Study on the solid solution of YMn1−xFexO3: Structural, magnetic and dielectric properties

The solid solution of YMn_1−xFe_xO₃ (x=0.0, 0.1, 0.2, 0.3, 0.5, 1.0) was synthesized from the citrate precursor route. The hexagonal crystal structure related to YMnO₃ was stable for x?0.3. Rietveld refinement was carried out on the composition for x=0.3 and was refined to a major hexagonal phase (∼97%) with 3% of orthorhombic Y(Fe/Mn)O₃ phase. The a-axis lattice constant increases and the c-axis lattice constant decreases with x for x?0.2. The increase in the c-axis lattice constant at x=0.3 could be due to the doping of significant amount of d⁵ ion (high spin Fe³⁺ ion) in a trigonal bipyramidal crystal field. The detailed structural, magnetic and dielectric properties are discussed.     

ESR study of the photo-induced decomposition of polypropylene in the presence of N,N,N′,N′-tetramethyl-p-phenylenediamine

The mechanism of uv (λ > 325 nm) photodegradation of polypropylene (PP) containing N,N,N′,N′-tetramethyl-p-phenylenediamine (T₄MPD) has been investigated by means of ESR spectroscopy. The observed spectra after uv irradiation of both isotactic-PP (IPP) and stereoblock-PP (SPP) samples in vacuum at 77 K consisted principally of a broad singlet which was assigned to a T₄MPD cation radical (T₄MPD^+·). On the other hand, the spectrum observed after irradiation of an atactic polypropylene (APP) sample at 77 K in vacuum was resolved into several components which decayed almost up to ca. 263 K to give rise to the broad singlet of T₄MPD^+·. One component was a sharp quartet which was assigned to a methyl radical, ·CH₃·. The other component, a singlet, was attributed to a trapped electron, e_t^?.By comparison of the ESR spectrum of deuterated T₄MPD with that of the normal compound it was found that 60 ～ 70% of the methyl radicals arose from the added T₄MPD due to β-scission, which also formed the N,N,N′-trimethyl-p-phenylenediamine radical, T₃MPD·. The T₃MPD· radical presumably captures an electron at lower temperatures to become a carbanion, T₃MPD^?, which releases the electron to reproduce the T₃MPD· radical at elevated temperatures. This production of the radical T₃MPD· due to the liberation of an electron provides an explanation for the observed increase in intensity of the decay curve in the temperature range from ? 168 K to 185 K. The remaining fraction, 30 ～ 40%, of the total methyl radicals was produced from the PP matrix by an energy transfer from the excited T₄MPD¹ to the PP matrix. The broad singlet which appeared in the temperature range near 195 K was attributed to an acyl radical ～CH₂CH(CH₃)CH₂?O from the observed g-value. By photoillumination of this sample this broad singlet was converted reversibly into the quartet which was assigned to the radical ～CH₂CH(CH₂·)CH₂CHO.     

Ab initio study of the mechanism of the formation of p-rosolic acid from trifluoromethoxybenzene under HF/Lewis acid conditions

The mechanism of the formation of p-rosolic acid from trifluoromethoxybenzene under HF/Lewis acid conditions was studied using ab initio methods. A series of cationic intermediates is initiated by the expulsion of a fluorine atom of trifluoromethoxybenzene with anomeric assistance of the trifluoromethoxy oxygen. The resulting difluorocarbonium ion is attacked by a second trifluoromethoxybenzene to generate the first carbon-carbon bond on what will be the central carbon of p-rosolic acid. Elimination of phenol results in the formation of cationic intermediate 4 which is susceptible to carbon-alkylation by the same phenol to form the second carbon-carbon bond. Attack on subsequent difluorocarbonium ions by trifluoromethoxybenzene strips trifluoromethyl groups from the downstream intermediates, eventually leading to p-rosolic acid, and continues the generation of 4, each of which initiate a fresh mechanistic series toward another p-rosolic acid molecule.     

p-Type thermoelectric properties of the oxygen-deficient perovskite Ca2Fe2O5 in the brownmillerite structure

Brownmillerite calcium ferrite was synthesized in air at 1573 K and thermoelectric properties (direct current electrical conductivity σ, Seebeck coefficient α, thermal conductivity κ, thermal expansion α_L) were measured from 373 to 1050 K in air. Seebeck coefficient was positive over all temperatures indicating conduction by holes, and electrical properties were continuous through the Pnma-Imma phase transition. Based on the thermopower and conductivity activation energies as well as estimated mobility, polaron hopping conduction was found to dominate charge transport. The low electrical conductivity, <1 S/cm, limits the power factor (α²σ), and thus the figure of merit for thermoelectric applications. The thermal conductivity values of ∼2 W/mK and their similarity to Ruddlesden-Popper phase implies the potential of the alternating tetrahedral and octahedral layers to limit phonon propagation through brownmillerite structures. Bulk linear coefficient of thermal expansion (∼14×10⁻⁶ K⁻¹) was calculated from volume data based on high-temperature in situ X-ray powder diffraction, and shows the greatest expansion perpendicular to the alternating layers.     

Investigation of structural and hydrogen absorption properties in the LaNi5−xPtx-H2 system

The substitution of nickel by platinum in the binary LaNi₅ compound (CaCu₅ structure type, a=5.019(1) Å, c=3.981(1) Å, space group P6/mmm) and its effect on the hydrogenation properties was studied. The phase LaNi_5−xPt_x has a homogeneity domain ranging from x=0 to 5. For x<3, platinum substitutes almost exclusively on site 3g and also replaces nickel on site 2c for x>3. Contrary to what is observed in other systems, the hydrogen absorption plateau pressure was found to increase as a function of the cell volume. Powder neutron diffraction experiments were conducted for two deuterated compounds with x=0.25 and 0.75. Deuterium partial ordering occurs in the case of x=0.25 leading to a symmetry decrease to the space group P6mm (LaNi_4.75Pt_0.25D_5.23, a=4.225(1) Å, c=5.357(1) Å, Z=1, R_Bragg=3.3%). For x=0.75, an orthorhombic superstructure based on the CaCu₅-type lattice was found (LaNi_4.25Pt_0.75D_2.61, a_orth=√3a_hex=9.089(1) Å, b_orth=b_hex=5.272(1) Å, c_orth=2c_hex=8.145(1) Å, Z=4, SG Ibam, R_Bragg=6.1%).     

Synthesis of homologously pure bacteriochlorophyll-e and f analogues from BChls-c/d via transformation of the 7-methyl to formyl group and self-aggregation of synthetic zinc methyl bacteriopheophorbides-c/d/e/f in non-polar organic solvent

Homologously pure methyl bacteriopheophorbides-e and f (BPhes-e/f_M) were prepared from modification of naturally occurring bacteriochlorophylls-c and d (BChls-c/d), respectively, by transformation of the methyl to formyl group at the 7-position. The absolute configuration of the 1-hydroxyethyl group at the 3-position of (Zn-)BPhes-e/f_M was determined from comparison with structurally known BChl-c/d epimers. Visible spectra of synthetic (Zn-)BPhe-c/d/e/f_M showed that the 7¹-oxidation and the 8²/12¹/20-methylation affected Soret, Q_x and Q_y bands of both the monomeric (in a polar organic solvent) and oligomeric species (in a non-polar solvent).     

Structural behavior of the ferromagnetic spinels AlxMo2S4 and GaxMo2S4, containing tetrahedral clusters of molybdenum atoms

The structure of the ferromagnetic spinels Al_xMo₂S₄ and Ga_xMo₂S₄ (x ～ 0.5) was determined from powder diffraction data. The Al and Ga atoms order on the tetrahedral sites. The space group is $\text{F}\text{4}\text{3m; a = 9.726}$ Å for Al_xMo₂S₄ and 9.739 Å for Ga_xMo₂S₄. The Mo atoms were found to shift towards the tetrahedral site vacancies, created by the lower Al and Ga concentrations. This results in tetrahedral clusters of Mo around the vacancies. Their semiconducting and magnetic behavior was explained on the basis of the structural behavior of the molybdenum lattice in these spinel compounds.     

Oriented growth of Srn+1TinO3n+1 Ruddlesden-Popper phases in chemical solution deposited thin films

Oriented thin films of perovskite-related Sr_n+1Ti_nO_3n+1 Ruddlesden-Popper phases (n=1, 2, 3) were grown on (001) single-crystalline SrTiO₃ substrates. Preparation of the films was carried out by wet chemical deposition from metalorganic Sr-Ti solutions (rich in Sr) and subsequent conversion into the crystalline state by thermal treatment in air atmosphere at a maximum temperature of 700 °C. Solutions were prepared by a modified Pechini method. The films were investigated by wide-angle X-ray scattering and high-resolution transmission electron microscopy. The phase content of powders prepared from the dried solutions and annealed under similar conditions differed from that present in the films, i.e. only polycrystalline SrTiO₃ was detected together with oxides of Ti and Sr.     

Effects of Pressure and Temperature on the Formation of Tungsten-Bronze-Related Phases, RExWO3+y, with RE=La and Nd

The influence of pressure (P) and temperature (T) on the formation of tungsten-bronze-related phases containing lanthanum and neodymium was investigated. A large number of samples with bulk compositions RE_xWO₃, prepared by solid-state reaction in the pressure and temperature regions P= 10-80 kbar and T= 1170-1620 K were examined by X-ray powder diffraction and electron microscopy, and a (P-T) diagram showing the phase relations was drawn. Three tungsten-bronze-related phases with perovskite (PTB)-, hexagonal (HTB)- and intergrowth (ITB)-type structures were identified. The PTB bronze RE_xWO₃ with x≈ 0.10 was formed at p≤50 kbar. The HTB-related phase with x≈ 0.10 was observed in samples prepared at P≥20 kbar, whereas phases of (n)-ITB-type were observed only in the 25-50 kbar region. In the latter pressure region, the PTB and ITB phases were only seen in samples prepared at T > 1520 K, while the HTB-related phase was found in almost all samples. The HTB- and ITB-related compounds are metastable, probably fully oxidized, high-pressure phases of composition RE_xWO_3+3x/2 with x≤0.13. They transform to a cubic PTB bronze during annealing in inert atmosphere under ambient pressure conditions. According to microanalysis studies of individual crystals, less than 40% of the hexagonal tunnel sites in the HTB and ITB structures are occupied by RE³⁺ ions. A superstructure of HTB-type with ≈60% occupancy of the hexagonal tunnel sites (x≈0.20) was observed in a few crystals from the samples prepared at P= 80 kbar. Ordered, defect and intergrowth structures are presented.     

The crystal structure of 3-R Nb1.06S2

The X-ray single crystal structure of 3-R Nb_1.06S₂ has been determined. The material crystallizes in the space group R_3m with a = 3.3285(4) and c = 17.910(4) Å when indexed on a hexagonal lattice. The structure was refined by full matrix least squares procedures to a final residual of R = 0.026 based on 79 observed (I > 3σ_I) reflections. The sulfurs form closest-packed layers with the majority of the metal in sheets of trigonal prismatic sites. A small portion of niobium was found to occupy octahedral sites, between the van der Waals gaps of the sulfur lattice. Niobium in the van der Waals region is trigonally distorted from octahedral symmetry, with niobium-sulfur distances of 2.234(8) and 2.577(11) Å, because of repulsion from niobium in adjacent trigonal prismatic layers.     

A direct route for the preparation of Fmoc/OtBu protected iodotyrosine

Herein the synthesis of an Fmoc/O^tBu orthogonally protected iodotyrosine derivative is reported. This has been achieved via a simple two-step process in an overall 58% yield from commercially available Fmoc-Tyr(^tBu)-OH. The Fmoc/O^tBu orthogonally protected iodotyrosine was also shown to be amenable to Suzuki-Miyaura cross-coupling to deliver a novel bi-aryl tyrosine derivative.     

An investigation of the Cr1−xMnxO2 series and the characterization of orthorhombic CrMnO4

Several members of the Cr_1?xMn_xO₂ series were prepared in the tetrahedral anvil press by the reaction of CrO₂ with MnO₂. The tetragonal, rutile-type products were single-phase and have been characterized by crystallographic and magnetic measurements. The results are consistent with the formulations Cr⁴⁺_1?2xCr³⁺ Mn⁵⁺O₂ for 0 ? x ? 0.5. At low manganese concentration, x < 0.20, the magnetic moments are consistent with ferromagnetic contribution from Mn⁵⁺. A two-phase product was noted at the composition x = 0.90. The CrMnO₄ composition was found to have a powder pattern similar to that of orthorhombic PtO₂.     

(Ca/Sr)AuxCd1−x: Stacking variants of the CrB-FeB series

The structural chemistry of binary 1:1 alkaline earth metallides A^IIM (M=p-block or late transition element) is dominated by planar M zig-zag chains, which are stacked in different orientations (CrB (c) to FeB (h) type) and with variable stacking distances (types I and II). As a case study of the electronic influences, the substitution of Au against Cd in the respective Ca and Sr aurides was examined by means of experimental, crystallographic and computational methods. Starting from CaAu, up to 11% of Au can be substituted by Cd without a change in the CrB structure type (orthorhombic, space group Cmcm, a=398.2(1), b=1122.6(6), , Z=4, R1=0.0303). Starting from SrAu (stacking sequence (hc)₂(h₂c)₂), depending on the proportion of the Cd substitution a successive change to structures with increased hexagonality is observed: In SrAu_0.93Cd_0.07 (monoclinic, space group P2₁/m, a=621.3(4), b=472.4(2), , β=96.97(5)^°, Z=6, R1=0.0467) the stacking sequence is h_2c, i.e. the hexagonality is 66.67%. A slightly more increased Cd content in SrAu_0.78Cd_0.22 (orthorhombic, space group Pnma, a=3243.3(8), b=474.17(8), , Z=16, R1=0.0682) drives the hexagonality to 75%, with a (h₃c)₂ stacking sequence known from several rare earth nickel compounds. Further Cd substitution is not possible. However, in the Cd-rich section of the two series, where the CsCl/β-brass structure type occurs for both alkaline earth elements, a small Au substitution, as determined from powder data by Rietveld refinements, is possible. The substitution limit and the stability ranges of the CsCl and the CrB type can be rationalized from the calculated band structures. Geometrical and electronic criteria are used to compare and discuss the stability ranges in a structural map.     

Phase Diagram and Phase Transitions in the Relaxor Ferroelectric Pb(Fe2/3W1/3)O3-PbTiO3 System

A complete solid solution between relaxor ferroelectric Pb(Fe_2/3W_1/3)O₃ (PFW) and ferroelectric PbTiO₃ (PT), (1−x)PFW-xPT, was synthesized by a B-site precursor method and characterized by X-ray diffraction, differential scanning calorimetry, and dielectric measurements. A phase diagram between PFW and PT has been established. The diffuse phase transition temperature (T_max≈180 K) of PFW was found to continuously increase with the increasing amount of Ti⁴⁺ ions on the B-site. At the same time, the relaxor ferroelectric behavior of PFW is gradually transformed toward a normal ferroelectric state, as evidenced by sharp and nondispersive peaks of dielectric permittivity around T_C for x≥0.25. At room temperature, a transition from a cubic to a tetragonal phase takes place with x increased up to 0.25. A morphotropic phase boundary is located within the composition interval 0.25≤x≤0.35, which separates a pseudocubic (rhombohedral) phase from a tetragonal phase.     

Investigation of the effect of epoxide structure on the initiation efficiency in isobutylene polymerizations initiated by epoxide/TiCl4 systems

The effect of the chemical structure of styrene-based epoxides, namely, styrene epoxide (SE), α-methylstyrene epoxide (MSE), p-methylstyrene epoxide (pM-SE) and α-methyl-p-methylstyrene epoxide (pM-MSE), in conjunction with TiCl₄, on the initiation efficiency (I_eff) in the carbocationic polymerization of isobutylene (IB) was investigated. SE yielded living polymerization, but the initiation efficiency was low when compared to MSE (I_eff=8% and 35%, respectively). pM-SE led to non-living IB polymerization, while pM-MSE revealed linear M_n-conversion plot and narrow MWD with a non-linear first order rate plot. Among the epoxides investigated, MSE was the best initiator to scale up the one-step synthesis of polyisobutylenes (PIBs) carrying one primary hydroxyl head group and one tertiary chloride end group. The hydroxyl functionality of these PIBs determined by ¹H-NMR was F_n=1.09±0.16 from 24 experiments.     

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.     

Heat capacity of iron-chromium spinels,Fe3−xCrxO4

The heat capacity of Fe_3?xCr_xO₄ with the composition x = 0.6, 0.8, and 1.0 was measured from 200 to 850 K. A γ-type heat capacity anomaly due to the ferri-paramagnetic transition was observed for all compositions. The transition temperatures were 652, 563, and 451 K for the compositions x = 0.6, 0.8, and 1.0, respectively. The variation of transition temperature with composition is discussed in terms of cation distribution. The magnetic contribution to the observed heat capacity was obtained by assuming that the heat capacity is expressed by the sum of the lattice heat capacity C_v(1), the dilation contribution d(d), and the magnetic contribution C(m). Entropy changes due to the transition were calculated from C(m) as 52.6, 49.7, and 46.3 J K^?1 mole^?1 for the compositions x = 0.6, 0.8, and 1.0, respectively, which are from 7 to 12 J K^?1 mole^?1 higher than the calculated values based on the assumption of randomization of unpaired spins on each ion. The difference between the observed and the calculated values is roughly explained by taking into account the orbital contribution of Fe²⁺ ions on octahedral and tetrahedral sites.     

On the conformational characteristics of the O,O,O-trimethyl selenophosphate molecule

Spectroscopic and theoretical studies were carried out for O,O,O-trimethyl selenophosphate molecule. DFT structural and vibrational calculations were performed at 6-311++G^∗∗ level. Ar/matrix-FTIR spectra were recorded. A coexistence of different conformers with C₃ and C₁ symmetries was detected at different temperatures. Spectral evidence of a lower energy C_s conformer was found. These conclusions are consistent with the results from DFT calculations. A tentative assignment of the features observed in the Ar/matrix-FTIR spectra is proposed.     

Double crystallization behavior in dry-jet wet spinning of cellulose/N-methylmorpholine-N-oxide hydrate solutions

The structure and resultant mechanical properties of fibers in the dry-jet wet spinning process of cellulose solutions in N-methylmorpholine-N-oxide (NMMO) hydrates were investigated in terms of molecular weight of cellulose, concentration, and hydration number (n) of NMMO hydrate. The value of n had an effect on the crystallization behavior of the cellulose solution system, which influenced the resultant fiber structure. Increasing cellulose concentration and decreasing the value of n retarded crystallization because of the increased interactions between cellulose and NMMO hydrate. Reducing the value of n from 1 to 0.72 produced more highly oriented cellulose fibers. However, incorporating n-propyl gallate, an antioxidant, had little effect on the fiber structure. When n=0.72 a double crystallization behavior was observed in the fiber spinning process irrespective of molecular weight of cellulose and concentration over the experimental ranges examined. It should be noted that such a double crystallization took place in the absence of foreign additives. The double crystallization behavior was more noticeable when the aspect ratio of spinning nozzle was greater. The double layer structure had a positive effect on the mechanical strength.     

Resolution enhanced Ni Kα and Kβ spectra obtained by the deconvolution method

Ni K_α, and K_β spectra of NiO, NiF₂, and NiS were measured using a vacuum two-crystal high resolution X-ray spectrometer. In order to make clear fine structures due to multiplet splitting, numerical deconvolution with the lifetime broadening factor as a deconvoluting function was carried out in the frequency domain using a successive pseudo-deconvolution technique. Based on extensive simulation studies, the most appropriate polynomial smoothing function was chosen from among those given by Savitzky and Golay [Anal. Chem.36. 1627 (1964)]. The quartic 17-point and 11-point smoothing convolute functions were used for processing observed K_β and K_α spectra. Results obtained by a simple jj-coupling model including exchange interactions between two incomplete shells do not seem to be consistent with the deconvolution results.