Special features of formation of nanocrystalline BiFeO<Subscript>3</Subscript> via the glycine-nitrate combustion method

Nanocrystalline bismuth orthoferrite has been synthesized under conditions of glycine-nitrate combustion. Temperatures of activation of formation and growth of BiFeO₃ nanocrystals have been shown to correlate with melting temperature of surface (non-autonomous) phases. Optimal temperature of synthesis of nanocrystalline bismuth orthoferrite has been determined.     

Thermophysical properties of gamma-irradiated LaFeO3 and YFeO3 orthoferrites

In the present work, LaFeO₃ and YFeO₃ orthoferrites were carefully prepared using the solid state high temperature ceramic and sintering technique. On these two materials, numerous measurements were applied before and after γ-absorbed dose (1.24 Mrad) namely, X-ray diffraction (XRD), DC-electrical conductivity, UV/Vis, IR, Mössbauer effect and EPR. Results obtained were explained, interpreted, and discussed in detail on the basis of lattice structure of the new perovskite investigated and the interaction of γ-radiation with the investigated materials. Finally the γ-induced functions are evaluated for electrical conduction, Mössbauer effect and EPR for the first time.     

Magnetic properties of lanthanum orthoferrites containing lattice defects

Polycrystalline lanthanum orthoferrites were prepared by firing coprecipitated hydroxides of La³⁺ and Fe³⁺ at elevated temperatures. The materials fired at temperatures below 1100°C were characterized by the coexistence of such lattice vacancies that the concentration ratio of V_La:V_Fe:V_o is always equal to that of the constituent atoms of LaFeO₃. They exhibited a large magnetic susceptibility, a low Néel temperature, and a small spontaneous magnetization, in comparison with lanthanum orthoferrite without these vacancies. These characteristics originated essentially from a local reduction in anisotropic superexchange interaction at vacant sites. The lowering of Néel temperature characteristic of these materials was interpreted in terms of a decrease in the number of Fe³⁺OFe³⁺ linkages due to the existence of oxygen and iron vacancies.     

Crystalline structure of polyamide 12 as revealed by solid‐state 13C NMR and synchrotron WAXS and SAXS

The crystalline structure of polyamide‐12 (PA12) was studied by solid‐state ¹³C nuclear magnetic resonance (NMR) as well as by synchrotron wide‐ and small‐angle X‐ray scattering (WAXS and SAXS). Isotropic and oriented PA12 showed different NMR spectra ascribed to γ‐ and γ′‐crystalline modifications, respectively. On the basis of the position of the first diffraction peak, the isotropic γ‐form and the oriented γ′‐form were shown to be with hexagonal crystalline lattice at room temperature. When heated, the two PA12 polymorphs demonstrated different behaviors. Above 140 °C, the isotropic γ‐PA12 partially transformed into α‐modification. No such transition was observed with the oriented γ′‐PA12 phase even after annealing at temperatures close to melting. A γ′–γ transition was observed here only after isotropization by melting point. Various structural parameters were extracted from the WAXS and SAXS patterns and analyzed as a function of temperature and orientation: the degree of crystallinity, the d‐spacings, the Bragg's long spacings, the average thicknesses of the crystalline (l_c) and amorphous (l_a) phases, and the linear crystallinity x_cl within the lamellar stacks. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3720–3733, 2005     

Kristallstruktur und Phasenumwandlungen von As(CH3)4I

Crystal Structure and Phase Transitions of As(CH₃)₄I The crystal structure of α-As(CH₃)₄I at room temperature was determined using single crystal data: cubic, space group Pa3 , a = 1 198.0(2) pm. Therefore α-As(CH₃)₄I displays a novel crystal structure, which is not comparable to known AB-Typ structures with respect to the arrangement of anions and the baricenters of the complex cations. Differential thermal analysis showed three phase transitions at 103, 175 and 215°C. The lattice parameters of the high temperature phases (temperature dependent Guinier measurements) are: β-As(CH₃)₄I (tetragonal): a = 845.2(2) pm, c = 615.0(2) pm; γ-As(CH₃)₄I (hexagonal): a = 737.7(2) pm, c = 1 082.2(3) pm; and to δ-As(CH₃)₄I (hexagonal): a = 705.8(2) pm, c = 1 147(1) pm. β-and γ-As[(CH₃)]₄I are isotypic to N(CH₃)₄Cl and As(CH₃)₄Br, respectively.     

ESR of the ozonide (O3−) radical in γ-irradiated AgClO3

The ozonide (O₃^?) radical obtained by γ-irradiation on AgClO₃ has been identified by its room temperature ESR spectra at X and Q band and by its optical absorption at 440 mμ. The complete study of the resolved Q band ESR spectra consisting of two lines with the same intensity revealed the presence of two inequivalent trapping sites for the O₃^? molecules.     

Thermal stability and glass transition behavior of PANI/γ-Al2O3 composites

Polyaniline/γ-Al₂O₃ (PANI/γ-Al₂O₃) composites were synthesized by in-situ polymerization at the presence of HCl as dopant by adding γ-Al₂O₃ nanoparticles into aniline solution. The composites were characterized by FTIR and XRD. The thermogravimetry (TG) and modulated differential scanning calorimetry (MDSC) were used to study the thermal stability and glass transition temperature (T _g) of the composites, respectively. The results of FTIR showed that γ-Al₂O₃ nanoparticles connected with the PANI chains and affected the absorption characteristics of the composite through the interaction between PANI and nano-sized γ-Al₂O₃. And the results of XRD indicated that the peaks intensity of the PANI/γ-Al₂O₃ composite were weaker than that of the pure PANI. From TG and derivative thermogravimetry (DTG) curves, it was found that the pure PANI and the PANI/γ-Al₂O₃ composites were all one step degradation. And the PANI/γ-Al₂O₃ composites were more thermal stable than the pure PANI. The MDSC curves showed that the nano-sized γ-Al₂O₃ heightened the glass transition temperature (T _g) of PANI.     

Formation mechanism of nanocrystalline yttrium orthoferrite under heat treatment of the coprecipitated hydroxides

Formation of nanocrystalline yttrium orthoferrite of ～30 nm average crystallite size from coprecipitated iron(III) and yttrium hydroxides was studied by thermo-X-ray diffractometry and simultaneous thermal analysis over 25–900°C temperature range. A mechanism of physicochemical transformations leading to the formation of YFeO₃ nanoparticles was suggested.     

Kinetics and mechanism of the dehydration of γ-FeOOH

The dehydration of γ-FeOOH to γ-Fe₂O₃ in vacuo has been investigated by thermoanalysis. Results have been checked by electron microscopy and diffraction and by x-ray diffraction. Authors find that formal kinetics are not conclusive. Electron micrographs, however, show directly that random nucleation occurs, producing perfectly oriented but disordered γ-Fe₂O₃ crystals of about 70 Å size. The results can be accomodated with an atomistic model of lattice collapse.     

Competitive Formation of Ylide and Carbene Intermediates from 1π,π*-Excited α,β-Unsaturated γ,δ-Epoxyketones

Laser flash photolyses (λ = 265mm) of the γ,λ-epoxyenones 1–3, 7 and 8 , the α,β-unsaturated γ,δ-epoxy ester 6 , and the epoxytriene 9 at ambient temperature produced short-lived transients with broad absorption maxima in the visible region, which are identified as carbonyl ylides. Comparison of the rather long-wavelength absorption maxima with the results of standard PPP SCF SCI calculations suggests that some degree of twisting is present in all the ylides studied. The lifetimes of the order of hundreds of ns of these intermediates and Stern-Volmer analysis of the trapping of the carbonyl ylide derived from 2 with CH₃COOH provide conclusive evidence that the carbene products are not formed via the carbonyl-ylide intermediate (Scheme 3).     

The iodates of scandium

A dihydrate, a monohydrate, and three polymorphs of the anhydride Sc(IO₃)₃ have been prepared and studied. Sc(IO₃)₃·2H₂O was prepared by gel growth and by precipitation and evaporation at room temperature. The γ-anhydride can be crystallized from boiling water or nitric acid solution. The other compounds are formed on heating the dihydrate, which also dehydrates very slowly at room temperature. The α-anhydride is amorphous. The β-anhydride generates second harmonics, with about twice the efficiency of quartz. In addition differential thermal analysis (DTA), thermogravimetric analysis (TGA), infrared absorption, and powder X-ray diffraction results are presented.     

Free-electron model for hyperpolarizability of the π-electron system in benzene

The hyperpolarizabilities of benzene have been calculated using a free-electron model for the π-electron system. The first hyperpolarizability is identically zero. For the second hyperpolarizability only the components γ_zzzz; γ_xxxx; γ_yyyy; γ_zzxx; γ_zzyy; γ_xxyy are non-zero. Some of these γ quantities show dispersion near the characteristic absorption band of the benzene molecule. The polarizability α of the π-electron system also shows similer dispersion.     

Magnetic hollow poly(N-isopropylacrylamide-co-N,N′-methylenebisacrylamide-co-glycidyl acrylate) particles prepared by inverse emulsion polymerization

To prepare functionalized magnetic polymer particles that are thermally responsive, inverse emulsion copolymerization of N-isopropylacrylamide, N,N′-methylenebisacrylamide and glycidyl acrylate (GA) was investigated in paraffin oil in the presence of γ-Fe₂O₃ nanoparticles dispersed in a water/glycerol mixture. The resulting polymer particles were characterized regarding the morphology, size, polydispersity, iron content, and the temperature-dependent phase transition using optical microscopy, transmission electron microscopy, scanning electron microscopy, atomic absorption spectroscopy, and differential scanning calorimetry. Magnetic properties were examined using hysteresis loop measurements and by analyzing the magnetic susceptibility with respect to temperature. We have also investigated the influence of the concentration of γ-Fe₂O₃ and GA in monomers on properties of the particles (morphology, size, and presence of oxirane groups). The particles possessed a hollow structure as a result of phase separation between water/glycerol hydrophilic solvents in the polymerization feed and the forming polymer. Depending on the concentration of γ-Fe₂O₃ in the monomer phase, the magnetic hollow particles contained 5–24 wt% iron. In water, the particles gradually collapsed when the temperature was raised to 40 °C because the elevated temperature weakened hydration and the PNIPAAm chains gradually became more hydrophobic.     

Coefficient of transverse deformation and anharmonism of lattice oscillations in quasi-isotropic solids

The dependence of the Grüneisen parameter γ _D on the anharmonism of lattice oscillations is controlled by γ₃ that appears to be a single-value function of the Poisson coefficient. In terms of γ _D -γ₃ relationship, all solids can be grouped into different structural types. Within each group, the above relationship is linear. The nature of correlation between the Grüneisen parameter and the Poisson coefficient is discussed.     

Thermodynamics,structure and kinetics in the system Ga–O–N

Within the ternary system Ga–O–N we performed experimental and theoretical investigations on the thermodynamics, structure and kinetics of new stable and metastable compounds.We studied the ammonolysis of β-Ga₂O₃ at elevated temperatures by means of ex situ X-ray diffraction, ex situ neutron diffraction, and in situ X-ray absorption spectroscopy (XAS). From total diffraction pattern refinement with the Rietveld method we analyzed the anionic occupancy factors and the lattice parameters of β-Ga₂O₃ during the reaction. Within the detection limits of these methods, we can rule out the existence of a crystalline oxynitride phase that is not derived from wurtzite-type GaN. The nitrogen solubility in β-Ga₂O₃ was found to be below the detection limit of about 2–3 at.% in the anionic sublattice. The kinetics of the ammonolysis of β-Ga₂O₃ to α-GaN and of the oxidation of α-GaN to β-Ga₂O₃ was studied by means of in situ X-ray absorption spectroscopy. In both cases the reaction kinetics could be described well by fitting linear combinations of β-Ga₂O₃ and α-GaN spectra only, excluding that other crystalline or amorphous phases appear during these reactions. The kinetics of the ammonolysis can be described well by an extended Johnson–Mehl–Avrami–Kolmogorow model with nucleation and growth of GaN nuclei, while the oxidation kinetics can be modeled by a shrinking core model where Ga₂O₃ grows as a layer. Investigations by means of TEM and SEM support the assumptions in both models.To investigate the structure and energetics of spinel-type gallium oxynitrides (γ-galons) we performed first-principles calculations using density-functional theory. In addition to the ideal cubic γ-Ga₃O₃N we studied gallium deficient γ-galons within the Constant-Anion-Model.In highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO_1.2 we found at a temperature around 670 K an insulator–metal transition, with a conductivity jump of seven orders of magnitude. We demonstrate through experimental studies and density-functional theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric β-Ga₂O₃ within the metastable oxide matrix. By doping with nitrogen the critical temperature and the conductivity in the highly conducting state can be tuned.     

Formation of crystalline inclusion compounds of poly (vinyl chloride) of different stereoregularity with γ-cyclodextrin

This work reports the formation and detailed characterization of the γ-cyclodextrin (γ-CD) inclusion compounds (ICs) formed with two poly (vinyl chloride) samples with different isotactic content. The ICs were characterized by X-ray diffraction, solid state ¹³C-NMR, solution ¹H-NMR, FT-infrared, differential scanning calorimetry, and thermogravimetric analysis. Experimental evidence of the inclusion of the guest polymer chains into the narrow channels created by the γ-CD crystalline host lattice has been obtained. Examination of coalesced poly (vinyl chlorides) (PVCs) obtained after the host γ-CD is removed reveals different characteristics specifically for the coalesced PVC sample with higher isotactic content. An increase in T_g was observed by DSC for this PVC. To the contrary, the T_g of the coalesced PVC sample with lower isotactic content is almost the same as that of the as-synthesized sample. Thermogravimetric analysis indicated that coalesced PVC with higher isotactic content acquires a degree of stabilization after modification by threading into and being extracted from its γ-CD IC. The results suggest that an irreversible conformational change takes place when PVC forms ICs with a solid host lattice like γ-CD. The PVC molecules extend and reorganize into a more stable conformation in the IC, consequently improving the properties of the coalesced sample. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2503–2513, 2007     

Influence of crystallite size on the oxidation kinetics of magnetite

The oxidation of magnetite yields the lacunar phase γ-Fe₂O₃, for sizes less than 5000 Å and the rhombohedral phase, α-Fe₂O₃, for sizes above 10 000 Å. For intermediate sizes, oxidation kinetics and X-ray analysis have confirmed that the γ-Fe₂O₃ phase forms at the beginning of the reaction, followed by phase α-Fe₂O₃ forming from γ-Fe₂O₃ and then directly from the still-unoxidized magnetite. Influence of size could be accounted for in terms of structure and stresses at the crystal lattice level.     

The Structure of the High Temperature Modification of Lithium Triflate (γ-LiSO3CF3)

The high temperature modification of lithium triflate (γ-LiSO₃CF₃) crystallises in the orthorhombic space group Cmca with the lattice parameters a = 6.3022(4), b = 8.6989(5), and c = 20.1262(14) Å (synchrotron powder diffraction data, T = 456 K). The crystal structure of γ-LiSO₃CF₃ is disordered, in the cationic as well as the anionic partial structure, and exhibits double layers of triflate anions with the lipophilic CF₃-groups facing each other. The lithium ions could not be located by the diffraction experiment due to their high mobility. The staggered triflate anion is twofold disorderd leading to two different positions on top of each other which are tilted to one another.     

Mutual annihilation of triplet excitons in a nearly one-dimensional system: 1,4-dibromonaphthalene

The analysis of the variation with incident flux of the time dependence of the delayed fluorescence in conjunction with the determination of the absolute ground state-first excited triplet absorption coefficients at room temperature, yields the value of γ_tot = (5.5 ± 2.0) × 10^?12 cm³ s^?1, for the total triplet-triplet annihilation rate constant in 1,4-dibromonaphthalene crystals. The one-dimensional mutual annihilation rate constant for the triplet exciton motion restricted to linear chains along the crystal c axis is γ¹_tot = (1.0 ± 0.4) × 10³ cm s^?1. The results are discussed in terms of recent theories of mutual annihilation of triplets in one-dimensional systems.     

Polarized i.r. spectra of 4-aminopyridine hemiperchlorate single crystal

The polarized i.r. spectra of 4-aminopyridine hemiperchlorate single crystals containing structurally asymmetric NHN⁺ bridges were studied at room and liquid nitrogen temperatures. It was shown that the broad intense absorption over the frequency range 200–3200 cm⁻¹ should be assigned to the protonic ν(NHN) stretching vibrations. The transmission windows can be ascribed most probably to a coupling of ν(NHN) vibrations with deformation δ(NHN) and γ(NHN) modes and their overtones as well as with internal vibrations of the pyridine ring which lead to a modulation of the bridge geometry. The behaviour of NH₂ groups in polarized i.r. spectra confirms the inequivalence of 4-aminopyridine moieties in homoconjugated cations both from the point of view of intramolecular charge distribution and environment of NH₂ groups in the lattice, in agreement with the diffraction data.