specific heat, on cooling to 8 K. The effective phonon mean free path λ _p, estimated from the thermal conductivity and known from the published values of the specific heat of fullerite, is comparable to the lattice constant of the crystal λ _p∼d=1.4 nm at temperatures T>200 K and reaches values λ_p∼50d at T<15 K, i.e., the maximum phonon ranges are limited by scattering on defects in the volume of the sample in the simple cubic phase. In the range T=25−75 K the observed temperature dependence k(T) can be described by the expression k(T)∼exp(Θ/bT), characteristic for the behavior of the thermal conductivity of perfect nonconducting crystals at temperatures below the Debye temperature Θ (Θ=80 K in fullerite), where umklapp phonon-phonon scattering processes predominate in the volume of the sample. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 651–656 (25 April 1997)     

Thermal conductivity of high-porosity biocarbon preforms of beech wood

This paper reports on measurements performed in the temperature range 5–300 K for the thermal conductivity κ and electrical resistivity ρ of high-porosity (cellular pores) biocarbon preforms prepared by pyrolysis (carbonization) of beech wood in an argon flow at carbonization temperatures of 1000 and 2400°C. X-ray structure analysis of the samples has been performed at 300 K. The samples have revealed the presence of nanocrystallites making up the carbon matrices of these biocarbon preforms. Their size has been determined. For samples prepared at T _carb = 1000 and 2400°C, the nanocrystallite sizes are found to be in the ranges 12–25 and 28–60 κ(T) are determined for the samples cut along and across the tree growth direction. The thermal conductivity κ increases with increasing carbonization temperature and nanocrystallite size in the carbon matrix of the sample. Thermal conductivity measurements conducted on samples of both types have revealed an unusual temperature dependence of the phonon thermal conductivity for amorphous materials. As the temperature increases from 5 to 300 K, it first increases in proportion to T, to transfer subsequently to ∼T ^1.5 scaling. The results obtained are analyzed.     

Spontaneous twisting of an incommensurable improper ferroelastic Ba2NaNb5O15

The spontaneous twisting of a y-orientation single-crystal sample of barium-sodium niobate Ba₂NaNb₅O₁₅ at a phase transition and into an incommensurable ferroelastic phase is investigated. Anomalies are found in the temperature dependence of the torsional deformation at temperatures 453, 533, and 573 K and anomalous hysteresis of the temperature dependences of the torsional deformation, measured on heating and cooling of the sample. The results obtained are discussed on the basis of an analysis of the temperature dependence of incommensurable 1q and 2q structures and ferroelastic twins. Fiz. Tverd. Tela (St. Petersburg) 41, 1848–1850 (October 1999)     

Thermal stability and impedance spectroscopy studies of (Cu, Ni) substituted Bi4V2O11 ceramics

Polycrystalline samples of the oxygen ion conductor Bi₂V_0.9Cu_0.1−xNi_xO_5.35 with various contents of nickel (0 ≤ × ≤ 0.1) were investigated. The X-ray powder diffraction revealed the tetragonal structure of all compositions. DTA curves exhibit effects due to phase transition, one endothermic effect during heating and one exothermic one during cooling. The impedance of the ceramics with Pt electrodes was measured in the frequency range 10⁻¹–10⁷ Hz at constant temperatures between 350 and 920 K. The conductivity was determined by nonlinear least-squares analysis of the impedance spectra. Separation of the total resistance into grain interior and grain boundary components was feasible at temperatures below 580 K. The transition temperatures observed in DTA coincide with those observed in conductivity measurements. A phase transition, involving a reordering process of the oxygen ions is considered to be responsible for this phenomenon. The frequency dependent part of the intragrain conductivity was modeled by a constant phase admittance. The effective hopping rate was estimated by comparing the frequency dependent part and the dc limit of the intragrain conductivity. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997.     

Ac electrical parameters of Al-ZnPc-Al organic semiconducting films

The ac electrical parameters of thermally evaporated zinc phthalocyanine, ZnPc, semiconducting thin films was measured in the temperature range of 180–390 K and frequency between 0.1 and 20 kHz. Aluminum electrode contacts were utilized to sandwich the organic ZnPc semiconducting films. Capacitance and loss tangent decreased rapidly with frequency at high temperatures, but at lower temperatures a weak variation is observed. An equivalent circuit model assuming ohmic contacts could qualitatively and successfully explains capacitance and loss tangent behavior. The ac conductivity showed strong dependence on both temperature and frequency depending on the relevant temperature and frequency range under consideration. Ac conductivity σ (ω) is found to vary with ω, as ω ^s with the index s ≤ 1.35 suggesting a dominant hopping conduction process at low temperatures (< 250 K) and high frequency. The conductivity of some samples did not increase monotonically with temperature. This behavior was attributed to oxygen exhaustion of the sample as its temperature is increased. The ac conductivity behavior at low temperatures of ZnPc films could be described well by Elliott model assuming hopping of charge carriers between localized sites.     

The contribution of anion disorder to ionic conductivity on single crystals of β-PbF2

The effect of the different cooling processes on the disorder of flourine ions and ionic conductivity in β-PbF₂ has been studied by X-ray method and ionic conductivity measurements on single crystals below the transition temperature T_c. The spike-like diffuse scattering was observed along the <111>¹ directions around the Bragg reflections. The activation energies for the conduction process are 0.40 eV for the sample quenched from 970 K and 0.54 eV for the one from 720 K. The higher the quenching temperature is, the higher the conductivity and the lower the activation energy become. The dependence of conductivity on the different cooling processes is more evident in single crystals than in polycrystalline samples. The contribution of the different cooling processes to ionic conductivity can be quantitatively explained by the extent of ordering of mobile fluorine ions. Time dependence of ionic conductivity has not been observed.     

Intrinsic metastability of low doped manganites: La<Subscript>0.8</Subscript>Ca<Subscript>0.2</Subscript>MnO<Subscript>3</Subscript> case

Transport properties of phase separated La_0.8Ca_0.2MnO₃ crystals in the aged highly resistive metastable state were studied. It was found that the coexistence of different ferromagnetic phases at low temperatures is sensitive to electric current/field. In a contrast with the previously studied low resistivity metastable states the high resistivity state exhibits positive magnetoresistance and significant current dependence of the resistivity even at temperatures much higher than the Curie temperature. Application of current pulses results in appearance of zero bias anomaly in the current dependent conductivity. Similarly to the low resistivity metastable states the memory of the resistivity can be erased only after heating of the sample to T_e ≈360 K. After one year storage at room temperature the La_0.8Ca_0.2MnO₃ samples show clear signatures of aging. The aged samples spontaneously evolute towards high resistivity states. The results are discussed in the context of a coexistence of two ferromagnetic phases with different orbital order and different conductivity. The metallic ferromagnetic phase seems to be less stable giving rise to the experimentally observed electric field effects and aging.     

Thermal expansion of orthorhombic and tetragonal phases of the highT c superconductors RE1Ba2Cu3O y (RE=Y,Gd, Dy)

Measurements of the coefficient of linear thermal expansion α have been carried out for the orthorhombic and tetragonal phases of RE₁Ba₂Cu₃O _y (RE=Y, Gd, Dy) compounds using a high resolution capacitance dilatometer in the temperature range 77–300 K. All the superconducting samples exhibit a jump Δα at their respective transition temperatures,T _c. Evidences of, sample-to-sample variation in α values and dependence of Δα on the sample preparation conditions, have been obtained. The non-superconducting samples, in general, exhibit lower values of α possibly because of lowering of oxygen content.     

Magnetic and thermal properties of CuFeS<Subscript>2</Subscript> at low temperatures

The magnetic moment M, the magnetic susceptibility χ, and the thermal conductivity of chalcopyrite CuFeS₂, which is a zero-gap semiconductor with antiferromagnetic ordering, have been measured in the temperature range 10–310 K. It has been revealed that the quantities χ(T) and M(T) increase anomalously strongly at temperatures below ∼100 K. The temperature dependence M(T) is affected by the magnetic prehistory of the sample. An analysis has demonstrated that the magnetic anomalies are associated with the presence of a system of noninteracting magnetic clusters in the CuFeS₂ sample under investigation. The formation of the clusters is most likely caused by the disturbance of the ordered arrangement of Fe and Cu atoms in the metal sublattice of the chalcopyrite, which is also responsible for the phase inhomogeneity of the crystal lattice. The inhomogeneity brings about strong phonon scattering, and, as a result, the temperature dependence of the thermal conductivity coefficient exhibits a behavior characteristic of partially disordered crystals.     

Dielectric relaxation and dc conductivity on the PVOH-CF3COONH4 polymer system

In the present paper, the ionic conductivity and the dielectric relaxation properties on the poly(vinyl alcohol)-CF₃COONH₄ polymer system have been investigated by means of impedance spectroscopy measurements over wide ranges of frequencies and temperatures. The electrolyte samples were prepared by solution casting technique. The temperature dependence of the sample’s conductivity was modeled by Arrhenius and Vogel-Tammann-Fulcher (VTF) equations. The highest conductivity of the electrolyte of 3.41×10^− 3 (Ωcm)^− 1 was obtained at 423 K. For these polymer system two relaxation processes are revealed in the frequency range and temperature interval of the measurements. One is the glass transition relaxation (α-relaxation) of the amorphous region at about 353 K and the other is the relaxation associated with the crystalline region at about 423 K. Dielectric relaxation has been studied using the complex electric modulus formalism. It has been observed that the conductivity relaxation in this polymer system is highly non-exponential. From the electric modulus formalism, it is concluded that the electrical relaxation mechanism is independent of temperature for the two relaxation processes, but is dependent on composition.     

Conductivity studies of starch-based polymer electrolytes

A proton-conducting polymer electrolyte based on starch and ammonium nitrate (NH₄NO₃) has been prepared through solution casting method. Ionic conductivity for the system was conducted over a wide range of frequency between 50 Hz and 1 MHz and at temperatures between 303 K and 373 K. Impedance analysis shows that sample with 25 wt.% NH₄NO₃ has a smaller bulk resistance (R _b) compared to that of the pure sample. The amount of NH₄NO₃ was found to influence the proton conduction; the highest obtainable room temperature conductivity was 2.83 × 10⁻⁵ S cm⁻¹, while at 100 °C, the conductivity in found to be 2.09 × 10⁻⁴ S cm⁻¹. The dielectric analysis demonstrates a non-Debye behavior. Transport parameters of the samples were calculated using the Rice and Roth model and thus shows that the increase in conductivity is due to the increase in the number of mobile ions.     

Low-temperature phase transitions in LiKSO4: A neutron study

A single crystal neutron diffraction study of the low temperature structural phase transitions in LiKSO₄ has been carried out. Detailed temperature dependence of the Bragg peak profiles and integrated intensities for a group of about 20 reflections has been investigated upon cooling and heating in the temperature range 300 K to 96 K. The crystal undergoes three clear phase transitions at 205 K (from sp. gr. P6₃ to sp. gr. P31c), at 189 K and at about 135 K on cooling. The corresponding temperatures are 250 K, 190 K and 138 K on the heating cycle. These transitions are extremely sluggish, the kinetics depending on the thermal treatment of the crystal.     

The temperature dependences of electromechanical properties of PLZT ceramics

The mechanical and electrical properties in lanthanum modified lead zirconate-titanate ceramics of 5/50/50 and 10/50/50 were studied by mechanical loss Q ^{- 1}, Young's modulus E, electric permittivity ε and tangent of dielectric loss of angle tgδ measurements. The internal friction Q ^{- 1} and Young modulus E measured from 290 K to 600 K shows that Curie temperature T_C is located at 574 K and 435 K (1^st cycle of heating) respectively for ceramic samples 5/50/50 and 10/50/50. The movement of T_C in second cycle of heating to lower temperature (561 K for 5/50/50 and 420 K for 10/50/50) has been observed. Together with Q ^{- 1} and E measurements, temperature dependences of ε=f(T) and tgδ=f(T) were determinated in temperature range from 300 K to 730 K. The values of T_C obtained during ε and tgδ measurements were respectively: 560 K for 5/50/50 and 419 K for 10/50/50. These temperatures are almost as high as the temperatures obtained by internal friction Q ^{- 1} measurements in second cycle of heating. In ceramic sample 10/50/50 the additional maximum on internal friction Q ^{- 1} curve at the temperature 316 K was observed.     

Pyroelectric properties of BiFeO3 ceramics prepared by a modified solid-state-reaction method

BiFeO₃ (BFO) ceramics were prepared by a modified solid-state-reaction method which adopts a higher heating/cooling rate during the sintering process than usually used. It was found that the calcination temperature T _cal (from 400 to 750°C) does not influence the BFO phase formation, while the sintering temperature T _sin (from 815 to 845°C) dominates the phase purity. The optimum sintering temperature was in the range from 825 to 835°C. The optimized samples exhibit saturated ferroelectric hysteresis loops with a remnant polarization of 13.2 μC/cm². The measured piezoelectric coefficient d ₃₃ was 45 pC/N. No remnant magnetization was observed in all of the samples. The pyroelectric properties were studied as a function of temperature and frequency. A pyroelectric coefficient as high as 90 μC/m² K was obtained at room temperature in the optimized sample. An abrupt decrease of the pyroelectric coefficient was observed at temperatures between 70 and 80°C. On the basis of our results, BFO may have the potential for pyroelectric applications.     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ∼v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

Harmonic analysis of the repolarization processes in a Rb<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystal near the temperature of domain structure freezing (<Emphasis Type="Italic">T</Emphasis>* ≈ 150 K)

Contributions from the repolarization of rubidium tetrachlorozincate Rb₂ZnCl₄ samples near the domain structure freezing temperature (T* ≈ 150K) and the resulting conductivity to amplitudes of the harmonic components of the Sawyer-Tower circuit output signal were separated. The nonlinear dependence of sample polarization (ignoring dielectric loss) and the dependence of instantaneous conductivity on the applied harmonic voltage were determined. It was shown that the shape of the potential profile of domain wall motion changes appreciably over a relatively narrow temperature range near T*.     

Kinetic coefficients of the semiconducting phase of FeSi2 at low temperatures

The results of studying the kinetic coefficients of β-FeSi₂ in the temperature range 4.2–300 K are considered. The resistivity decreases upon heating in the entire temperature range under investigation. The temperature dependences of the resistivity and thermal conductivity exhibit a break at ～20 K. In the range of 4.2–20 K, the resistivity is a linear function of temperature. The thermo-emf increases rapidly upon cooling and attains values exceeding 15 mV/K. The temperature dependence of the thermo-emf exhibits a break at ～40 K. The observed set of temperature dependences of the kinetic coefficients apparently cannot be explained by a superposition of the known effects only. A new effect probably exists that is associated with a strong electron-phonon interaction in FeSi₂ and which requires a further investigation.     

Thermal conductivity and magnetic properties of the B substituted Ca3Co4O9

In this study, we reported the effects of the boron (B) substitution into the Ca site in the Ca₃Co₄O₉ system on the electrical, thermal and magnetic properties between 300 K and 5 K. The results indicated that the B-substitution into the system caused an increase of resistivity due to the decrease on carrier concentration. Thermal conductivity decreased for the x = 0.5 B-substituted sample and then increased with increasing the B-content. Analysis on the thermal conductivity of samples showed that the phonon–phonon interaction term is the dominant component in the total thermal conductivity for all the samples. It was found that the point defect contribution to the thermal conductivity increased by increasing the B-content. The temperature dependence of magnetic susceptibility showed a paramagnetic behavior at room temperature and ferrimagnetic behavior below 20 K for unsubstituted sample. But, the magnetization decreased in the B-substituted samples. The substitution of B into the Ca site destroyed the interlayer coupling, which resulted in the decrease of the ferromagnetic behavior. The susceptibility data was fitted using Curie–Weiss law with temperature independent term and the μ_eff values were calculated to be 1.42 μ_B and 3.89 μ_B for unsubstituted sample and the highest B-substitution, respectively.     

Cryogenic absorption cells operating inside a Bruker IFS-125HR: First results for CH4 at 7 μm

New absorption cells designed specifically to achieve stable temperatures down to 66 K inside the sample compartment of an evacuated Bruker IFS-125HR Fourier transform spectrometer (FTS) were developed at Connecticut College and tested at the Jet Propulsion Laboratory (JPL). The temperature stabilized cryogenic cells with path lengths of 24.29 and 20.38 cm were constructed of oxygen free high conductivity (OFHC) copper and fitted with wedged ZnSe windows using vacuum tight indium seals. In operation, the temperature-controlled cooling by a closed-cycle helium refrigerator achieved stability of ±0.01 K. The unwanted absorption features arising from cryodeposits on the cell windows at low temperatures were eliminated by building an internal vacuum shroud box around the cell which significantly minimized the growth of cryodeposits. The effects of vibrations from the closed-cycle helium refrigerator on the FTS spectra were characterized. Using this set up, several high-resolution spectra of methane isotopologues broadened with nitrogen were recorded in the 1200-1800 cm⁻¹ spectral region at various sample temperatures between 79.5 and 296 K. Such data are needed to characterize the temperature dependence of spectral line shapes at low temperatures for remote sensing of outer planets and their moons. Initial analysis of a limited number of spectra in the region of the R(2) manifold of the ν₄ fundamental band of ¹³CH₄ indicated that an empirical power law used for the temperature dependence of the N₂-broadened line widths would fail to fit the observed data in the entire temperature range from 80 to 296 K; instead, it follows a temperature-dependence similar to that reported by Mondelain et al. [17] and [18]. The initial test was very successful proving that a high precision Fourier transform spectrometer with a completely evacuated optical path can be configured for spectroscopic studies at low temperatures relevant to the planetary atmospheres.