Pressure and temperature dependences of the dielectric constant,Raman spectra and lattice constant of SnI42

The pressure and temperature dependences of the Raman frequencies, static dielectric constant and lattice constant of the molecular crystal SnI₄, were investigated. These results are combined to evaluate the pure volume and pure temperature (i.e. volume-independent) dependences of the molecular polarizability and of the Raman-active phonon self-energies. The pressure results also allow the separation of the Raman modes into external and internal modes of the crystal; the external modes exhibit a much stronger pressure dependence, and thereby larger Grüneisen parameters, than do the internal modes. The Raman frequencies increase (decrease) with increasing pressure (temperature) whereas the dielectric constant increases with both increasing pressure and temperature, emphasizing the importance of non-volume effects. In fact, it is found that the isobaric temperature dependence of the dielectric constant is dominated by the pure temperature dependence of the molecular polarizability. The pure volume dependence of the polarizability, on the other hand, is relatively small as is perhaps typical of most molecular crystals.     

The lattice polarizability of PbI2

We have measured the far-i.r. reflectivity spectrum of the layer crystal PbI₂ for the in-plane polarization. A strong reststrahlen band is observed, with TO and LO phonon frequencies located at 52 and 108 cm^?1. The large lattice contribution accounts for three-quarters of the low-frequency dielectric constant of 26. The deduced infrared effective charge and electronic polarizability of PbI₂ indicates that the intralayer bonding is closely related to the chemical bonding in the IV–VI rocksalt-structure crystals.     

Radiation damage in polyacrylonitrile

Radiation damage in polyacrylonitrile (PAN) has been studied in the temperature range 273 to 420 K using thermoluminescence, electron spin resonance and optical absorption techniques. ATL glow peak has been observed with maximum at 362 K. The species responsible for theTL glow peak has been identified to be due to the free radical -CH₂-CH(CN)-ĊH-CH(CN)-CH₂-by correlation with the data obtained by the temperature variation ofESR and optical absorption spectra of thex-irradiated, pre- and post-x-irradiated and thermally-treated samples. A mechanism of formation of network structures accelerated by the free radicals formed on irradiation has been investigated to explain the optical absorption spectra. TheTL glow peak is analysed by various methods and corresponds to an activation energy of 1 eV, following second order kinetics.     

The pressure dependence of theE 2 reflectivity peak and of the dielectric constant in III–V semiconductors

The functional dependence on pseudopotential form factors of the Penn gap related to theE ₂ reflectivity peak is used to calculate the pressure dependence ofE ₂ and of the static electronic dielectric constant in III–V semiconductors. In a one-parameter approximation only the pressure coefficient of one Fourier coefficient of the pseudopotential is fitted to experimental values of the pressure dependence ofE ₂ in group-IV semiconductors and then taken over to III–V compounds. We find a decrease of the dielectric constant with pressure. The comparison of our results with those experimentally known supports strongly our method.This work was performed in the Arbeitsgemeinschaft A_III-B _v -Halbleiter of the Karl-Marx-Universität Leipzig.     

Evidence for vacancy-driven phase transitions in BaMnF 4 at elevated temperatures

Anomalies near 370 K and/or 400 K in dielectric constant, diffuse x-ray scattering, C₁₁ elastic coefficient, electrical conductivity, and piezoelectric resonance have been reported for BaMnF ₄. Particularly curious is the dielectric constant along the ac-diagonal, which rises from 15 at 300 K to a sharp peak of 73 at 400 K. These anomalies are discussed in terms of 2D ionic conduction mechanisms (fluorine vacancy hopping), and the disappearance of short-range antiferroelectric order at temperatures above 400 K. At 650 K the crystals lose all piezoelectricity, which is interpreted as a 3D vacancy-driven C_2v – D_2h, transition.     

Variational calculations of the static polarizability of H2 and of the dipole-dipole contribution to the dispersion energy between two hydrogen molecules

The convergence of variation-perturbation calculations using non-linear variation parameters has been examined for the static polarizability of H₂ and for the coefficient (C₆) of the R ^-6 term in the multipole expansion of the H₂-H₂ dispersion energy. Using a correlated ground state wavefunction, and one excited state for each of the parallel and perpendicular components of the polarizability, a single variational parameter for each state gave 96 per cent of the exact value of α_⊥ and 91 per cent of the exact value of α_‖. A second variational parameter for the parallel component gave 94 per cent of α_‖. The same procedure gave the spherical average and anisotropy of C₆ to an accuracy of about 90 per cent. By extending the calculations into the region where overlap of the orbitals of the two molecules becomes significant, it is shown for the end-on configuration that the C₆ R ^-6 term underestimates the dispersion energy near the van der Waals' minimum by roughly 10 per cent.     

Temperature dependent EPR of octahedral S-state ions in garnets

Temperature dependence of EPR of the octahedral Fe³⁺ and Mn²⁺ ions in four different garnets is studied between 77 K and the room temperature. The observed change of the spin hamiltonian parametersB ₂ ⁰ ,B ₄ ⁰ ,B ₄ ³ is of the order of several %, while theg-factor is temperature independent within the experimental errors. The role of thermal expansion and the effect of phonons on the temperature variation of B ₂ ⁰ is analyzed. It is shown that the inhomogeneous thermal expansion may account for theB ₂ ⁰ (T) dependence observed.The authors wish to thank Dr. B. V.Mill of Moscow State University (USSR) for supplying the single crystals of Na₃Sc₂V₃O₁₂, Ca₃Al₂Ge₃O₁₂ and Ca₃Ga₂Ge₃O₁₂ garnets.     

Iminodiacetic acid doped ferroelectric triglycine sulphate crystal: Crystal growth and characterization

Single crystals of iminodiacetic acid (HN(CH₂COOH)₂) doped triglycine sulphate (IDATGS) crystals have been grown from aqueous solution containing 1-10 mol% of iminodiacetic acid at constant temperature by slow evaporation technique. The effects of different amounts of doping entities on the growth habit have been investigated. X-ray powder diffraction pattern for pure and doped TGS was collected to determine the lattice parameters. The grown crystals were subjected to Fourier transform infrared (FTIR) spectroscopy studies to find the presence of various functional groups qualitatively. The dielectric permittivity has been studied as a function of temperature. An increase in the transition temperature (49.2-49.7 °C) of IDATGS crystals is observed. The dielectric constant (ε′_max) of IDATGS crystals vary in the range 922-2410 compared to pure TGS (T_c=49.12 °C and ε′_max=3050). Curie Weiss constants C_p and C_f in the paraelectric and ferroelectric phases were determined. The transition temperature (T_c) is found to decrease with increase in dopant concentration. P-E hysteresis studies show the presence of internal bias field in the crystal. Piezoelectric measurements were also carried out at room temperature. Domain patterns on b-cut plates were observed using scanning electron microscope. The micro hardness studies reveal that the doped crystals are harder than the pure TGS crystals. The low dielectric constant, higher transition temperature, internal bias field and hardness suggest that IDATGS crystals could be a potential material for IR detectors.     

Anion polarizability functions in alkali halide crystals

Anion polarizabilities in alkali halide crystals are analysed as a function of interionic separation R. The anion polarizability is treated as a function of the anion and cation radii, with its partial derivatives approximated by those with respect to R for fixed cation and anion, respectively. With pressure derivatives of the ionic radii deduced from the crystal compressibility, assuming transferability among crystals, the polarizability derivatives with respect to ionic radius yield pressure derivatives of the polarizability that agree with experiment to within a factor of two. These results offer a useful means of predicting the pressure dependence of dielectric data.     

The dielectric behaviour of doped near-stoichiometric lithium niobate in the terahertz range

The dielectric properties of near-stoichiometric LiNbO₃:Fe and LiNbO₃:Ce single crystals have been investigated using terahertz time domain spectroscopy in a frequency range of 0.7-1.6 THz at room temperature. When coupled with an applied external optical field, obvious photorefractive effects were observed, resulting in a modulation of the complex dielectric constant for the crystals. The variation in refractive index, |Δn|, had a linear relationship with the applied light intensity, accompanied by a step-like decrease at high intensity. The findings were attributed to the internal space charge field of the photorefraction and the light-induced domain reversal in the crystals.     

等静压下0.75Pb（Mg1/3Nb2/3）O3-0.25PbTiO3陶瓷的介电性能研究

研究了等静压对0.75Pb（Mg_1/3Nb_2/3）O₃-0.25PbTiO₃（PMN-25PT）陶瓷介电温谱的影响,PMN-25PT剩余极化随等静压变化和等静压压致相变.结果表明,随着压力增加,PMN-25PT的介电峰值温度T_m降低,/+{dT_m}/-{dP}≈-4℃/kbar,极化弛豫增强;剩余极化随压力增加连续减小;介电常数对压力的依赖关系与对温度场的依赖相似,压力诱导PMN-25PT发生弛豫铁电—顺电相变,相变为宽化的渐变过程,频率色散和极化弛豫更加强烈和普遍. 关键词： 铌镁酸铅-钛酸铅 等静压 介电弛豫 压致相变     

Dielectric behaviour of anisotropic ionic crystals

In the case of cubic ionic crystals Havinga has shown that the temperature variation of dielectric constant could be described in terms of volume and temperature effects. By extending his formalism to anisotropic, ionic crystals it has been shown that unlike in cubic ionic crystals where the volume effect consists of a change in the number per unit volume of the polarizable particles and their polarizability with volume, in the case of anisotropic ionic crystals, in addition to these, a variation in the anisotropy of polarizability due to uneven thermal expansion also has to be taken into account. This method of analysis has been examined by taking rutile as an example.     

Temperature and pressure dependences of the properties and phase transition in paratellurite (TeO2: Ultrasonic,dielectric and Raman and Brillouin scattering results

The effects of temperature and pressure on the ultrasonic propagation properties, dielectric constants and the Raman and Brillouin spectra in paratellurite (TeO₂) were investigated with emphasis on the behavior in the vicinity of the newly-discovered, pressure-induced phase transition. The transition is found to be second-order and purely strain-induced, driven by a soft shear acoustic mode propagating along a <110〉 and polarized along a (110) crystal direction. Such pure-strain transitions were previously discussed by Anderson and Blount and the transition in paratellurite is the first observation of this kind of transition. No evidence was found for any coupling of the soft mode to any other acoustic or optic mode, although small anomalies associated with lattice strains accompanying the transition were observed in some of the elastic and dielectric constants. Analysis of the effective elastic constant C, governing the soft mode velocity indicates that, within experimental uncertainty, the transition can be described by mean-field theory. Although the apparent attenuation of the soft mode increased significantly near the transition, it is concluded that this effect is probably due to the fact that the phase and group velocities are not parallel rather than to intrinsic dissipative processes in the crystal. With the exception of C₄₄, the remaining elastic constants and Raman-active phonon frequencies displayed normal increases with pressure. No soft Raman-active modes were observed in either phase. The static dielectric constants ?₁ are large, due to the large electronic polarizability of TeO₂, and the anisotropy in ? results almost entirely from the anisotropy in the optical dielectric constants ?_∞. In the low pressure tetragonal phase both ?_a and ?_c exhibit normal temperature dependences and ?_c decreases with pressure; however, ?_a exhibits an anomalous increase with pressure. Temperature, pressure and uniaxial stress measurements are combined to evaluate the various contributions to the temperature and pressure dependences of ?. Combining the ? data with available i.r. measurements demonstrated that the generalized Lyddane-Sachs-Teller relation is well obeyed for TeO₂. Finally, the Szigetti effective charge ratios were determined for the lowest frequency IR-active modes. These ratios were found to be quite low, being 0.27 and 0.18 for the a-axis responses, respectively, indicating that the bonding is highly covalent.     

Non-ohmic conduction and electrical switching under pressure of the charge transfer complexo-tolidine-iodine

The current voltage characteristics ofo-tolidine-iodine, with stoichiometry 1:1 grown from benzene, have been studied under high pressures upto 6 GPa atT=300 K andT=77 K. The characteristics show a pronounced deviation from ohmicity beyond a certain current for all pressures studied. At room temperature, beyond a threshold field the system switches from a low conductingOFF state to a high conductingON state with σ_ON/σ_OFF ∼ 10³. TheOFF state can be restored by the application of an a.c. pulse of low frequency. The temperature dependence of the two states studied indicates that theOFF state is semiconducting while theON state, beyond a certain applied pressure is metallic. The characteristics atT=77 K do not show any switching.     

Pressure and volume dependence of thelo-to phonons in InAs

The pressure dependence of thelo-to phonons in InAs has been investigated by Raman scattering using the diamond anvil cell. Indium arsenide transforms, presumably to the rock-salt structure at 70±1 kbar. The mode Grüneisen parameters for thelo-to phonons are γ _lo =0.99±0.03, γ _to =1.2±0.03 respectively. The effective charge,e* _T , for InAs decreases slightly with pressure and this trend is in accordance with the behaviour of other III–V zinc blende structured semiconductors: The structural phase transition is discussed in the light of theoretical calculations for phase stability of III–V compounds, as well as recent high pressure x-ray diffraction studies.     

Electroluminescence in lead titanate single crystals

Electroluminescence in PbTiO₃ single crystals is studied with variation in applied electric field, frequency (20 Hz to 5 kHz) and temperature. The EL onset depends on the rate at which the dipole switches. Extremely sharp upward rising nature of the pulses of micro second duration suggest that there is a self maintained discharge in the dielectric due to secondaryγ _p mechanism. Frequency dependence of EL suggests that both the secondary mechanisms, viz. theγ _p andγ _i are active after the application of a high field and the critical field at which this occurs decreases with increase in the frequency of the applied voltage. Similarly the onset voltage decreases with increase in frequency. The temperature dependence of EL at the applied frequency of 50 Hz shows that the onset voltage is intimately connected with the coercive field of the crystal and it is minimum at the Curie point. The study suggests that EL occurs in the bulk and there is a breakdown in the dielectric due to an avalanche formation.     

Piezĭlectric and elastic properties of Sr<Subscript>3</Subscript>NbGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> (SNGS) single crystals

The elastic, piezoelectric, and dielectric constants of new piezoelectric single crystals Sr₃NbGa₃Si₂O₁₄ (SNGS) are measured. The elastic moduli C ₁₁ and C ₆₆ and the piezoelectric coefficient e ₁₁ are determined over a wide temperature range. It is demonstrated that the piezoelectric activity of the crystal is retained up to the highest temperatures (900°C).     

Dielectric and photoelectric properties of Pb(Mg1/3Ta2/3O3 crystals

The temperature dependence of the dielectric constant ɛ and the spectral dependence of the photoconductivity of Pb(Mg_1/3Ta_2/3)O₃ crystals are investigated. The width of the band gap (3.4 eV) is determined. It is found that above the temperature of the maximum of the dielectric constant, its reciprocal 1/ɛ varies with temperature first quadratically and then according to a linear law, as is characteristic of ferroelectrics with a smeared phase transition. The smearing parameter of the transition estimated from the experimental data is close to the value calculated assuming the absence of long-range order in the arrangement of the Mg and Ta ions. Fiz. Tverd. Tela (St. Petersburg) 40, 109–110 (January 1998)     

Calorimetric studies of C70S48 crystals

A comparative study of the thermodynamic properties of the C₇₀ fullerene, sulfur, and C₇₀S₄₈ crystals has been made by differential scanning calorimetry. It is shown that only C₇₀S₄₈ has an endothermic δ phase transition with a peak at 430 K, which lies in the temperature region exhibiting conductivity and dielectric anomalies. A correlation between the behavior of the sulfur sublattice in the C₇₀S₄₈ crystals and the thermodynamic parameters of the transition has been established. Fiz. Tverd. Tela (St. Petersburg) 41, 360–363 (February 1999)     

Temperature variation of the elastic constant C44 of VCr alloys

The elastic constant C₄₄ has been measured for eight VCr single crystals from 4.2 K to room temperature. The observed temperature variation is totally similar to the 4d and 5d systems.