Thermally stimulated current observation of trapping centers in an undoped Tl4Ga3InSe8 layered single crystal

Thermally stimulated current measurements are carried out on as‐grown Tl₄Ga₃InSe₈ layered single crystal in the temperature range 10–160 K with different heating rates. Experimental evidence is found for two shallow trapping centers in Tl₄Ga₃InSe₈. They are located at 17 and 27 meV. The trap parameters have been determined by various methods of analysis, and they agree well with each other. It is concluded that in these centers retrapping is negligible, as confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies of EPR g factors for tetragonal and rhombic Cr5+ centers in ferroelectric PbTiO3

The anisotropic g factors (g_x, g_y, g_z) at low temperature and the motionally averaged isotropic g factor at high temperature for two Cr⁵⁺ centers, the tetragonal t‐Cr⁵⁺ center and the rhombic O‐Cr⁵⁺ center, in ferroelectric PbTiO₃ are calculated from the high‐order perturbation formulas based on a two‐mechanism model. In the model, the contributions to g factors from both the crystal‐field (CF) mechanism concerning the CF excited states and the charge‐transfer (CT) mechanism (which is neglected in CF theory) concerning the CT excited states are contained. The calculated results are in reasonable agreement with the experimental values. From the calculations, the defect models of t‐Cr⁵⁺ center (which is attributed to Cr⁵⁺ in the tetragonally‐compressed octahedron caused by Jahn‐Teller effect rather than in the tetragonally‐elongated octahedron in the host PbTiO₃) and O‐Cr⁵⁺ center (which is due to the t‐Cr⁵⁺ center perturbated by the electrical polarization perpendicular to the C₄ axis) are confirmed, and the defect structure of t‐Cr⁵⁺ center is obtained. It is found that in the precise calculations of g factors for the high valence state d¹ ions in crystals, both the contributions due to CF and CT mechanisms should be taken into account. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time‐dependent dielectric behaviour in incommensurate TlInS2 crystal

The results of measurements of temperature and time dependencies of the dielectric constant of TlInS₂ in the temperature interval of successive incommensurate and commensurate phase transitions have been presented. Considerable decrease of the dielectric constant in a temperature interval including successive phase transitions was observed after annealing the sample at a fixed temperature in the ferroelectric state. Additionally, it has been revealed that the time dependencies of the real part of dielectric susceptibility and dielectric losses angle tangent of TlInS₂ crystal in the incommensurate phase exhibits an unusual behaviour, which is accompanied by exponential decreasing of the dielectric constant and increasing of dielectric losses angle tangent by time. The observed effects are qualitatively explained using defect‐density‐wave model and phenomenological theory of incommensurate systems containing discommensurations undergoing nucleation and evolution processes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and crystal structure of the layered compound TlGaSe2

The semiconducting compound TlGaSe₂ was grown by solid state reaction technique. The crystal structure of this material was confirmed by single‐crystal X‐ray diffraction. TlGaSe₂ crystallizes in the monoclinic system with space group C2/c (No. 15), Z = 16 and unit cell parameters a = 10.779(2) Å, b = 10.776(1) Å, c = 15.663(5) Å, β = 99.993(6)°. The structural refinement converged to R(F) = 0.0719, R(F²) = 0.0652 and S = 1.17. The structure consists of a three‐dimensional arrangement of distorted TlSe₈ and GaSe₄ polyhedrons. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

RbCdF3:Ni2+晶体结构相变、光谱精细结构和EPR谱的研究

本文采用半自洽场(semi-SCF)3d轨道模型和μ-κ-α模型,利用完全对角化方法,建立了Oh、D4h对称晶体场局部结构参数与光谱精细结构、EPR谱之间的定量关系,统一解释了RbCdF3∶Ni2+晶体的光谱精细结构和EPR谱。所得理论结果与实验值符合很好。     

Bias field induced impedance relaxation of TlGaSe2 semiconducting layered crystal at room temperature

Complex Impedance Spectroscopy has been employed for the study of absorption kinetics at TlGaSe₂ layered crystal under Bias voltage at room temperature. Two relaxations mechanisms have been observed. The crystal has “slow” and “fast” relaxation mechanisms in low and high frequency region, respectively. The complex impedance spectra were fitted by the superposition of two Cole‐Cole types of relaxations. The fast relaxation in the higher frequency region may be due to dipolar relaxation and the slow relaxation in the low frequency region may be due to Maxwell‐Wagner type relaxation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure,dielectric properties of (K0.5Na0.5)NbO3 single crystal grown by flux method using B2O3 flux

Lead free piezoelectric single crystals of sodium potassium niobate (K_0.5Na_0.5)NbO₃ (KNN) were grown by high‐temperature solution method using two different fluxes; one with a mixture of NaF and KF and other with addition of B₂O₃ along with the mixture. In the present study, the growth of KNN crystals without B₂O₃ flux and the same with B₂O₃ flux were compared. It was found that additions of small amounts of B₂O₃ lowered the melting temperature of the solid solution and enabled better dielectric properties. Phase analysis showed that all samples were crystallized in pure orthorhombic perovskite phase. AFM morphological studies showed that the addition of B₂O₃ flux increased the roughness of the grown crystal. Further, addition of B₂O₃ flux slightly decreased the orthorhombic to tetragonal phase transition temperature T_(O—T) and the Curie temperature T_C. The ferroelectric behaviour of KNN single crystal has been investigated at room temperature. The crystal grown using B₂O₃ flux exhibited a remanent polarization (P_r) ∼ 32 μC/cm² and coercive field (Ec) of ∼11.8 kV/cm whereas the crystal grown without the use of B₂O₃ flux had a remanent polarization (P_r) ∼ 36 μC/cm² and coercive field (Ec) of ∼14.6 kV/cm.     

Dielectric properties of thallium gallium diselenide layered crystal in the incommensurate phase

The dielectric measurements of the layered crystal were studied in temperature range of successive phase transitions. The measurements revealed that the phase transition occurred in 242 K is an incommensurate phase transition. When the sample is annealed at a stabilized temperature in the incommensurate phase, a remarkable memory effect has been observed on cooling run. The mechanism of the memory effect in the incommensurate phase of the semiconducting ferroelectric TlGaSe₂ can be interpreted in the frame of the theory of defect density waves. This theory claims that the memory effect is the result of pinning of the incommensurate structure by the lattice inhomogeneities. With decreasing the annealing temperature the phase transition temperature shifts to lower temperatures gradually. Moreover, the peak intensities also increase gradually. In addition to these effects, the phase transition temperature shifts to lower temperatures with increasing annealing time. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of CuInS2 single crystals from Sb2S3 and Bi2S3 solutions

The phase diagrams of the CuInS₂‐Sb₂S₃ and CuInS₂‐Bi₂S₃ systems were investigated using X‐ray powder diffraction and differential thermal analysis. Based on these results, the compositions for the growth of the CuInS₂ single crystals from CuInS₂‐Sb₂S₃ and CuInS₂‐Bi₂S₃ melts were selected and Bridgman crystal growth process was performed. The investigation of the obtained single crystals using X‐ray powder diffraction and optical absorption spectra indicates that the incorporation of the solvent atoms into the crystal lattice is absent. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron paramagnetic resonance of Gd3+ in Eu2Zn3(NO3)12·24H2O single crystals

Electron paramagnetic resonance of Gd³⁺ in Eu₂Zn₃(NO₃)₁₂·24H₂O single crystals is studied at ≈9.45 GHz and at 298 and 77 K. Gd³⁺ substitutes for the Eu³⁺ site. In addition to the allowed fine structure lines, forbidden transitions (ΔM = ±2,±3,±4,±5) are observed. The superposition model is applied to the zero‐field splitting parameter b₂⁰. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical and dielectric studies on pure and Ni2+, Co2+ doped single crystals of bis thiourea cadmium chloride

Good quality single crystals of Ni²⁺, Co²⁺ ions doped Bisthiourea Cadmium Chloride (BTCC) are some of the excellent and efficient non‐linear optical materials grown from aqueous solution by slow evaporation method. The lattice parameters of the grown crystals are determined by single crystal X‐ray analysis. UV spectral analyses on these samples reveal the improved transparency of the doped crystals ascertaining the inclusion of metal ion in the lattice. FTIR spectral analysis carried out on the materials confirm the presence of functional groups. Dielectric measurements reveal that the dielectric constant of pure and doped crystals decreases with increase of frequency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

YAG:V2+激光材料晶格畸变及其EPR参量研究

利用Newman的晶场叠加模型,建立了晶体微观结构与电子顺磁共振(EPR)参量之间的定量关系.采用全组态完全对角化方法,对YAG:V2+晶体的局域晶格畸变及其EPR参量进行了系统的研究,结果表明:V2+离子掺入YAG晶体后,V2+离子的局域结构产生压缩三角晶格畸变,沿[111]晶轴方向V2+离子上方的三个O2-配体与下方的三个O2-配体均偏离[111]轴1.96°,从而成功地解释了YAG:V2+晶体的EPR参量.同时研究也表明,SS与SOO磁相互作用对EPR参量的贡献不可忽略.     

Facile solvothermal synthesis,growth mechanism and thermoelectric property of flower‐like Bi2Te3

Hierarchical flower‐like Bi₂Te₃ was synthesized through a facile solvothermal method. The crystal structure and morphology of the as‐prepared samples were characterized by X‐ray diffraction (XRD), filed emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high resolution TEM. The reaction parameters such as reaction time, the amount of glucose, concentration of NaOH and the reaction temperature were systematically investigated. Based on the FESEM observations, a possible mechanism defined as a self‐assembly process accompanied by anisotropic growth mechanism was proposed. Moreover, the thermoelectric properties were measured at the temperature range of 300–600 K. The hierarchical flower‐like Bi₂Te₃ presented good thermoelectrical properties. The maximum ZT value reached up to 0.6 at 600 K, which was higher than that of Bi₂Te₃ nanoparticles.     

Investigations of spin‐Hamiltonian parameters and defect structures for two tetragonal Cu2+ centers in KTaO3 crystal

Based on the defect models that the two tetragonal Cu²⁺ centers in KTaO₃ are due to Cu²⁺ ions at Ta⁵⁺ sites associated, respectively, with one and two oxygen vacancies (V₀) along C₄ axis because of charge compensation, the spin‐Hamiltonian parameters (g factors g_‖, g_⊥and hyperfine structure constants A_‖, A_⊥) of both Cu²⁺ centers in KTaO₃ are calculated from the perturbation theory method (PTM) and the complete diagonalization (of energy matrix) method (CDM). The calculated results from both theoretical methods are not only close to each other, but also in reasonable agreement with the experimental values. This suggests that both methods are effective in the explanations of spin‐Hamiltonian parameters for 3d⁹ ions in tetragonal symmetry. From the calculations, the defect models are confirmed and the defect structural data are obtained for both Cu²⁺ centers in KTaO₃. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermoelectric properties of Tl‐doped Bi2Se3 single crystals

The single crystals of the ternary system based on Bi_2‐xTl_xSe₃ (nominaly x = 0.0‐0.1) were prepared using the Bridgman technique. Samples with varying content of Tl were characterized by the measurement of lattice parameters, electrical conductivity σ_⊥c , Hall coefficient R_H (B∥c), and Seebeck coefficient S (ΔT ⊥c). The measurements indicate that by incorporating Tl in Bi₂Se₃ one lowers the concentration of free electrons and enhances their mobility. This effect is explained in terms of the point defects in the crystal lattice – formation of substitutional defects thallium on the site of bismuth Tl_Bi and the decrease of concentration of selenium vacancies V_Se⁺². We also discuss the temperature dependence of the power factor σS² of the samples. Upon the thallium doping we observe a significant increase of the power factor compare to the parental Bi₂Se₃. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical properties of Sr3TaGa3Si2O14 single crystal

The specific rotation ρ of strontium tantalum gallium silicate Sr₃TaGa₃Si₂O₁₄ (STGS) piezoelectric single crystal was determined from 350 to 850 nm by measuring the optical transmission between parallel polarisers in Z direction. It is shown that Sr₃TaGa₃Si₂O₁₄ has quite large a value of ρ which is a little smaller than that of strontium niobium gallium silicate Sr₃NbGa₃Si₂O₁₄ (SNGS). The crystal with ordered structure which is isostructural to calcium gallium germanate Ca₃Ga₂Ge₄O₁₄ (CGG) was grown by Czochraiski technique. And its birefringence was also determined. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low‐temperature urea‐assisted hydrothermal synthesis of Bi2S3 nanostructures with different morphologies

Different morphologies of single‐crystalline orthorhombic phase bismuth sulfide (Bi₂S₃) nanostructures, including sub‐microtubes, nanoflowers and nanorods were synthesized by a urea‐assisted hydrothermal method at a low temperature below 120 °C for 12 h. The as‐synthesized powders were characterized by X‐ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high‐resolution transmission electron microscopy (HRTEM) and UV‐vis spectrophotometry. The experimental results showed that the sulfur sources had a great effect on the morphology and size of the resulting powders. The formation mechanism of the Bi₂S₃ nanostructures with different morphologies was discussed. All Bi₂S₃ nanostructures showed an appearance of blue shift relative to the bulk orthorhombic Bi₂S₃, which might be ascribed to the quantum size effect of the final products. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

EPR spectra of (L-aspartate)di(2-pyridyl)aminezinc(II) hydrate substitutionally copper(II) doped complex

The [(L-aspartate)di(2-pyridyl)aminezinc(II)]hydrate doped with traces of copper was prepared. Electron paramagnetic resonance spectra of the polycrystalline sample were recorded at room temperature by using the X-band of the EPR spectrometer. The observedg factors for the complex areg _‖=2.258 andg _⊥=2.066. TheA _‖ value is 165.10⁻⁴ cm⁻¹. The crystal and molecular structure of the complex are also reported. The complex crystallizes in the monoclinic system; space groupP2₁ witha=7.2053(11),b=10.447(2),c=10.812(2) Å, β=108.489(14)° andZ=2. The structure can be described in the crystal lattice as polymeric chains.     

EPR spectra of (L-aspartate)di(2-pyridyl)aminezinc(II) hydrate substitutionally copper(II) doped complex

The [(L-aspartate)di(2-pyridyl)aminezinc(II)]hydrate doped with traces of copper was prepared. Electron paramagnetic resonance spectra of the polycrystalline sample were recorded at room temperature by using the X-band of the EPR spectrometer. The observedg factors for the complex areg _‖=2.258 andg _⊥=2.066. TheA _‖ value is 165.10⁻⁴ cm⁻¹. The crystal and molecular structure of the complex are also reported. The complex crystallizes in the monoclinic system; space groupP2₁ witha=7.2053(11),b=10.447(2),c=10.812(2) Å, β=108.489(14)° andZ=2. The structure can be described in the crystal lattice as polymeric chains.     

Single crystal EPR study of Mn(II) doped MgRb2(SO4)2·6H2O: A case of interstitial site

Single crystal electron paramagnetic resonance studies on magnesium rubidium sulphate hexahydrate doped with Mn(II) impurity, in the form of 0.1% of manganous chloride, are carried out at room temperature. The angular variation spectra of the crystal in the three orthogonal planes show that the paramagnetic impurity Mn(II) has entered the host lattice interstitially. The spin Hamiltonian parameters calculated are: g_xx = 1.974, g_yy = 1.928, g_zz = 1.980; A_xx = 9.28, A_yy = 8.58, A_zz = 9.87 mT and D_xx = ‐18.71, D_yy = ‐5.59, D_zz = 24.30 mT. The room temperature and liquid nitrogen temperature study for polycrystalline sample indicate no phase transition. The percentage covalency of Mn–O bond has been estimated to be 11. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)