Synthesis, crystal structure and blue electroluminescence of a new zinc complex based on 2,6-bis(benzimidazolyl)pyridine

A very stable and blue luminescent complex Zn(bbp)Cl₂ (bbp: 2,6-bis(benzimidazolyl)pyridine) was synthesized. X-ray crystal structural analysis for the complex revealed that there are intermolecular π?π interactions in the solid state. The fluorescence properties for this complex were studied. The similar devices with the structure of [ITO/CuPc(31 nm)/NPB(80 nm)/[Zn(bbp)Cl₂] (or L) (85 nm)/LiF15/Al] were constructed to investigate their electroluminescent performance. Both the complex and the ligand can be fabricated as blue-emitting materials. The complex shows emission peak at 555 nm, electroluminescent efficiency 0.017 cd A⁻¹ and turn-on voltage 7 V, compared to 470 nm, 0.036 cd A⁻¹ and 9 V for the ligand.     

Relationship between average and local crystal structure and the ferroelectric properties of a Sr-Bi-Ta-Si-O ferroelectric material

We investigated the relationship between the average and local crystal structures and the ferroelectric properties of Bi₂SiO₅, Bi₄Si₃O₁₂, or Bi₂O₃ added Sr_1−xBi_2+xTa₂O₉ (x=0, 0.2) produced by a solid-state reaction. By measuring the P-E hysteresis, we found that Sr_1−xBi_2+xTa₂O₉ (x=0, 0.2) has higher P_r and E_c than SrBi₂Ta₂O₉ (SBT). P_r increased and E_c decreased by adding Bi₂SiO₅, Bi₄Si₃O₁₂, or Bi₂O₃ to Sr_1−xBi_2+xTa₂O₉. The average crystal structures were determined by the Rietveld method. On the other hand, the local structure is important, because the ferroelectric property is related to the distortion, and ferroelectric complex oxides have domains. We also investigated the local crystal structure using atomic pair distribution function (PDF) analysis. Based on the results, the bond angle variance, σ², of each TaO₆ octahedron increased by substituting Si for the Ta site. The tilting angle, α_a,α_b, of each TaO₆ octahedron increased relative to that of the average structure, and the symmetry of the TaO₆ octahedron in the local structure deteriorated in comparison to that of the average structure. This distortion and symmetry of TaO₆ contributes to the remanent polarization.     

CO adsorption on oxygen-modified molybdenum surfaces

Structures of carbon monoxide layers on the oxygen-modified Mo(1 1 0) and Mo(1 1 2) surfaces have been investigated by means of density-functional (DFT) calculations. It is found that CO molecules adsorb at hollow sites on the O/Mo(1 1 0) surface and nearly atop Mo atoms on the O/Mo(1 1 2) surface. The favorable positions for adsorption are shown to be near protrusions of electron density above the Mo surface atoms. The presence of oxygen on the molybdenum surface significantly reduces the binding energy of the CO molecule with the substrate; on the oxygen-saturated Mo(1 1 0) surface, the adsorption of CO is completely blocked. The calculated local densities of states (LDOS) demonstrate that the O 2s peak for O adsorbed on Mo(1 1 0) surface is at −19 eV (with respect to the Fermi level), while for the oxygen atom of an adsorbed CO molecule the related 3σ molecular orbital gives rise to a peak at −23 eV. This difference stems from the bonding of the O atom either with Mo surface for adsorbed O or with C atom in adsorbed CO, and therefore the position of the O 2s peak in photoemission spectra can serve as a convincing argument in favor of either the presence or absence of the CO dissociation on Mo surfaces.     

Characterization of electrodeposited nickel film surfaces using atomic force microscopy

Microstructures of nickel surfaces electrodeposited on indium tin oxides coated glasses are investigated using atomic force microscopy. The fractal dimension D and Hurst exponent H of the nickel surface images are determined from a frequency analysis method proposed by Aguilar et al. [J. Microsc. 172 (1993) 233] and from Hurst rescaled range analysis. The two methods are found to give the same value of the fractal dimension D∼2.0. The roughness exponent α and growth exponent β that characterize scaling behaviors of the surface growth in electrodeposition are calculated using the height-difference correlation function and interface width in Fourier space. The exponents of α∼1.0 and β∼0.8 show that the surface growth does not belong to the universality classes theoretically predicted by statistical growth models.     

Conical parametric scattering of a single extraordinary beam in BaTiO3 crystal

In this paper, the parametric scattering of a single extraordinary polarized beam of laser in BaTiO₃ photorefractive crystal has been investigated experimentally and theoretically. The resulting pattern consists of beam fanning, isotropic ring, and anisotropic one. Among all parts of scattering pattern, isotropic ring has not been studied as much as beam fanning and anisotropic ring, and there still are some differences in reports about it. Therefore, the study has mainly focused on this part. In this experimental configuration, isotropic ring is just visible in positive angles although the other parts of parametric scattering pattern can be visible from behind and in front of the crystal. In addition to steady state pattern in forward and backward directions, its transient behavior with the rotation of crystal has been studied. The results of experiments have been analyzed carefully, and their theoretical explanations have been presented based on the standard theory of parametric scattering in photorefractive crystals. It has been shown that this configuration corresponds to the so called parametric B-process scattering.     

YB48 the metal rich boundary of YB66; crystal growth and thermoelectric properties

It was discovered that the well-known higher boride YB₆₆, one of the first reported phonon glass electron crystals (PGEC), could be obtained in a much more metal-rich composition than previously thought possible. Using the floating zone growth method, YB₄₈ single crystals with YB₆₆ crystal structure could be obtained, and their thermoelectric properties measured. This expansion of the homogeneity range of the well-known YB₆₆ compound is surprising and a new Y atomic site was discovered. YB₄₈ exhibits much higher power factors than YB₆₆ which increase rapidly with increasing temperature. The obtained dimensionless figure of merit of this compound at 990 K is approximately 30 times higher than that of previously reported YB₆₆ samples, and higher than any other pristine higher boride. This discovery reveals YB₄₈ as a promising high temperature thermoelectric material.     

Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films

Ion beam sputtering process was used to deposit n-type fine-grained Bi₂Te₃ thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 °C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 °C. Thermoelectric properties of Bi₂Te₃ thin films were investigated at room temperature. The Bi₂Te₃ thin films, including as-deposited, exhibit the Seebeck coefficients of −90 to −168 μV K⁻¹ and the electrical conductivities of 3.92×10²-7.20×10² S cm⁻¹ after annealing. The Bi₂Te₃ film with a maximum power factor of 1.10×10⁻³ Wm⁻¹ K⁻² is achieved when annealed at 300 °C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties.     

Crystal engineering in two- and three-dimensional systems based on cyanomolybdates: structures, magnetism and intercalation properties

A design of infinite two- and three-dimensional cyano-bridged networks with useful properties has attracted a great deal of attention in contemporary science. A proposal described some investigations in crystal engineering, performed over the past few years on supramolecular coordination polymers based on [Mo(CN)_n]⁴⁻ (n=7,8) building blocks and on their magnetic and intercalation properties.     

Synthesis, crystal structure and photoconductivity of new molecular complex of C60 with tetrabenzo(1,2-bis[4H-thiopyran-4-ylidene]ethene): Bz4BTPE C60

A new molecular complex of C₆₀ with tetrabenzo(1,2-bis[4H-thiopyran-4-ylidene]ethene), Bz₄BTPE C₆₀ (1) has been obtained. The complex has a layered structure in which closely packed hexagonal layers of C₆₀ alternate with the layers composed of Bz₄BTPE molecules. The complex has a neutral ground state according to UV-vis-NIR spectrum. It has been found that single crystals of 1 show low ‘dark’ conductivity of σ∼10⁻¹⁰ (Ω cm)⁻¹. A 10² increase in photocurrent has been observed upon illuminating the crystal with white light. Photoconductivity of 1 is sensitive to magnetic field with B₀<1 T and increases up to 5% in magnetic field. The photoconductivity spectra of the complex indicate that free charge carriers are generated in the UV-visible range mainly by the Bz₄BTPE excitation (the peaks at 622, 562, 472 and 348 nm) with a possible contribution of charge transfer excitations between neighboring C₆₀ molecules (the peak at 472 nm).     

Synthesis, growth and characterization of novel second harmonic nonlinear chalcone crystal

Nonlinear optical (NLO) materials are useful in many of the industrial applications. New NLO chalcone derivative (2E)-3-[4-(methylsulfanyl)phenyl]-1-(4-nitrophenyl)prop-2-en-1-one (4N4MSP) crystals have been grown by slow evaporation technique at ambient temperature. The grown crystals were subjected to single crystal X-ray diffraction study. The crystal has noncentrosymmetric structure in the orthorhombic system with space group Aba2 and unit cell parameters a=14.0647(15) Å, b=33.738(4) Å and c=6.0039(6) Å. To confirm the presence of various functional groups in the compound, FT-IR spectrum was recorded. The crystal was subjected to TGA/DTA analysis to find its thermal stability. The grown crystals were characterized for their optical transmission and mechanical hardness. The second harmonic generation (SHG) efficiency of the crystal is obtained by classical powdered technique using Nd:YAG laser and its value is 28.57 times that of urea. The laser damage threshold for 4N4MSP crystal was determined using Q-switched Nd:YAG laser. The refractive index values for green and red wavelengths were measured by Brewster angle technique. The dielectric and electrical measurements were carried out to study the different polarization mechanism and conductivity of the crystal. Good thermal, mechanical, transmission and SHG response make it desirable for the NLO applications.     

Crystal structure,NMR study,dielectric relaxation and AC conductivity of a new compound [Cd3(SCN)2Br6(C2H9N2)2]n

The crystal structure, the ¹³C NMR spectroscopy and the complex impedance have been carried out on [Cd₃(SCN)₂Br₆(C₂H₉N₂)₂]_n. Crystal structure shows a 2D polymeric network built up of two crystallographically independent cadmium atoms with two different octahedral coordinations. This compound exhibits a phase transition at (T=355±2 K) which has been characterized by differential scanning calorimetry (DSC), X-rays powder diffraction, AC conductivity and dielectric measurements. Examination of ¹³C CP/MAS line shapes shows indirect spin–spin coupling (¹⁴N and ¹³C) with a dipolar coupling constant of 1339 Hz. The AC conductivity of this compound has been carried out in the temperature range 325–376 K and the frequency range from 10⁻² Hz to 10 MHz. The impedance data were well fitted to two equivalent electrical circuits. The results of the modulus study reveal the presence of two distinct relaxation processes. One, at low frequency side, is thermally activated due to the ionic conduction of the crystal and the other, at higher frequency side, gradually disappears when temperature reaches 355 K which is attributed to the localized dipoles in the crystal. Moreover, the temperature dependence of DC-conductivity in both phases follows the Arrhenius law and the frequency dependence of σ(ω,T) follows Jonscher's universal law. The near values of activation energies obtained from the conductivity data and impedance confirm that the transport is through the ion hopping mechanism.     

Semi-ab initio calculations of superposition model and crystal field parameters for Co ions using the exchange charge model

This paper attempts for the first time to establish a reliable linkage between the two well-known and independent models of crystal field (CF), namely the exchange charge (ECM) and superposition models (SM). Our approach aims to show that the SM parameters can be reliably extracted from the distance dependence of the CF invariants for Co²⁺ as derived from the ECM through some semi-ab initio calculations which involved a single fitting parameter and a set of newly constructed procedures. Complete sets of the numerical values of SM parameters and t_k for Co²⁺ in its own host lattices of Li₂Co₃(SeO₃)₄, CoSO₄·H₂O, CoSeO₄·H₂O, and Co(OH)₂ are obtained and they are found to be around 13,000-16,000 cm⁻¹ for , 4100-5700 cm⁻¹ for , 4.1-5.0 for t₂ and 6.2-6.5 for t₄. The present results generally agree with but should be much better than those incomplete sets of results found by previous researchers using the conventional fitting approach. Plausible explanations for some noticeable discrepancies are also discussed together with the effects of different CF contributions on values of the SM parameters.     

Inter-relationship between optical dielectric constant and interatomic separation of mixed crystals in ionic and covalent binary compound families

An ion dependent dielectric model is developed for mixed binary crystals. The interatomic separation (R) and optical dielectric constant (ε_∞) of the mixed crystals are computed from the measured values of R and ε_∞ of pure crystals. An empirical relation between ε_∞ and R is therefore found for mixed crystals by using the computed values. It is found that the dielectric behaviour of ionic mixed crystals is cation dependent while that of covalent mixed crystals is anion dependent. The prescribed theory can therefore be used to form different mixed crystals with particular values of ε_∞ required in any specific opto-electronic devices.     

Optical and electric properties of Nb-doped anatase TiO2 single crystal

Nb-doped anatase TiO₂ single crystal has been grown by chemical vapour transport method. Raman spectra shows that the obtained crystal with Nb of 0.08 wt% has typical anatase structure. An absorption band was observed at around 2.2 eV, which seems to be due to the d-d transition in the conduction band. The electron paramagnetic resonance and electric resistivity measurements show that the doped niobium makes quite shallow donor level whose orbital is d_xy-like centered at the titanium position of anatase.     

Syntheses, crystal structure and luminescence property of novel 4f-3d heterometallic one-dimensional coordination polymers

Two 4f-3d heterometallic one-dimensional coordination polymers [ZnEu₂(Fur)₈·2H₂O]n and [ZnTb₂(Fur)₈·2H₂O]n (HFur=α-furancarboxylic acid) with good solubility were synthesized and characterized by infrared spectra, X-ray single crystal and powder diffractions, thermogravimetric and differential thermal analysis, and excitation and emission spectra. The X-ray diffraction analyses revealed that the two complexes are isomorphous. The Zn and Ln atoms are linked into one-dimensional chains by furancarboxylate ligands and the metal atoms are arranged in the ···Zn···Ln···Ln···Zn···Ln···Ln···order. Under the excitation of UV light, these coordination polymers emit intense characteristic luminescence of Eu³⁺ or Tb³⁺ ion, respectively. The influence of Zn²⁺ ion on the luminescence properties was also discussed here.     

Ab initio study of ultra-incompressible ternary BeCN2 polymorph

The structural, mechanical, thermodynamic, and electronic properties calculated by projector-augmented wave method are presented for BeCN₂ in chalcopyrite and wurtzite-like structures. The calculated high bulk modulus (321 and 309 GPa) and large shear modulus (302 and 298 GPa) suggest that they are ultra-incompressible and hard materials. The ultra-incompressibility is attributed to a stacking of strongly three-dimensional covalent bonded CN₄ and BeN₄ tetrahedrons connected by corners. Thermodynamic study demonstrates that these two structures can be synthesized at ambient condition. Furthermore, the structural transformation from the wurtzite-like to the chalcopyrite phase was predicted at about 17 GPa according to the enthalpy difference calculations.     

Preparation and properties of disordered NaBi(XO4)2, X=W or Mo, crystals doped with rare earths

NaBi_1−xRE_x(XO₄)₂, X=W or Mo and RE=Pr, Nd, Ho, Er and Yb single crystals have been grown by the Czochralski technique. Rare earth concentrations about 3.5×10²⁰ cm⁻³ have been achieved in crystals with good optical quality. Melt stability is obtained by synthesising NaBi(XO₄)₂ from the precursor Na₂X₄O₁₃ phase and minimising Mo volatility. The strength of W and Mo compounds to chemical attack and thermal annealing in several atmospheres is reported. Mo compound is etched by inorganic acids and becomes coloured after vacuum annealing. The optical absorption, photoluminescence and refractive indices of the hosts are characterised and show a dichroic character. The lattice disorder induces broadening of the 10 K optical absorption of the rare earth impurities.