Calorimetric and dielectric studies of a phase transition in thallium dihydrogen phosphate: A nearly critical case

The heat capacity of thallium dihydrogen phosphate was measured from 12 to 300 K. A lambda anomaly with a small first order discontinuity was found at 229.76 K with the integrated enthalpy and entropy changes equal to 370 J mol^-1 and 1.8 J K^-1 mol^-1, respectively. The entropy discontinuity at the first order transition is 0.25 J K^-1 mol^-1. The Landau theory of phase transitions of the second kind reproduces closely the temperature dependence of the anomalous heat capacity. This, together with the small discontinuity in the entropy, implies that the phase transition is close to a classical critical point of higher order. The relative dielectric permittivity ?_r(b) along the b-axis at 1 kHz is anomalously large and strongly temperature dependent, while the ?_r(a¹) and ?_r(c) are not. A broad absorption centered at 1700 cm^-1 was observed in the infrared absorption spectrum, indicating presence of hydrogen bonding of the length ～ 0.25 nm. Occurrence of a phase transition at 127 ± 2 K was suggested by differential thermal analysis of thallium dideuterium phosphate.     

Determination of the protonic transference number for KH2PO4 by electromotive force measurements

The protonic transference number (t_H⁺) of KH₂PO₄ has been determined at 293 K as a function of hydrogen partial pressure (1>p_H2(atm)>10^-3) using EMF measurements. The results are consistent with earlier work which indicated that t_H⁺ is essentially unity (>0.97) for this material under the relevant experimental conditions.     

Electrical contacts on organic crystals: TTF-TCNQ

Room temperature plots of the electrical potential measured on the surfaces of TTF-TCNQ crystals which carry current are presented. It is shown that silver paint contacts behave electrically as though thin resistive layers ($\text{? 20}\text{mhos}\text{mm}{}^2$ conductance) were inserted between the crystal and the current-injecting electrodes. The contact resistance is microscopically nonuniform, and can lead to erroneous results in 4-probe conductivity measurements. By measurement and calculation (using the method of image charges) it is demonstrated that measurement techniques which introduce nonuniform current densities into highly anisotropic crystals are subject to error unless the contacts are kept very small. The principal conductivities of TTF-TCNQ at room temperature were found to be: σ_b = 440 ± 15, σ_c ≈ 13, σ_a, = 1.30 ± 0.05 Ω^?1 cm^?1, independent of contact effects.     

Magnetic and dielectric study of Bi2CuO4

Magnetic and dielectric properties have been investigated for Bi₂CuO₄, which has the same chemical formula as that of the parent materials of cuprate superconductors R₂CuO₄ (R: rare earths). Magnetization measurements show the antiferromagnetic transition of the Cu²⁺ spins at ～42 K, as reported previously. Dielectric measurements for the frequencies of 1 kHz to 1 MHz show that the dielectric constants are 100–500 at room temperature. The dielectric dispersion reveals that the dielectric response lacks spatial coherence, a property which indicates the possible existence of phase separation as suggested for La₂CuO₄. The imaginary part of dielectric response gives the activation energy of 0.22 eV, suggesting that the dielectric response is governed by the electron hopping between the Cu ions.     

Structure and dielectric behavior of TlSbS2

A comparison of structure and dielectric properties of TlSbS₂ thin films, deposited in different thicknesses (400–4100 Å) by thermal evaporation of TlSbS₂ crystals that were grown by the Stockbarger–Bridgman technique and the bulk material properties of TlSbS₂ are presented. Dielectric constant ε ₁ and dielectric loss ε ₂ have been calculated by measuring capacitance and dielectric loss factor in the frequency range 20 Hz–10 KHz and in the temperature range 273–433 K. It is observed that at 1 kHz frequency and 293 K temperature the dielectric constant of TlSbS₂ thin films is ε ₁=1.8–6 and the dielectric loss of TlSbS₂ thin films is ε ₂=0.5–3 depending on film thickness. In the given intervals, both of dielectric constant and dielectric loss decrease with frequency, but increase with temperature. The maximum barrier height W _m is calculated from the dielectric measurements. The values of W _m for TlSbS₂ films and bulk are obtained as 0.56 eV and 0.62 eV at room temperature, respectively. The obtained values agree with those proposed by the theory of hopping over the potential barrier. The temperature variation of ac conductivity can be reasonably interpreted in terms of the correlated barrier hopping model since it obeys the ω ^s law with a temperature dependent s (s<1) and going down as the temperature is increased. The temperature coefficient of capacitance (TCC) and permittivity (TCP) are evaluated for both thin films and bulk material of TlSbS₂.     

Transport and dielectric properties of lisicon

The ionic and electronic conductivities of Lisicon with compositions Li₁₄Zn(GeO₄)₄-(A) and Li₁₂Zn₂(GeO₄)₄-(B) have been determined in the temperature range 306–578 K. It has thus been shown that the ionic transport number is nearly unity throughout the temperature range. The activation energies for lithium motion were found to be 0.6 eV and 0.47 eV for compounds A and B. These values are very close to the values of the heat of transport q¹_Li⁺ 0.59 eV and 0.50 eV respectively, obtained from thermoelectric power (θ_t) measurements. The total ionic conductivity of the pellets of Lisicon was found to decrease with increasing average grain size in the range 20–78 μ. Electrode effects on the ionic conductivity are also reported for silver and TiS₂ electrodes. The electron/hole conductivity was studied using the Wagner cell technique. The activation energies for electron and hole transport for A were estimated as 1.02 eV and 1.39 eV and for B as 0.89 eV and 1.49 eV, respectively. The variation of the dielectric parameters ?', ?” and tan δ have been studied for sample A between 10 Hz-100 kHz. The high frequency dielectric constant (at 100 kHz) was 21.4. The ?' versus ?” (Cole-Cole plot) is also presented.     

Magnetostriction of monocrystalline URu2Si2

We report magnetostriction measurements on a monocrystalline sample of the heavy-fermion compound URu₂Si₂, below (at 4.2 K) and above (at 20 K) the Néel temperature. For a field direction along the tetragonal axis the magnetostriction is strongly anisotropic; the volume effect amounts to 1.4 × 10^-6T^-1 at 8 T and 4.2 K. For a field direction in the tetragonal plane the magnetostriction is nearly two orders of magnitude smaller, and less anisotropic.     

AC conductivity and dielectric behavior of bulk Furfurylidenemalononitrile

AC conductivity and dielectric behavior for bulk Furfurylidenemalononitrile have been studied over a temperature range (293–333 K) and frequency range (50–5×10⁶ Hz). The frequency dependence of ac conductivity, σ_ac, has been investigated by the universal power law, σ_ac(ω)=Aω^s. The variation of the frequency exponent (s) with temperature was analyzed in terms of different conduction mechanisms, and it was found that the correlated barrier hopping (CBH) model is the predominant conduction mechanism. The temperature dependence of σ_ac(ω) showed a linear increase with the increase in temperature at different frequencies. The ac activation energy was determined at different frequencies. Dielectric data were analyzed using complex permittivity and complex electric modulus for bulk Furfurylidenemalononitrile at various temperatures.     

DFT(B3LYP/LanL2DZ), non-linear optical and electrical studies of a new hybrid compound: [C6H10(NH3)2]CoCl4·H2O

A new organic-inorganic material [C₆H₁₀(NH₃)₂]CoCl₄·H₂O was reported. The title compound was synthesized at room temperature by slow evaporation and then characterized by a single X-ray diffraction, spectroscopic measurements, thermal analysis and dielectric technique. It crystallizes in the non-centrosymmetric space group Pna2₁ with the following unit cell parameters: a=12.5328(1) Å, b=9.0908(1) Å, c=11.7440(1) and α=β=γ=90°. The structure can be described by the alternation of two different cationic-anionic layers. It consists of isolated H₂O, isolated [CoCl₄]²⁻ tetrahedral anions and diammoniumcyclohexane [C₆H₁₀(NH₃)₂]²⁺ cations, which are connected via N–H…Cl, N–H…O and O–H…N hydrogen bonds. The Hirshfeld surface analysis was conducted to investigate intermolecular interactions and associated 2D fingerprint plots, revealing the relative contribution of these interactions in the crystal structure quantitatively. Theoretical calculations were performed using DFT/B3LYP/LanL2DZ method for studying the molecular structure and vibrational spectra and especially to examine the non-linear optical behavior of the compound. Solid state ¹³C NMR spectrum shows three signals correspond to three different carbon environments. Thermal analysis discloses a phase transition at the temperature 315 K and the evaporation of water molecule at 327 K. A detailed dielectric study was reported and shows a good agreement with thermal measurements.     

Decrease of loss in dielectric properties of TbMnO3 by adding TiO2

Low-frequency (10²-10⁵ Hz) dielectric properties of TbMnO₃+xTiO₂ (x=0.33, 1, 3) ceramic composites, which were fabricated by conventional solid-state reaction, were investigated from 360 to 77 K. Very high dielectric constants and interesting temperature dependence of the dielectric properties were observed in the present composite ceramics. When compared to the high dielectric loss of the polycrystalline TbMnO₃, the loss of TbMnO₃+xTiO₂ (x=0.33, 1, 3) decreased with the increasing TiO₂. Especially for TbMnO₃+1TiO₂, the dielectric loss decreased remarkably, while the dielectric constant was still very high, which are more favorable for practical applications.     

Rotational and vibrational temperatures of electronically excited BiN radicals in a chemiluminescent flame

Rotational and vibrational temperatures of electronically excited BiN radicals in a low-pressure Bi_x+N/N₂*/N₂+Ar chemiluminescent flame have been deduced from high-resolution Fourier-transform emission spectra. Bands of three electronic transitions, a³Σ⁺(a₁1)→X¹Σ⁺(X0⁺), b⁵Σ⁺(b₁0⁺)→X¹Σ⁺(X0⁺), and b⁵Σ⁺(b₁0⁺)→a ³Σ⁺(a₁1), were analysed to determine the optical temperatures in the a³Σ⁺(a₁1) and b⁵Σ⁺(b₁0⁺) states. The rotational temperatures characterising the rotational populations in the a₁1, v=0 and 1 states were determined from the a₁→X, 0-2, 0-3, 0-4, 1-1, and 1-2 bands. The b₁→X, 0-8 and 0-11 bands, and the b₁→a₁, 0-0 bands served to determine the rotational temperature of the radicals in the b₁0⁺, v=0 state. The temperatures derived from the various bands and transitions were well consistent and the mean rotational temperature was determined to be 353±18 K, which is close to the translational temperature of the gas.Vibrational temperatures of the radicals in the a₁1 and b₁0⁺ states were derived from band intensities of the a₁→X and from the b₁→X as well as b₁→a₁ systems, respectively. The Franck-Condon factors needed were calculated with RKR potentials deduced from literature values of the rotational and vibrational constants in the three states involved. The a₁1 vibrational temperature (336±21 K) was close to the rotational temperature, while the b₁0⁺ vibrational temperature (438±36 K) differed, likely due to the previously observed perturbation of the b₁0⁺ state.     

Anisotropy in the magnetic susceptibility of 2H-TaSe2

The magnetic susceptibility of 2H-TaSe₂ is temperature dependent and anisotropic. The susceptibility with H parallel to the c axis is approximately 2.5 times that perpendicular. The decrease in susceptibility below the charge density wave transition at 122 K is also anisotropic with [χ₆(122 K) ? χ₆(4.2 K]?[χ_⊥(122 K) ? χ_⊥(4.2 K)] ≈ 4. Consequently, this change cannot be simply interpreted as a change in the density of states at the Fermi level, unless very anisotropic electron g values are assumed.     

Low-temperature dielectric behavior of disordered and ordered langasite family single crystals LGS,LGT, SNGS and STGS

Dielectric measurements of disordered La₃Ga₅SiO₁₄ (LGS), La₃Ga_5,5Ta_0,5O₁₄ (LGT) and ordered Sr₃NbGa₃Si₂O₁₄ (SNGS), Sr₃TaGa₃Si₂O₁₄ (STGS) single crystals of the langasite family performed at frequencies from 10 Hz to 1 MHz at temperatures between 4.2 and 300 K are reported. Temperature dependences of dielectric permittivity ε₃₃ and ε₁₁ are obtained. It is shown that ε₃₃ in LGS and LGT exhibits incipient ferroelectric-like behavior. SNGS and STGS demonstrate ordinary dielectric temperature dependences as dielectric permittivity decreases down to helium temperatures.     

Transport and dielectric properties of thin polycrystalline CoO films

Transport and dielectric properties of polycrystalline CoO films were studied as functions of the applied field, frequency and temperature. TheI–V plots showed that the Poole-Frenkel field emission mechanism is responsible for conduction at fields>10⁵ V/cm. The ac conductivity σ(ω), the imaginary part of the dielectric constantε ₂, and tan δ plots as functions of frequency revealed three dispersion regions. The σ(ω) andε ₂ frequency dependence indicates a non-adiabatic hopping of charge carriers at low frequencies and adiabatic hopping at high frequencies. The activation energy of a dielectric oscillator is 0.15 eV. Work supported by the Office of Naval Research.     

Temperature and pressure dependences of EPR spectra of Gd ion doped in the EuAl3(BO3)4 monocrystal

The ground state of Gd³⁺ ions substituting for trivalent europium in the EuAl₃(BO₃)₄ single crystal was studied by electron paramagnetic resonance (EPR) over the temperature range of 300-4.2 K and at pressures up to 9 kbar. The EPR spectra were analysed using the spin Hamiltonian of axial symmetry. The following parameters are reported: g=1.981±0.002, b₂⁰=280.18±0.12, b₄⁰=−12.95±0.08 and b₆⁰=0.61±0.12 (at Т=298 K). The distortions of the nearest environment of Gd³⁺ ion were analysed within the framework of the superposition model of crystal field.     

Crystal structure,ionic conductivity and dielectric relaxation studies in the (C5H10N)2BiBr5 compound

The synthesis and crystal structure of the bis (3-dimethylammonium-1-propyne) pentabromobismuthate(III) salt are given in the present paper. After an X-ray investigation, it has been shown that the title compound crystallizes at 298 K in a centrosymmetric monoclinic system, in the space group C₂/c with the following lattice parameters a=12.9034(3) Å, b=19.4505(6) Å, c=8.5188(2) Å, β=102.449(2). Not only were the impedance spectroscopy measurements of (C₅H₁₀N)₂BiBr₅ carried out from 209 Hz to 5 MHz over the temperature range of 318 K–373 K, but also its ac conductivity evaluated. Besides, the dielectric relaxation was examined using the modulus formalism. Actually, the near values of activation energies obtained from the impedance and modulus spectra confirms that the transport is of an ion hopping mechanism, dominated by the motion of the H⁺ ions in the structure of the investigated material.     

Ionic conductivity in tysonite-type solid solutions La1−xBaxF3−x

The ionic conductivity of single crystals of tysonite-type solid solutions La_1?xBa_xF_3?x(0?x?0.095) has been studied parallel and perpendicular to the crystallographic c axis in the temperature range 293–1300 K. Three regions can be discerned in the compositional dependence of the ionic conductivity: (i) the “pure” crystal, in which at room temperature no exchange occurs between different types of anion sites in the tysonite structure; (ii) an intermediate region(0 < x < 7 × 10^-2) which reveals changes in both the conductivity activation enthalpy and the magnitude of the conductivity; (iii) a concentrated solid-solution region (x > 7 × 10^-2), where fluoride ions interchange easily among the different anion sublattices. Diffusion coefficients calculated from ionic conductivity results, are in good agreement with those calculated from ¹⁹F NMR measurements. Using the present data, along with ¹⁹F NMR data, dielectric relaxation data and structural considerations, mechanisms governing the ionic conductivity are proposed.     

Phase transition of [(C6H5)3PCH3]+(TCNQ)-2 and electrical transport properties

The electrical conductivity at 10GHz, the dielectric constant, and the thermoelectric power (TEP) of [(C₆H₅)₃PCH₃]⁺(TCNQ)^-₂, from 230 up to 400 K, have been measured. This organic quasi-one-dimensional solid undergoes a first order phase transition at 314 K. At the transition the conductivity increases by a factor of 2.2 and the activation energy drops to 0.26 from 0.31 eV. At 314 K TEP decreases abruptly from -75 to -60μVK^-1 and remains almost constant for T > 314 K. The dielectric permeability ?₀ is constant and equal to 5 in the low temperature phase, increases abruptly by 7% at the transition, and then depends strongly on temperature in the high temperature phase. Results of the high temperature phase are interpreted in terms of a strongly correlated salt.     

AC conductivity and dielectric properties of bulk tungsten trioxide (WO3)

AC conductivity and dielectric properties of tungsten trioxide (WO₃) in a pellet form were studied in the frequency range from 42 Hz to 5 MHz with a variation of temperature in the range from 303 K to 463 K. AC conductivity, σ_ac(ω) was found to be a function of ω^s where ω is the angular frequency and s is the frequency exponent. The values of s were found to be less than unity and decrease with increasing temperature, which supports the correlated barrier hopping mechanism (CBH) as the dominant mechanism for the conduction in WO₃. The dielectric constant (ε′) and dielectric loss (ε″) were measured. The Cole–Cole diagram determined complex impedance for different temperatures.     

The electron mobility and the static dielectric constant of Cd3As2 at 4.2 K

The ionized-impurity mobility of Cd₃As₂ at 4.2 K with carrier concentration lying in the range 10¹⁸–10¹⁹ cm^?3 has been calculated taking into account the inverted HgTe-type energy band structure of this material. A comparison of the results with experimental mobility data leads to a static dielectric constant of ?₀ = 36. This value represents a lower limit since any compensation effects would increase the fitted estimate of ?₀.