Phonon Side Bands of Specular Optical Transitions in Molecular Crystals

Quantitative investigations of phonon side bands (PSB) in the small radious centers absorption and luminescence spectra of some impurity molecular crystals are presented. Three pairs of specular spectra were considered corresponding to three impurity systems: benzene-d ⁰ in benzene-d ⁶, phenol in benzene-d ⁰ and β-methylnaphthalene-d ¹⁰ in naphthalene-d ⁸. Analysis of the observed spectra revealed such PSB regularities.

1) The most common characterization of PSB spectra is the deviation of the PSB shape from specular symmetry.

2) The non-specularness of the PSB shape does not depend on whether the PSB integral intensities are equal or different.

3) There is an unambiguous correlation between the expression of the PSB non-specularness and the temperature variation of the phononless band half width and shift.

4) For weak transitions, the non-specularness of the PSB integral intensities is observed in addition to the PSB shape non-specularness.

For the first time the observed characteristics were explained quantitatively in terms of the deviation from the Condon approximation on the basis of one-phonon spectra substracted from the PSB observed.     

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)     

Water-soluble ferroelectric crystals with inhomogeneous impurity distribution

Stabilization of the unipolar state via the formation of inhomogeneous impurity distribution in crystal bulk is considered. Possible growth of crystals with stable characteristics is demonstrated on triglycine sulfate (TGS) crystals with a regular inhomogeneous impurity distribution. The properties of TGS crystals with an inhomogeneous distribution of chromium ions grown above and below the Curie temperature T_C are studied. Inhomogeneous TGS crystals of three types are obtained: type-I crystals with a smooth variation of the concentration gradient along the growth direction, type-II crystals with a periodic layer variation of the impurity concentration, and type-III crystals with a sawtooth-like variation of the impurity concentration along the sample length. The TGS crystals with the regular inhomogeneous impurity distribution in the ferroelectric phase are characterized by higher values of the internal bias field E _b , unipolarity coefficient k, and pyroelectric coefficient γ than the inhomogeneous crystals in the paraelectric phase and the crystals with the statistic impurity distribution grown by the conventional method.     

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.

     

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.     

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)     

On the dependence of the critical crystallization rate on the initial impurity concentration in melt

The dependence of the critical crystallization rate on the initial impurity concentration in melt is derived by determining the condition at which a nonplanar solution to the stationary diffusion problem arises. It is suggested that the conditions at which defects arise at the interface differ from those obtained when crystallization becomes stationary. The initial transient process of binary melt solidification under crystal pulling at a constant rate has been studied numerically within a 1D model. It is shown that the character of the time dependence of the crystallization rate is determined by the ratio of the crystal pulling rate to solidification rate V _c, when a constitutional supercooling zone is formed in the melt.     

Distribution of aluminum and indium impurities in crystals of Ge-Si solid solutions grown from the melt

The problem regarding the distribution of aluminum and indium impurities in bulk crystals of solid solutions with a variable composition Ge_1−x Si _x (0 ≤ x ≤ 0.3) is solved in order to establish regularities of the changes in the segregation coefficients of impurities with variations in the composition of the host lattice in the germanium-silicon system. Aluminum-and indium-doped crystals of Ge_1−x Si _x (0 ≤ x ≤ 0.3) solid solutions with a silicon content decreasing along the crystallization axis are grown by a modified Bridgman method with the use of a silicon seed. The concentration distribution of impurities over the length of the crystals is determined from Hall measurements. It is demonstrated that the experimental data on the concentration distribution of impurities in the crystals are in good agreement with the results obtained from the theory according to which the equilibrium segregation coefficients of impurities vary linearly with a change in the composition of Ge-Si solid solution crystals.

     

Electromagnetic stirring and retention to improve segregation in silicon for photovoltaics

The segregation of impurities was investigated during the crystallization of upgraded metallurgical grade silicon (UMG Si). An experimental solidification system has been developed, including a strong electromagnetic stirring in order to mix the melt, to increase the effective segregation coefficient.Our crucible has a slit-like opening at the bottom of a face, to be adapted to ribbon pulling for future developments of a crystallization system for the photovoltaic industry. The electromagnetic system allowed retaining the liquid silicon inside the crucible during the crystallization of an ingot (thickness ～5 cm), or make it flow out after the partial crystallization of a bottom layer (thickness ～2 mm).Starting from UMG Si of known composition, the chemical analysis of the impurity concentrations in that multicrystalline material shows an efficient purification, despite the rather high crystallization speed (estimated between 9 and 20 cm/h). Phosphorous concentration was reduced from 6 to 1.7 ppm during the segregation, which is difficult to achieve for this impurity, which has a segregation coefficient close to unity (k₀=0.3).     

A New Relation Between the Melting Points of Organic Solids and Their Reactivity

Abstract

A new relation between the melting points of organic solids and their reactivity has been established. The relation follows √? = - k log Mp + C where ? is the thickness of the colored boundary Mp is the melting point of the organic solid and k and C are constants.     

Effect of cationic impurities on solubility and crystal growth processes of ammonium oxalate monohydrate: Role of formation of metal‐oxalate complexes

The effect of different bi‐ and trivalent cationic impurities on the solubility of ammonium oxalate and the composition and distribution of chemical complexes formed in saturated ammonium oxalate aqueous solutions as a function of impurity concentration are investigated. The knowledge of the composition and stability of complexes formed in saturated aqueous solutions is then employed to explain the appearance of dead zones of supersaturation for growth and the difference in the effective segregation coefficient of the impurities. Analysis of the experimental results revealed that: (1) at a constant temperature, the dependence of concentration of complex species formed in saturated solutions on the concentration of different impurities can be described by an equation similar to that of the concentration dependence of density of solutions, (2) the dominant metal‐containing species present in saturated solutions are negatively‐charged, most stable oxalato complexes like Cu(C₂O₄)₂²⁻, Mn(C₂O₄)₃⁴⁻, Zn(C₂O₄)₃⁴⁻, Cr(C₂O₄)₃³⁻ and Fe(C₂O₄)₃³⁻, (3) in the investigated range of impurity concentration, the solubility of ammonium oxalates increases linearly with the concentration of all impurities and the increase is associated with the stability of dominant complexes, (4) appearance of dead supersaturation zones in the presence of impurities is associated with instantaneous adsorption of all growth sites by dominant oxalato complexes in relatively short adsorption time, and (5) the segregation coefficient of an impurity cation M of charge z + increases with a decrease in the solubility product constant K_sp for the hydrolysis products of reactions of the type: M^{z +} ↔ M₁^{(z −1)+} + H⁺ (where the cation M has z + charge, and H⁺ is hydrogen ion). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal Structures and Phase Transformations of the Fluorinated Fullerenes

Abstract

In situ x-ray diffraction experiments of the fluorinated fullerenes, C₆₀F _x (x = 0, 36 and 48) at high temperatures have been undertaken. The structural phase transformation of C₆₀F₄₈ was observed at about 358 K. It was also found that the lattice parameters of the low temperature phase of C₆₀F₄₈ show anomalous change at about 310 K. The thermal expansion coefficients of the fluorinated fullerenes were determined by using the change of the lattice parameters with temperature.     

Crystal structure analyses of divalent metal cation complexes with neutral noncyclic ionophores. I. Mn2+ and Ca2+ complexes withN,N,N′,N′-tetrapropyl-1,2-benzodioxydiacetamide

The crystal structure of three divalent metal complexes withN,N,N,N-tetrapropyl-1,2-benzodioxydiacetamide are discussed: (1) (C₂₂H₃₆O₄N₂)₂ Mn·MnBr₄,M _r =1214.6, tetragonal, space group (No. 82),a=b=13.661(5),c=14.994(4) Å,V=2798.2 ų,Z=2,d _m =1.42,d _x =1.44gcm^–3,R=0.051 for 660 unique diffractometer data; (2) (C₂₂H₃₆O₄N₂)₂CaBr₂,M _r =985.0, monoclinic, space groupC2/c (No. 15),a=14.530(4),b=35.565(10),c=12.384(3) Å, =122.47(2)°,V=5399.2 ų,Z=4,d _x =1.21g cm^–3,R=0.056 for 2436 unique diffractometer data; (3) (C₂₂H₃₆O₄N₂) Ca(NCS)₂·solvent,M _r =548.8 + solvent, triclinic, space groupP (No. 2),a=9.341(2),b=13.203(2),c=14.237(2) Å, =75.69(1),=78.38(1), =76.66(1)°,V=1635.9 ų,Z=2,d _m =1.19,d _x (without solvent)=1.11g cm^–3,R=0.112 for 3450 unique diffractometer data. Crystals (1) and (2) are 21 complexes with the metal cations at special positions, (1) at a site of local ¯4(S ₄ ), and (2) at a site of local 2(C ₂ ) symmetry, in both cases coordinated to all eight O atoms of the two ligand molecules. Crystal (1) also contains complex anions MnBr ₄ ^2/s- . Crystal (3) is a 11 complex with a seven-coordination of the Ca²⁺, involving four O atoms of the ligand molecule, two N atoms from thiocyanate anions, and a solvent molecule.The authors thank Prof. I. J. Borowitz for the samples and Profs. J. D. Dunitz and W. Simon for valuable discussions.     

Local symmetry breaking for negatively charged impurity centers in SrTi<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript>

First-principles calculations of the geometry and electronic structure of the impurity center in SrTi_{1 − x} Mn _x O₃ have been performed. Neutral and negatively charged defects are considered. It is found that the doubly charged impurity center is polar; it has C _4v symmetry and electronic state ⁴ B ₁ with electron polaron localized at one of the neighboring titanium atoms. It is shown that this state is due to the spontaneous breaking of the defect local symmetry: O _h (⁴ A _1g ) → D _4h (⁴ B _1g ) → C _4ν(⁴ B ₁).     

Crossover temperature (TCO) and temperature gradient of refractive indices and length to breadth ratio of 4O.m mesomorphic compounds – an optical study

Refractive indices as a function of temperature are measured in a number of liquid crystals which belong to the N-(p-n-butoxy benzylidene)-p-n alkyl anilines, 4O.m compounds of the famous Schiff’s base nO.m liquid crystal compounds with m = 4 to 10 and 12. The temperature gradient of refractive indices, dn_e/dT and dn_o/dT, of these compounds are estimated. It is well known that in the case of dn_o/dT, there exists a crossover temperature, T_CO, which exhibits an odd–even effect with the alkyl chain length as expected. Further, using the birefringence data, the length to breadth ratio, k, for these compounds is estimated, which shows not exactly even–odd effect but irregular change with the chain number. The results are discussed with the body of the data.     

Isochronal crystallization kinetics of Cu60Zr20Ti20 bulk metallic glass

Isochronal crystallization kinetics of Cu₆₀Zr₂₀Ti₂₀ bulk metallic glass has been investigated by differential scanning calorimetry. By means of the Kissinger, Ozawa, Kempen, Matusita and Gao methods, average effective activation energies for the first and second crystallization reactions in Cu₆₀Zr₂₀Ti₂₀ are calculated to be about 375 ± 9 and 312 ± 11 kJ mol⁻¹, respectively, which are smaller than the values deduced from isothermal experiments. Meanwhile, average Avrami exponents, 3.0 ± 0.1 and 3.4 ± 0.2, for two crystallization reactions in isochronal anneals, differ from the value about 2.0 in isothermal anneals. The nonidentity of the Avrami exponents and effective activation energies may be contributed to different crystallization mechanisms and the nature of non-isokinetic between isochronal and isothermal experiments. The values of frequency factor k₀ for the first and second crystallization reactions of Cu₆₀Zr₂₀Ti₂₀ are (1.7 ± 0.3) × 10²⁴ and (7.0 ± 0.8) × 10¹⁸ s⁻¹, respectively, and the large value of k₀ has been discussed in terms of the atomic configuration and interaction.     

A luminescent heteroleptic five-coordinate cadmium(II) complex containing a bridging dithiolate ligand

The luminescent heteroleptic Cd(II) complex bis (-1,2-benzenedithiolato)-1 kS, 1:2 k ² S 2 kS, 1:2 k ² S-bis[(1,10-phenanthroline-k ² N ¹, N ¹)cadmium], [Cd₂(C₆H₄S₂)₂(C₁₂H₈N₂)₂], containing benzenedithiolate (bdt) and 1,10-phenanthroline (phen) has been synthesized and found to have the dinuclear formulation [Cd(bdt)(phen)]₂. The complex crystallizes in the space group P2₁/n with a = 10.3400(4), b = 11.5110(4), c = 13.6230(5) Å, and = 106.828(2)°. The dinuclear complex has crystallographically imposed centrosymmetry with one S atom of the bdt ligand bridging the Cd atoms in an asymmetric fashion (Cd–S = 2.5743(5) and 2.6817(5) Å) while the second S atom is bound in a terminal mode (Cd–S = 2.4955(5) Å). The mean interplanar spacing between the phen ligand and the phenyl ring of the bdt ligand is 3.291 Å while that between the phen ligands of adjacent molecules is 3.363 Å, suggesting the presence of both intra- and intermolecular -stacking. The complex is emissive in the solid state at room temperature (_em = 569 nm) with a luminecsent lifetime of 369 ns. The unstructured emission is believed to be a ligand-to-ligand –* charge transfer transition.     

Gedunin,ad-seco limonoid

Chloroform/Methanol extraction of the bark ofCedrela salvadorensis (Meliaceae), followed by chromatography afforded the crystalline C₂₈H₃₄O₇ terpene. The structure first proposed by spectroscopy is confirmed by this X-ray analysis as 7-acetyloxy-14,15:21,23-diepoxy-4,4,8-trimethyl-d-homo-24-nor-17-oxachola-1,20,22-triene-3,16,-dione. The title compound crystallized in the orthorhombic space groupP2₁2₁2₁ with unit cell parametersa=10.243(2),b=13.356(3) andc=18.058(4) Å. The molecule is comprised of a six-membered tetracyclic skeleton, with a circular arc overall conformation. No intermolecular hydrogen bonding is indicated.     

New refinement of the crystal structure of 1,3-diaminopropane dihydrochloride

The structure of C₃H₁₀N₂·2HCl,M _r =147.05,a=8.567(3),b=9.341(2),c=9.444(9) Å,V=755.7 ų,Pbcn,d ₀=1.30,d _c =1.29 Mg m^–3,Z=4,F(000)=312e, which had been previously solved (Hirokawa etal., 1968), has been refined by full matrix to a finalR value of 0.034 as compared to 0.196 in the work cited above.     

Growth dynamics of Ge1?xSix single crystals obtained by directional constitutional supercooling of the melt

The problem of growth dynamics for crystals of binary solid solutions, obtained by the constitutional supercooling of the melt with a silicon feeding rod, has been solved within the Pfann approximation. The dependences of the change in the axial growth rate of Ge_1−x Si _x crystals (0 ≤ x ≤ 0.3) are calculated. It is shown that the Ge_1−x Si _x crystallization rate significantly changes during growth. The results make it possible to determine the optimal conditions and technological parameters for growing Ge_1−x Si _x single crystals (0 ≤ x ≤ 0.3) with a specified concentration gradient along the crystallization axis.