Structure refinement, dielectric, pyroelectric and Raman characterizations of Ba1−xLax(1−y)/2Euxy/2Nax/2TiO3 solid solution

The perovskite-type oxides Ba_1−xLa_x(1−y)/2Eu_xy/2Na_x/2TiO₃ (0?x?0.5 and xy=0.04) were synthesized and characterized by X-ray diffraction as well as dielectric measurements and Raman spectroscopy. The crystal structure of these ceramics has been determined by the Rietveld refinement powder X-ray diffraction data at room temperature. These compounds crystallize at room temperature in tetragonal space group P4mm for 0?x?0.1 and in the cubic group for 0.2?x?0.5. The phase transition temperature T_C (or T_m) decreases as x content increases. The degree of diffuseness of the phase transition is more pronounced for higher x content, implying the existence of a composition-induced diffuse phase transition of the ceramics with x?0.1. The evolution of the Raman spectra was studied as a function of various compositions at room temperature. The polarization state was checked by pyroelectric measurements.     

Structural characterisation of the perovskite series Sr0.9−xCaxCe0.1MnO3: Influence of the Jahn-Teller effect

Fifteen perovskite-type compounds Sr_0.9−xCa_xCe_0.1MnO₃, x=0-0.9 in steps as fine as 0.05, have been synthesised by solid state methods, and the room temperature structures characterised using X-ray synchrotron powder diffraction. At low Ca contents (x?0.45) the structures are tetragonal in space group I4/mcm and at high Ca contents (x?0.55) the compounds are orthorhombic in space group Pbnm. At room temperature these two phases co-exist in the compound with x=0.5. XANES measurements show the Ce to be present as Ce⁴⁺ in all the oxides. High temperature structures are reported for selected members.     

Doping studies of the magnetic cobaltocuprate CoSr2Y2−xCexCu2O9±δ

A structural, magnetic and electronic study of the cobaltocuprate CoSr₂Y_2−xCe_xCu₂O_9±δ (x=0.5-0.8) has been performed. All materials crystallise in the orthorhombic Cmcm symmetry space group in which chains of corner linked CoO₄ tetrahedra run parallel to the 1 1 0 direction. An antiferromagnetic transition is observed for x=0.5-0.8; T_M increases with x. A change in the dimensionality of the magnetic order occurs at x=0.8 as the interchain distance increases to a critical value. There is charge transfer between the cuprate planes and cobaltate layer as Ce doping increases, so that Co³⁺ is partially oxidised to Co⁴⁺ with a concomitant reduction in the valence of Cu. Superconductivity is not observed in any of the samples and a crossover from Mott to Efros and Shklovskii variable range hopping behaviour is evidenced as x increases from 0.5 to 0.8.     

Identification and investigation of impurities in undoped Hg1−xCdxTe

The effect of impurities on electrical properties has been the most sought information for the well-known infrared sensing material mercury-cadmium telluride. The relationship between the excess holes (p-type) or excess electrons (n-type) of undoped mercury-cadmium tellurideHg_1?xCd_xTe(0.23 ? x ? 0.4)and the residual impurities in the crystals were investigated. The impurities in the undoped material were determined using emission spectrometry, atomic absorption, and spark sources mass spectrometry.Trace analysis indicates that impurities such as Cu and Ag consistently appear inp-type samples. Although these elements are not frequently observed inn-type substance, occasionally they are found in the material in a minute amount. Impurities such as Si, Cr, Pb, Li, Rb, Co, and Sn have been detected in bothp-type andn-type materials. The possible correlation of these impurities with thep-type orn-type behavior of the undoped material is discussed.     

Crystal structure of Ln1/3NbO3 (Ln=Nd, Pr) and phase transition in Nd1/3NbO3

The crystal structure of the A-site deficient perovskite Ln_1/3NbO₃ (Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO₆ octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd_1/3NbO₃ has also been studied in situ. This compound undergoes a continuous phase transition at around 650 °C to a tetragonal structure in space group P4/mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature.     

The fragmentation reaction of 16R-bromopregnane-3S,20S-diol

16R-Bromopregnane-3S,20S-diol reacted with potassium t-butoxide to afford androst-16-en-3S-ol in a moderate yield via fragmentation reaction. The latter is a key intermediate for the synthesis of 5α-androst-16-en-3-one, as boar sex pheromone, and other steroidal drugs. In addition, 16R,20S-epoxypregnane-3S-ol was also obtained as a major product by changing the reaction solvent.     

Tolerance factor rules for Sr1−x−yCaxBayMnO3 perovskites

Synthesis of new perovskite Sr_1−x−yCa_xBa_yMnO_3−δ compounds is described in detail and dependence of their phase stability and structural distortions on the tolerance factor is discussed. Oxygen contents have been measured over extended temperature and composition ranges. Neutron powder diffraction was used to precisely measure the A-O and Mn-O bond lengths and derive accurate interatomic distances [Ca-O], [Sr-O], [Ba-O], and [Mn-O]. By using these parameters instead of tabulated ionic radii we have derived the functional dependence of the tolerance factor t=t(x,y,T,δ) on composition, temperature, and oxygen content. At a fixed oxygen content, the tolerance factor is an increasing function of temperature as a result of greater thermal expansion of the average 〈A-O〉 bond relative to the 〈Mn-O〉 bond. We find that the stability of the perovskite phase at high temperature is governed, as expected, by the magnitude of tolerance factor (t?1) which can be adjusted by controlling the oxygen content 3−δ. This dependence of the tolerance factor on oxygen content and temperature can be utilized to design synthesis conditions for the controlled formation of the new, kinetically stable, perovskite phases.     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

Synthesis, crystal structure and magnetic properties of the spin ladder compounds La2(Cu1−xZnx)2O5 and La8(Cu1−xZnx)7O19

The influence of Zn-doping on the crystal structure and magnetic properties of the spin ladder compounds La₂Cu₂O₅ (4-leg) and La₈Cu₇O₁₉ (5-leg) have been investigated. The La₂(Cu_1−xZn_x)₂O₅ and La₈(Cu_1−xZn_x)₇O₁₉ solid solutions were obtained as single phases with x=0-0.1 via the solid-state reaction method in the temperature range between 1005-1010 °C and 1015-1030 °C in oxygen and air atmospheres, respectively. The lattice parameters a and c of the monoclinic crystal structures as well as the unit cell volume V increase with increasing x, while b and β decrease for both series. The magnetic susceptibilities χ of both series show a very similar behavior on temperature as well as on Zn-doping, which is supposed to be due to the similar Cu-O coordination in both La₂Cu₂O₅ and La₈Cu₇O₁₉. For low Zn-doping (x?0.04), a spin-chain like behavior is found. This quasi-one-dimensional behavior is strongly suppressed in both series for x?0.04. Here, the maximum (characteristic for spin chains) in χ(T) disappears and χ(T) decreases monotonically with increasing temperature.     

Structural and magnetic characterization of Mn3IrGe and Mn3Ir(Si1−xGex): experiments and theory

The structural and magnetic properties of a new ternary Ir-Mn-Ge phase, Mn₃IrGe, as well as the solid solution Mn₃Ir(Si_1−xGe_x), 0?x?1, have been investigated by means of X-ray and neutron powder diffraction, magnetization measurements and first principles calculations. The crystal structure is cubic, of the AlAu₄-type (an ordered form of the β-Mn structure), Z=4, space group P2₁3, and the unit-cell dimension varies linearly with the silicon content. For all compositions, antiferromagnetic ordering is found below a critical temperature of about 225 K. The magnetic structure is noncollinear, as a result of frustrated magnetic interactions on a triangular network of Mn atoms, on which the moments rotate 120° around the triangle axes. The magnitude of the magnetic moment at 10 K is 3.39(4) μ_B for Mn₃IrGe. The theoretical calculations reproduce with very good accuracy the magnitudes as well as the directions of the experimentally observed magnetic moments.     

Synthesis and properties of the Co7Se8−xSx and Ni7Se8−xSx solid solutions

We report the synthesis and elementary properties of the Co₇Se_8−xS_x (x=0-8) and Ni₇Se_8−xS_x (x=0-7) solid solutions. Both systems form a NiAs-type structure with metal vacancies. In general, the lattice parameters decrease with increasing x, but in the Ni₇Se_8−xS_x system c increases on going from x=5 to 7. Magnetic susceptibility measurements show that all samples exhibit temperature-independent paramagnetism from 25-250 K. Samples within the Co₇Se_8−xS_x system, as well as Ni₇Se₈ and Ni₇SeS₇, were found to be poor metals with resistivities of ∼0.20 and ∼0.06 mΩ cm at 300 K, respectively. The Sommerfeld constant (γ) was determined from specific heat measurements to be ∼13 mJ/mol_CoK² and ∼7 mJ/mol_NiK² for Co₇Se_8−xS_x and Ni₇Se_8−xS_x, respectively.     

Structural characterization and physical properties of the system La1.33LixCrxTi2−xO6

New phases which arise from partial substitution of Ti⁴⁺ by Cr³⁺ and Li⁺ of the compound La_2/3TiO₃ have been obtained, giving rise to the series La_1.33Li_xCr_xTi_2−xO₆ (x=0.66, 0.55 and 0.44). These phases adopt a perovskite-type structure as deduced from their structural characterization. Rietveld's analyses of neutron diffraction data show that it is orthorhombic (S.G. Pbnm) with ordered domains. Conductivity has been examined by complex impedance spectroscopy and it increases with increasing lithium and chromium content. These materials behave as mixed conductors with low activation energies. Magnetic susceptibility variation with temperature shows antiferromagnetic interactions at the lowest temperatures.     

Asymmetric reactions of axially chiral amides: use of removable ortho-substituents in radical cyclizations of o-iodoacrylanilides and N-allyl-N-o-iodoacrylamides

Radical cyclizations of enantiomerically enriched o-iodoacrylanilides and N-allyl-o-iodoanilides bearing a removable ortho-substituent such as trimethylsilyl or bromine provide oxindoles and indoles in good yields and with good to excellent levels of chirality transfer from the N-Ar axis to the new stereocenter. Transition state models for the chirality transfer are suggested. Chemoselectivity of the radical cyclization in favor of the iodine in the case of 2-iodo-6-bromo-N-allylacrylamides has been exploited for the synthesis of chiral pyrroloquinolinones by a one-pot sequence of 5-exo and 6-endo cyclizations.     

Atomistic simulation on the site preference and mechanical properties of Th3Co4+xAl12−x and U3Co4+xAl12−x

An atomistic study is presented on the phase stability, site preference and lattice constants of the actinide intermetallic compounds Th₃Co_4+xAl_12−x and U₃Co_4+xAl_12−x. Calculations are based on a series of interatomic pair potentials related to the actinides and transition metals, which are obtained by a strict lattice inversion method. The lattice constants of Th₃Co_4+xAl_12−x and U₃Co_4+xAl_12−x are calculated for different values of x. The site preference of Co atoms at Al sites is also evaluated and the order is given as 6h, 4f, 2b and 12k for Th₃Co_4+xAl_12−x, and 6h, 4f, 12k and 2b for U₃Co_4+xAl_12−x. In addition, some simple mechanical properties such as the elastic constants and bulk modulus are investigated for the actinide compounds with complex structures.     

Fabrication and Optical Properties of Aligned Zn1-xMgxO Nanowire Arrays

Aligned Zn_1-xMg_xO nanowire arrays were successfully prepared on Si(111) substrates via the chemical vapor deposition (CVD) method with a mixture of ZnO, Mg, and activated carbon powders as reactants. The microstructures and optical properties of the synthesized Zn_1-xMg_xO nanowire arrays were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis X-ray (EDAX), and photoluminescence (PL) spectrum analytic apparatus, respectively. While Mg content was achieved or less approximately 0.29 (x, atomic ratio) in ZnO lattice, the crystal lattice of the synthesized samples exhibited wurtzite structure. The Mg atoms were distributed into the ZnO crystal as the interstitial and displaced atoms, and there was no phase separation in preparing Zn_1-xMg_xO nanowire arrays. However, as the Mg content was up to 0.53 (x) in the fabricated Zn_1-xMg_xO samples, a clear phase separation phenomena appeared in the Zn_1-xMg_xO crystal. Compared with the PL spectrum of the pure ZnO nanowire arrays, the analytic results showed that a blue-shift of the near-band edge emission with increasing Mg content was observed in the Zn_1-xMg_xO arrays. And the relative intensities of green peak at ca 535 nm and UV emission peak at ca 376 nm were all restrained.     

Catalytic and chemoselective deprotection of S,S- and S,O-acetals and ketals in the presence of their O,O-analogs with electrophilic halogens under neutral conditions

A novel and catalytic method is described for the selective deprotection of S,S- and S,O-acetals and ketals in the presence of their O,O-analogs to their corresponding carbonyl compounds based on the use of N-bromosuccinimide (NBS), N-chlorosuccinimide (NCS), 2,4,4,6-tetrabromo-2,5-cyclohexadiene-1-one (TABCO), trichlorocyanuric acid (TCCA) or molecular bromine as sources of electrophilic halogens in the presence of DMSO as the source of oxygen in CHCl₃.     

CuI/N,N-dimethylglycine-catalyzed synthesis of N-aryloxazolidinones from aryl bromides

CuI/N,N-dimethylglycine catalyzed coupling of aryl bromides with substituted oxazolidinones took place at 120 °C in DMF, affording the corresponding N-arylation products with good to excellent yields. A number of functional groups, such as ketone, nitrile, nitro, methoxy, and hydroxyl were tolerated under these conditions, thereby allowing diversity synthesis of N-aryloxazolidinones.     

Effects of Rh on the thermoelectric performance of the p-type Zr0.5Hf0.5Co1−xRhxSb0.99Sn0.01 half-Heusler alloys

We show that Rh substitution at the Co site in Zr_0.5Hf_0.5Co_1−xRh_xSb_0.99Sn_0.01 (0≤x≤1) half-Heusler alloys strongly reduces the thermal conductivity with a simultaneous, significant improvement of the power factor of the materials. Thermoelectric properties of hot-pressed pellets of several compositions with various Rh concentrations were investigated in the temperature range from 300 to 775 K. The Rh “free” composition shows n-type conduction, while Rh substitution at the Co site drives the system to p-type semiconducting behavior. The lattice thermal conductivity of Zr_0.5Hf_0.5Co_1−xRh_xSb_0.99Sn_0.01 alloys rapidly decreased with increasing Rh concentration and lattice thermal conductivity as low as 3.7 W/m*K was obtained at 300 K for Zr_0.5Hf_0.5RhSb_0.99Sn_0.01. The drastic reduction of the lattice thermal conductivity is attributed to mass fluctuation induced by the Rh substitution at the Co site, as well as enhanced phonon scattering at grain boundaries due to the small grain size of the synthesized materials.     

DFT study of the reaction sites of N,N′-substituted p-phenylenediamine antioxidants

The geometry of N,N′-diphenyl-p-phenylenediamine (DPPD), N-phenyl-N′-(1′-methylbenzyl)-p-phenylenediamine (SPPD), N-phenyl-N′-(1,3-dimethyl-butyl)-p-phenylenediamine (6PPD), N-phenyl-N′-isopropyl-p-phenylenediamine (IPPD), and N-(1-methyl-1-phenylethyl)-N′-phenyl-p-phenylenediamine (CPPD) as well as of their dehydrogenation products has been optimized at B3LYP/6-31G^∗ level of theory. Our results support the idea of formation of stable ketimine Ph-NC structures (instead of quinonediimine structures) during consecutive dehydrogenation of SPPD, 6PPD, and IPPD antioxidants despite the formation of tertiary carbon-centered radicals in the first dehydrogenation step is energetically preferred for SPPD only.     

Far-infrared and X-ray investigations of the mixed platinum dichalcogenides PtS2−xSex, PtSe2−xTex, and PtS2−xTex

Polycrystalline samples of the CdI₂-type mixed crystals PtS_2?xSe_x, PtSe_2?xTe_x, and PtS_2?xTe_x were prepared and characterized by far-infrared (4000-40 cm^?1) and X-ray techniques. A change in the behavior of the c/a-ratio and in the x dependence of the plasma resonance frequency of the free carriers present is observed in the system PtS_2?xSe_x near PtS_0.6Se_1.4, which is interpreted as a critical composition for (p,d)-band overlap in the platinum dichalcogenides. Optical phonon frequencies are given for the two-mode system PtS_2?xSe_x.