Electrical conductivity and dielectric properties of potassium sulfamate single crystals

Single crystals of potassium sulfamate are grown by the method of slow evaporation at constant temperature. AC electrical conductivity of potassium sulfamate is measured in the temperature range 300–430 K and in the frequency region between 100 Hz and 3 MHz along the a, b and c‐axes. Complex impedance spectroscopy is used to investigate the frequency response of the electrical properties of the potassium sulfamate single crystal. Temperature variation of AC conductivity and dielectric measurements show a slope change around 345 K for both heating and cooling run and this anomaly is attributed as phase transition, which is well supported by the DSC measurements. Value of loss tangent in the temperature region 330–400 K is found to be very low. Activation energies for the conduction process are calculated along the a, b and c‐axes. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical conductivity of sulfamic acid single crystals

Single crystals of sulfamic acid have been grown by the method of slow evaporation at constant temperature. DC electrical conductivity was measured in the temperature range 300 ‐ 440 K along a, b and c‐axes. Conductivity measurements show slope change near 330 K and 410 K. The slope change observed around 330 K may be attributed as due to a phase transition which has been well supported by the DSC and DTA measurements. Slope change observed around 410 K is attributed as the onset of the thermal decomcoposition as evidenced by TGA curve. TGA studies show the crystal is very stable up to 440 K. Activation energies for the conduction process are calculated for all measured crystallographic directions. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth,composition, ferroelectric and magnetic properties of new multiferroic Pb3.3Mn4.8Ni1.1Ti0.56O15.3 single crystals

Multiferroic single crystals in the novel system Pb‐Mn‐Ni‐Ti‐O have been grown by the high temperature solution growth method. At room temperature the crystals are indexed in the hexagonal space group P6₃cm. The dielectric and magnetic properties along with the temperature dependence of the c‐lattice parameter have been studied in the temperature range 2 K ‐ 500 К. The magnetic measurements reveal a paramagnetic to antiferromagnetic phase transition around 48 K. The dielectric permittivity exhibits a maximum at 430 K, indicating ferroelectric to paraelectric phase transition. The temperature dependent Raman and XRD measurements around 430 K reveal an anomaly and abrupt change of the lattice parameter along the z‐axis respectively, thus confirming the ferroelectric‐to‐paraelectric phase transition.     

Thermal and Nonlinear Optical Properties of Ca4YO(BO3)3

Ca₄YO(BO₃)₃ (YCOB) crystals have been grown using the vertical Bridgman method. The thermal properties of YCOB were measured for the first time to our knowledge. The specific heat is 729.7 J/kg K at 373K. The average thermal expansion coefficients along the a, b and c axes are 9.9 × 10^‐6 /K, 8.2 × 10^‐6 /K and 12.8 × 10^‐6 /K, respectively, in the temperature range of 293‐1173 K. The thermal conductivities along the a, b and c axes are 1.83 W/mK, 1.72 W/mK and 2.17 W/mK at 373 K. The anisotropy in the measured thermal conductivities of YCOB is consistent with the experimental results of the thermal expansion. The SHG of a Nd: YAG laser was compared with that of a KDP crystal. The effective nonlinear coefficients (d_eff) of YCOB in type I phase matching directions of (θ, ϕ) = (66.3°, 143.5°) and (65.9°, 36.5°) were estimated to be 1.45 pm/V and 0.91 pm/V, respectively. The bulk damage threshold was observed as 85 GW/cm² for single pulse of a Nd:YAG laser with 10 ns pulseduration.     

Transport Properties of (NH4)2 CuCl4.2H2O Low Dimensional Single Crystals

The X‐ray diffraction and Infrared (IR) spectral studies of (NH₄)₂ CuCl₄.2H₂O single crystals reveals that these crystals contains tetragonal crystal structure with the unit cell dimensions of a = 7.58Å, c = 7.95Å, z= 2, β =90° and two water molecules in the unit cell. The temperature dependence of thermally stimulated depolarization current (TSDC) and dc electrical conductivity (σ) studies of this two‐dimensional (NH₄)₂ CuCl₄.2H₂O single crystal have been carried out in 77K–300K temperature region. The TSDC thermograph shows only one sharp peak at 248K with a peak current of 130nA, which is attributed to the Maxwell‐Wagner peak. The activation energy (U), relaxation time (τ) are calculated as 0.78eV and 3.44×10^‐15 s respectively. Dc electrical conductivity studies of these crystals show a first order phase transition at about 248K.     

Thermal conductivity of Tb2(MoO4)3 crystals at low temperatures

Thermal conductivity of the improper ferroic Tb₂(MoO₄)₃ was measured in the range 0.3 K–100 K by the method of the stationary heat flow. An additional maximum in thermal conductivity observed at 0.45 K is due to the anomaly of specific heat corresponding to the antiferromagnetic phase transition. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ferroelectric and ferroelastic properties of Rb2MnCl4·2H2O

Rb₂MnCl₄·2H₂O crystal belonging to the family of tetrahalogeno‐metallates dihydrates has been subjected to thermal, optical and dielectric studies. At room temperature the optical study under a polarization microscope has revealed a ferroelastic domain structure. At 240 K a dielectric anomaly, characteristic of the week ferroelectric phase transition, has been also observed. Moreover, below the temperatures of the phase transitions dielectric dispersion has appeared. Ferroic properties of Rb₂MnCl₄·2H₂O crystal have been compared to those of other tetrachlorides dehydrates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Superconducting Transition of (TMTSF)2CLO4 in Magnetic Fields

Abstract

Superconducting transition of (TMTSF)₂ClO₄ was studied by conductivity measurements along the most conductive a-axis in magnetic fields applied along three different principal crystallographic axes. The GL coherence lengths at OK along the a-, b-, and c*-axes, ?_a(0)≈ 600A, ?(0) ≈ 540A, and ?_c*(0) ≈ 60A were obtained from the measurements of the temperature dependence of the upper critical field H_C2 near the transition temperature. The anisotropy is discussed in terms of the dimensionality arising from the crystal structure and of the conductivity anisotropy in the normal state.     

Synthesis,characterization and impedance spectroscopy of the new material [(CH3) (C6H5) 3P] 2CoBr4: a member of the A2BX4 family

The crystal structure of bis‐(methyltriphenylphosphonium) tetrabromocobaltate (II), [(C₁₉H₁₈P)₂ CoBr₄] is determined: M_r = 933.203, monoclinic, P2₁, a = 9. 6977 (3) Å, b = 12.5547 (4)Å, c = 16.4503 (6)Å, β = 105.603 (2)°, V = 1929.04 (11)ų, Z = 2, D_x = 1.607 Mg m^‐3, T = 298 K. Differential thermal analysis at high temperatures shows three endothermic peaks characterizing four phases, with onset temperatures at T₁= 313±2 K, T₂ = 320±4 K and T₃= 360±1 K. The structural instability detected via the temperature dependence of permittivity at T₁ is ascribed to order‐disorder transition associated with cation dipole reorientation. Permittivity and ac conductivity studies as a function of temperature (295 K‐375 K) and frequency (0.11 kHz < f <100 kHz) are presented. The results indicate the importance of the cation size and shape on the phase transitions in the system. Bulk conductivity behavior is thermally activated. The associated activation energies are in the range 2.9 to 1.0 eV depending on the temperature regime. Two contributions to the ac conductivity, one dominating at low temperatures and high frequencies which are characterized by superlinear frequency exponent and the second dominates at high temperatures characterized by a sublinear frequency exponent. The behavior is interpreted in terms of the jump relaxation model. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New technique for crystal growth and Phase Transition in TlInS2 Crystals

New technique for crystal growth was designed in the present work. The electrical conductivity and Hall effect of the TInS₂ single crystals, grown by this technique, were measured in the temperature range 150–500 K. From the measurements, the conductivity type, the energy gap, ionization energy of the impurity level, Hall mobility and carrier concentration were determined. Also the measurements revealed a presence of phase transition in the crystal at 189 and 220 K.     

Some special dielectric characteristics of TlGaSe2 layered crystals

TlGaSe₂ layered crystals were characterized with dielectric spectroscopy measurements. We have studied the memory effect within soliton regime, time relaxation in annealing temperature, and the effect of different cooling rate on conductivity and dielectric constant. The effects connected with the annealing time and cooling rate processes may be explained the theory of DDW. The measurements were carried out in a narrow temperature region (95‐135 K). The temperature dependence of dielectric constant measurements show that the crystal reveals a weak dielectric anomaly at 101 K. A well‐defined ferroelectric behaviour of first order transition was observed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of Inclusion Free KTP Crystals by Top Seeded Solution Growth and their Characterization

KTiOPO₄ single crystals of dimensions 34 × 15 × 15 mm³ have successfully been grown by Top Seeded Solution Growth (TSSG), using phosphate flux (K₆P₄O₁₃), employing a home made vertical cylindrical three zone resistive heated furnace. Growth experiments were carried out with <001> seed orientation. A method to grow inclusion free crystal with less dislocation density is proposed, by employing cooling rate of 0.5‐2 K/day and rotation rate of 60‐30 rpm. UV‐VIS transmission studies were performed and the dependence of absorption coefficient with wavelength is discussed. SHG elements of dimension 8 × 7.5 × 7.5 mm³ were fabricated and conversion efficiency of 58 % was achieved without antireflection coating. Nature of ionic conductivity and ferroelectric phase transition behavior of KTP crystals along the crystallographic polar axis in the frequency range of 100 Hz – 10 MHz are discussed.     

Structural phase transitions in the LaMnO3+λ system

Structural phase transitions in the LaMnO_3+λ system are studied at various temperatures and with different Mn⁴⁺ ion concentrations. Two structural phase transitions are established. The O′-orthorhombic – O-orthorhombic transition (O′ O transition) is due to removing the cooperative Jahn-Teller distortions and occurs in the Mn⁴⁺ concentration range from 10 to 14% at room temperature. The temperature of this transition varies from 710 K for the stoichiometric LaMnO₃ to room temperature for LaMnO_3.07. The O-orthorhombic – rhombohedral transition (O R transition) is due to the change of the rotation axis of the undistorted MnO₆ octahedra from [100] and [110] to [11 1]. The temperature of this transition varies from 1010 K for LaMnO₃ to 230 K for LaMnO_3.135. The structural phase diagram of temperature vs composition is plotted, indicating the existence of regions of the phase with different structures.     

A neutron diffraction study of the phase transition of fully deuterated triglycine sulphate (ND2CD2COOD)3.D2SO4

Using neutron single crystal and powder diffraction, the first thorough investigation of the structure of fully deuterated triglycine sulphate, (ND₂CD₂COOD)₃.D₂SO₄ is presented, including its evolution with T, through its structural phase transition. This includes new precise structural parameters determined at several key temperatures above and below T_C using single crystal diffraction, and for the first time a parametric study has been undertaken over a wide temperature range — from 4 to 500 K in 2 K steps. It was found that fully deuterated TGS shows a structure consistent with hydrogenous TGS and partially deuterated TGS. The evolution of several key hydrogen bond lengths suggests that weakening of the H‐bond network with T is crucial in decoupling the polarising glycine molecules from the other glycines and allowing the long‐range ferroelectric order to break down. A new parameterisation of the phase transition is demonstrated. Contrary to results of physical properties measurements, there is no evidence of a second low temperature phase transition in TGS – no low temperature anomalies were observed in the crystal structure.     

Strontium-based Mercury Cuprates with Perovskite-like Crystal Structures

Conditions of synthesis, structure, and temperatures of superconducting transitions of strontium-based mercury cuprates Hg—Sr—Me—Cu—O (Me = Ca, La, Ce, Sm, Ho) with perovskite-like crystal structure are described. The samples belong to two homological series: HgSr₂ (Ln, Ce)_n–1CunO_2n + _{2 + δ} with the Hg—O bond directed along <110> and (Hg, Cu) Sr₂Ln_n–1Cu_nO_{2n + 2 + δ} with the (Hg, Cu)—O bond directed along <100>. The variation of composition leads to rotation of the Hg—O bond along the hk0 direction and consequently to the increase of T_c from 28 K to 81 K. An anomaly of the resistivity was observed around 200 K for most of the investigated samples, independent of their content and phase composition.     

Dielectric constant,loss factor and ac conductivity of Ni2+‐doped K2ZnCl4 crystals in the ferroelectric‐commensurate,incommensurate and normal phases

The dielectric constant (ϵ), dielectric loss (tanδ) and ac conductivity (σ_ac) of virgin and thermally cycled undoped and Ni²⁺‐doped K₂ZnCl₄ (KZC) crystals in the ferroelectric‐commensurate (FC), incommensurate (IC) and normal phases (N) have been studied. Anomalous behaviour at the two‐phase transition points was observed while measuring ϵ along the polar a‐axis for the undoped sample. With increasing Ni²⁺concentration a systematic shift of the phase transition temperature towards lower values and a continuous inhibition of the peak height were detected. ϵ changed linearly with lnf up to f =10⁵ Hz. tanδ along the a‐axis declared the phase transitions by peak changes. After Ni²⁺‐doping this behaviour was preserved at the FC‐IC phase transition point while the IC‐N phase transition was manifested by a change in the slope of the straight line representing the tanδ‐T dependence. The ac conductivity changed lineally with frequency according to a relation of the form σ_ac = σ_o f ^β where 0>β>1.9. σ_ac increased monotonically with increasing temperature and doping concentration in the low‐temperature phases tending to merge in one straight‐line with high activation energy that might be due to superionic dc conduction in the high temperature N‐phase. Doping with Ni²⁺ pinned stripples, decreased the soliton‐soliton interaction, weakened discommensuration effects, shortened the IC‐phase and strongly affected the ‘chaotic' behaviour at the T_C‐IC. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Unusual Glassy Smectic-G Liquid Crystal: Heat Capacity of N-p-n-Pentyloxybenzylidene-p'-n-butylaniline between 11 and 393 K

Abstract

The heat capacities of the title compound (C₃H₁₁,O—C₆H₄,- CH=N—C₆H₄,—C₄H₉, abbreviation 5O ? 4) with a purity of 99.92 mole percent have been measured with an adiabatic-type calorimeter between 11 and 393 K. The transition temperature and the enthalpy and entropy of phase transition for stable crystal → S_G, S_G → N and N → isotropic liquid were T _c = 299.69 K/ΔH = 22.68 kJ mol^?1/ΔS = 75.70 JK^?1 mol^?1, 325.72/7.11/21.79 and 342.48/1.78/5.22, respectively. The crystal which melts at 285.5 K is a metastable modification. The S_A phase hitherto reported in between S_G and N does not exist. The glassy So state was realized by rapid cooling of the specimen from the So phase. The molar enthalpy of the glassy S_G state at 0 K was by (10.1±0.1) kJ mol^?1 higher than that of the stable crystalline state and the residual entropy of the glassy state was (9.40±0.83) JK^?1 mol^?1. The relaxational heat-capacity anomaly was observed from as low as 100 K and double glass transition phenomenon occurred around 200 K; a quite unusual phenomenon which has never been observed for the glassy states of nematic and cholesteric liquid crystals. The present results give a fair evidence that the unusual glass transition phenomenon previously found for the S_G state of 6O?4 (a homologous compound) is not exceptional at all but common to the smectic glasses; at least common to the glassy S_G states. Two possible origins responsible for the double glass transitions have been discussed.     

Electrical properties of PbxSn1‐xS thin films prepared by hot wall deposition method

The Pb_xSn_1‐xS (x = 0 – 0.25) thin films were prepared on glass substrates by hot wall vacuum deposition. The films were polycrystalline monophase in nature and had orthorhombic crystal structure. The thickness of the films was about 2‐3 μm. The temperature dependences of the conductivity were measured in the temperature range from 150 to 420 K. The films revealed p‐type of conductivity. The Seebeck coefficient and conductivity values of the films was in the range of α = 6 – 360 μV/K and σ = 4.8×10^‐5 – 1.5×10^‐2 Ω^‐1·cm^‐1, respectively, at room temperature depending on concentration of the lead in the films. The lead atoms created the substitution defects Pb_Sn in the crystal lattice of the Pb_xSn_1‐xS. These defects formed the donor energy levels in the band gap. The activation energy of the films increased in the range ΔE_a = 0.121 – 0.283 eV with increasing of the lead concentration. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High‐quality LaCoO3 single crystal grown by optical floating zone method and its electromagnetic properties

A LaCoO₃ single crystal with 4 mm in diameter and 30 mm in length has been grown by optical floating zone method. The as‐grown crystal is highly crystalline with the rhombohedral perovskite structure (R3c) and grows parallel to the (121) direction. The room temperature resistivity of the as‐grown crystal is 0.12 Ω·cm and the insulator‐metal transition occurs around 500 K. The coercivity and the remanence of the as‐grown crystal are 5 Oe and 6.61×10^–5 μ_B/f.u. at 5 K, respectively. In 1000 Oe under zero‐field cooling, the magnetic susceptibility of the as‐grown crystal shows an upturn in a Curie tail fashion below 35 K, and appears a wave crest over the interval 55 K≤T≤90 K. In addition, a slope change of 1/χ(T) at about 12 K is observed in 50000 Oe under zero‐field cooling. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectroscopic and Transport Properties of (NH4)2CuCl4 · 2 H2O Single Crystal

Electron paramagnetic resonance (EPR), Optical absorption and Transport properties of (NH₄)₂CuCl₄ 2 H₂O single crystals have been studied. An anisotropic g tensor was observed with gl = 2.241 and g∥ = 2.081 by EPR method. The spin orbit coupling constant is found to be 540 cm⁻¹. The optical absorption in UV region are characterized by charge transfer band, in the visible and near infrared region at 13,333, 4,480, 4,336, and 3,998 cm⁻¹ attributed to the transitions between the (d-d) stark energy levels of the copper (II) ion in an extended octahedral crystal field. Dc electrical conductivity measurements with temperature reveal an anisotropic characteristic of a two-dimensional layered structure, and exhibits two first order structural phase transitions at about 383 K and 413 K. These transitions are attributed to loss of the two water inolecules of hydration and free rotation of NH⁺₄ ion from a state of torsional oscillation.