On the dependence of the surface tension at the crystal-melt interface on the impurity concentration

The stationary task of impurity diffusion in a melt has been solved within a two-dimensional crystallization model in a second-order approximation with respect to the amplitude of deviation from a smooth crystallization front. The dependence of the surface tension Γ at the interface on the impurity concentration C is taken into account in the form Γ = Γ₀ + ζ _d C, where Γ₀ and ζ _d are constants. The variational method is used to obtain the condition for the transition from a smooth crystallization front to a cellular one. It is shown that calculated cell sizes are in agreement with the experimental data in the literature only when the parameter ζ _d ≠ 0. For binary systems with distribution coefficients k < 1 and k > 1, ζ _d should be positive and negative, respectively.

     

Infrared Spectroscopic Studies of the Nematogenic Homologous Series p−(p−ethoxyphenylazo) phenyl alkanoates

Abstract

Infrared (500–2000 cm^?1) spectra of p-(p-ethoxyphenylazo)phenyl alkanoates (valerate, hexanoate, heptanoate and undecylenate) have been examined in the solid state. An attempt has been made to correlate the variations in the intensities and frequency shifts with the known crystal structures determined by X-ray diffraction methods. The gradual shift towards smaller wavelength in the carbonyl (C=O) stretching frequency and the relative increase in the intensity of the carbonyl frequency as the chainlength increases may be correlated with the increased deviation from planarity of the molecule.     

The impurity ion off-center behavior induced by a localized vibronic Frenkel exciton in a soft-mode crystal

The 3d-4p Frenkel exciton localized at a Mn⁴⁺ impurity ion in BaTiO₃ is considered. The electron and hole of this exciton are correlated, degenerate, and involved in the pair Jahn-Teller effect. Such an exciton is characterized by the significant contribution of indirect electron-hole attraction through lattice (in particular, soft) vibrations to the coupling energy, which is comparable with the Coulomb attraction. Thus, the exciton under consideration is both Coulomb and vibronic excitation. The vibronic contribution increases when approaching the point of soft phonon condensation. It is shown that a vibronic Frenkel exciton allows the off-center behavior of an impurity ion, which is significantly enhanced when approaching the point of soft phonon condensation. As a result, the vibronic Frenkel exciton is a low-lying excitation.

     

Paramagnetic defects in diamond films synthesized by the hot filament chemical vapour deposition

Defects in nitrogen‐doped diamond films, produced by hot filament chemical vapour deposition have been studied by Electron Spin Resonance (ESR), Raman spectroscopy and Scanning Electron Microscopy (SEM). The peak‐to‐peak ESR line width (ΔH _pp) varies in the range 0.36‐0.52 mT and depends on the nitrogen concentration in the process gas. In the case of nitrogen‐doped diamond films ESR spectrum shows a hyperfine structure typical of N_S⁰ paramagnetic centre. The shape of the central ESR line shows that it is a superposition of two components: a narrower Lorentzian and a broader Gaussian one, characterized by different saturation behaviour. With increasing nitrogen concentration in process gas the ratio of integral intensities A _G/A _L (Gausian to Lorentzian) of ESR spectrum also increases. The Raman spectra show that with increasing doping level the diamond Raman line at 1332.5 cm^‐1 broadens, the broad band at about 1530 cm^‐1 becomes more pronounced what indicate on degradation of diamond crystallinity and it is in agreement with SEM observation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Assignment of the Lowest Singlet and Triplet States of the Monoaldehyde of Durene

Abstract

The monoaldehyde of durene has been synthesized. It has been shown to be the impurity frequently encountered in spectral studies involving durene crystals. Both the polarized absorption and emission spectra of the protonated and deuterated aldehyde in crystals of durene^h-14 and durene^d-14 have been obtained. Although the lowest excited singlet and triplet states, separated by 2200 cm^?1, are shown to be nπ* a number of features in the phosphorescence spectrum are more characteristic of a ³π* state. The polarization of the absorption spectrum, in particular the 0-0 band and bands involving the excitation of totally symmetric vibrations, is inconsistent with the nπ* nature of the transition. A number of explanations is proposed.     

Electrical Conductivity of Phenoxazine-Iodine (2:3) and Phenothiazine-Iodine (2:3) Crystals

Abstract

Single crystals of phenoxazine-iodine (2 : 3) and phenothiazine-iodine (2 : 3) salts are found to be highly conducting ([sgrave] _R.T. = 5–20 ohm^?1 cm^?1). The observed deviation from the exponential temperature dependence of the conductivities is ascribed to the degenerate semiconducting phases or alternatively to the metallic phases with impurities. However, phenoxazine-iodine and phenothiazine-iodine are perfect semiconductors below 220° K with activation energies of 0.12 eV and 0.14 eV, respectively. The absorption features related with (phenoxazine)⁺ ₂ and (phenothiazine); cations are observed in the infrared spectra of the salts.     

Optical Properties,Conductivity and Structure of the DTT-TCNQ CT Complex

Abstract

The crystal structure, the optical spectra and electrical conductivity of dithieno(3,2-b; 2′,3′-d)thiophene-tetracyanoquinodimethane (DTT-TCNQ) charge-transfer complex have been measured. DTT-TCNQ crystallizes in the monoclinic space group P2₁/c with a = 7.206(2), b = 7.574(2), c = 32.324(9), β - 92.18(4)°, Z - 4, and the donor and acceptor molecules are arranged in alternated stacks.

The low conductivity (a = 10^?2 S cm^?1 for the single crystal, along the stacking axis a) is characteristic of a semiconductor with activation energy of .61-.76 eV, and is related to the alternate stack structure.

Despite the poor condctivity of the DTT-TCNQ CT complex we estimated, from structure and IR data, a low degree of ionicity (p = .2–.3) which characterizes the DTT molecule as an interestign donor.

The polarized IR spectra shows the effect of vibronic activation of some of the a_g modes of TCNQ.     

Photoconductivity and photoluminescence in chemically deposited (Cd‐Zn)S:CdCl2,Tb films

Results of SEM, EDX and XRD studies, Optical Absorption spectra, Photoconductivity (PC) rise and decay, PC excitation spectra and Photoluminescence (PL) emission spectra are presented for (Cd‐Zn)S:CdCl₂, Tb films prepared by chemical deposition method on glass substrates either at room temperature (RT) or at 60 °C in a water bath (WB). SEM studies show ball type structure which is related to layered growth. EDX measurements show excess of Cd in such preparations along with the presence of Tb. XRD studies show prominent diffraction lines of CdS and ZnS along with lines of CdCl₂ and impurity. The values of strain (ε), grain size (D), and dislocation density (δ) are evaluated from XRD studies and the nature of crystallinity of the films are discussed. Optical Absorption spectra also show the presence of Tb in the lattice corresponding to the transition ⁷F₆(4f⁸) → ⁵D₀(5d¹4f⁷) of Tb³⁺ ions. From results of optical absorption spectra, the band gaps are determined, whose values are quite similar to those obtained from PC excitation spectra. Sufficiently high photo current (I_pc) to dark current (I_dc) ratios with a maximum value of the order of 10⁶ are observed. This high photosensitisation is related to increase in mobility and lifetime of carriers due to photo excitation. PL emission spectra consist of peaks due to transitions in Tb levels. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the origin of supersaturation barriers during the growth of single crystals from aqueous solutions containing impurities – a review

A generalised treatment of the appearance of supersaturation barriers σ_d, σ* and σ** during the growth of single crystals is outlined from the standpoint of well‐defined critical values of relative step velocities on a face. The final theoretical expressions are based on the premise that: (1) there are critical values of the relative step velocities associated with different average distances between adsorbed impurity particles during instantaneous, time‐dependent and time‐independent adsorption of the impurity on the growing surface, (2) the growth rate of a face is proporptional to velocity of steps on the growing face, and (3) Freundlich and Langmuir adsorption isotherms apply for different impurities. The theoretical expressions are then used to critically analyse the experimental data on supersaturation barriers observed during the growth of ammonium oxalate monohydrate and potassium dihydrogen phosphate single crystals from aqueous solutions containing different impurities. It was found that: (1) Langmuir adsorption isotherm is more practical for the analysis of the experimental data of the dependence of supersaturation barriers σ_d, σ* and σ** on the concentration c_i of an impurity, and (2) the ratios σ_d/σ* and σ*/σ** of successive supersaturation barriers for an impurity either increases or remains constant with an increase in impurity concentration c_i, and may be explained in terms of the mechanism of adsorption of impurity particles. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Application of the optical method for determining of the RMS roughness of porous glass surfaces

The application of an optical method for characterizing surface roughness is presented. This method was used for an examination of porous glass surfaces. The expressions relating the root mean square rms (σ) of a surface to its specular reflectance at normal incidence are used for σ ? λ, (λ - wavelength). For light of sufficiently long wavelength the decrease in the measured specular reflectance due to the surface roughness depends only on the root mean square (rms) height of the surface irregularities. On the basis of reflectance spectra, one can determine σ for the porous glass surfaces after technological processes. The measured reflectance spectra were compared with calculated ones for which the scattered component of light was taken into account. The parameters rms determined from the optical method are comparable to those obtained from atomic force microscopy examinations.     

A new equation for the temperature dependence of density of saturated aqueous solutions of electrolytes

A new empirical three-parameter equation d = d₀ exp[β′(T – T₀)²] (where β′, d₀, and T₀ are constants for a salt) has been proposed to describe the dependence of density d (g/ml³) of saturated aqueous solutions of electrolytes on temperature T (K) in the whole range of temperature in which solubility increases or decreases with an increase in temperature.     

Composition,morphology, and properties of sodium-bismuth tungstate crystals

A correlation has been revealed between the formation conditions, composition, morphology, and physicochemical properties of NaBi_{1 − x} (W_{1 + y} O₄)₂ crystals (0 ≤ x ≤ 0.16, 0 ≤ y ≤ 0.11). The effect of melt deviation from stoichiometry on the decomposition rate of grown crystals under electron beam irradiation is shown by transmission electron microscopy. A nonuniform dopant distribution over the crystal boule cross-section is found by microprobe analysis. The Raman spectra of the samples depend on their crystallographic orientation.

     

Band intensities in the circular dichroism and absorption spectra and the states of <Emphasis Type="Italic">d</Emphasis> ions in absorbing media: I. Octahedrally coordinated positions of <Emphasis Type="Italic">d</Emphasis> ions

The factors affecting the band intensity in circular dichroism (CD) and absorption spectra (selection rules, point group of structural position, and the electron density of the ground state of octahedrally coordinated d ion in absorbing medium) are considered. The intensities of CD and absorption bands are calculated and compared with the experimental data. It is shown that the differences in the spatial inversion of the electric- and magnetic-dipole moment transformations lead to a more rigorous validity of the symmetry selection rules and increase the resolution of peaks and transparency window of medium absorption in CD spectra (in contrast to absorption ones). Some criteria for estimating the valence and structural states of d ions are formulated.     

Magnetic and Raman Scattering Studies on Intercalation Compounds FexNbS2

Abstract

Raman scattering spectra and magnetic susceptibility are measured in intercalation compounds Fe^xNbS² (0.159≤x≤0.325). A strong modification of Raman spectra is observed in a sample with x=0.239 (～ 0.25) which is accompanied by 2a × 2a structural change and antiferromagnetic ordering. Some correlations between magnetic ordering (antiferromagnetic or spin glass) and structural change are also discussed for wide range of Fe contents x.     

Growth kinetics of ammonium oxalate monohydrate single crystals from aqueous solutions containing Co(II) and Ni(II) impurities

The growth kinetics of different faces of ammonium oxalate monohydrate (AO) single crystals from aqueous solutions containing different concentrations of Co(II) and Ni(II) ions at a constant temperature are described and discussed. It was found that: (1) at a given supersaturation σ, both Co(II) and Ni(II) ions lead to a decrease in the growth rates R of different faces of AO crystals, (2) the growth of a particular face of the crystals occurs above a critical supersaturation σ_d, but there is also another supersaturation barrier σ* when the rate abruptly increases with σ, and (3) the values of σ_d and σ* increase with increasing concentration c_i of the impurity. The experimental R(σ) data for different concentrations c_i of the impurities were analysed according to the model involving complex source of cooperating screw dislocations and concepts of instantaneous and time‐dependent impurity adsorption. Analysis of the data showed that: (1) adsorption of Co(II) and Ni(II) impurities occurs on the surface terrace of AO crystals, (2) there is a simple relationship between Langmuir constant K and the impurity concentration c_i* corresponding to maximum surface coverage, and (3) the ratio σ_d/σ* of the supersaturation barriers observed in the presence of both impurities increases with an increase in impurity concentration c_i, and may be explained from the standpoint of the mechanism of adsorption of impurity particles at kinks and ledges. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Absorption and circular-dichroism spectra of iron-doped berlinite crystals

Absorption and circular-dichroism spectra of iron-doped AlPO₄ crystals have been studied. For the first time, the experimental data on the electronic states of the [FeO₄]^5? complex are obtained in the range from 190 to 350 nm with due regard for the interactions that cannot be described within the one-electron approximation. The bands observed in the absorption and circular-dichroism spectra are attributed to corresponding electronic transitions. The advantages of the use of crystal-field-induced circular dichroism in comparison with other spectroscopic methods in the analysis of the electronic states of impurity ions in gyrotropic crystalline matrices are considered on the [FeO₄]^5? complex.     

Peculiarities of resonant interaction in paired impurity centers of 2-fluoronaphthalene in naphthalene

Abstract

Fluorescence and absorption spectra of the 2-fluoronaphthalene admixtures in naphthalene were studied at low temperature (T?=?4.2 К). Two types of pairwise impurity centers were formed at admixture concentrations of more than 1?wt%. Polarization of absorption bands was detected; these spectra were determined by resonance interactions between molecules of the impurity center. Resonant splitting of electronic levels for the translationally nonequivalent molecules in the unit cell of the naphthalene crystal was analyzed for the case, when one molecule was in the similar phase with the incident light wave and the other one was in antiphase.     

Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: II. Growth kinetics

The experimental results of the effect of concentration of Mn(II) ions on the growth kinetics of different faces of ammonium oxalate monohydrate single crystals from aqueous solutions at a constant temperature and different predefined supersaturations are described and discussed. It was observed that: (1) at a given supersaturation σ, Mn(II) ions lead to a decrease in the growth rates of different faces of AO crystals, (2) the growth of a particular face of the crystals occurs above a critical supersaturation σ_d but there is also another supersaturation barrier σ* when the rate abruptly increases with σ, (3) the values of σ_d and σ* increase with increasing concentration of the impurity, and (4) the values of σ_d depend on the growth kinetics of a face but those of σ* are independent of face growth kinetics. The experimental R(σ) data for different Mn(II) concentrations c_i were analysed according to the model involving complex source of cooperating screw dislocations and concepts of instantaneous and time‐dependent impurity adsorption. It was found that: (1) for a given face the differential heat of adsorption Q_diff is higher during instantaneous impurity adsorption than that during time‐dependent adsorption, and (2) the values of Q_diff involved during instantaneous adsorption are related with face growth kinetics but those during time‐dependent adsorption are independent of face growth kinetics.     

Etude par Calorimetrie,Diffraction des Rayons X et Diffusion Raman d'une Transition de Phase dans (C4 H4P) Mn (CO)3

Abstract

Heat capacity measurements (95-300K), X-ray diffraction (78-300K) and low frequency Raman spectroscopy (10-350K) have evidenced an order-disorder phase transition in phosphacymantrene, (C₄ H₄P) Mn (CO)₃. This transition has been characterized by a monoclinic ←→ triclinic structural change at about 110 K and by a pretransitional phenomenon. The measured transition enthalpy and entropy are 480 ± 10J.mil^?1 and 4.17 ± 0.08J.K^?1 mol^?1, at 115 K, respectively.

A complete assignment of the observed Raman bands in h₄ and d₄ derivatives is proposed. From the temperature dependence of frequencies, intensities and half-widths of some Raman bands we have discussed the order, the nature and the mechanism of the phase transition: intermolecular interactions appear to be mainly involved in the mechanism and an activation energy roughly equal to 2100 ± 840 J. mol^?1 has been determined.