Phase-transition studies of 5O.5 and 9O.4

As part of our detailed comparative studies of groups of liquid-crystalline compounds that belong to a homologous series, we present phase-transition studies of the compounds N-(4-n-pentyloxybenzylidene)4'-n-pentylaniline (5O.5) and N-(4-n-nonyloxybenzylidene)4'-n-butylaniline (9O.4) using different experimental techniques. The compound 5O.5 is reported to exhibit a phase sequence N, S_A, S_C, S_B and S_G, while 90.4 shows the sequence S_A, S_F and S_G. The salient features of our work on 5O.5 are (i) a new smectic F phase is found in place of the reported smectic B phase, which is confirmed by both miscibility and X-ray studies; (ii) the formation of smectic-C-like short-range order in the nematic phase well above the S_A-N transition; and (iii) a large tilt-angle variation in the smectic C phase (0-23·5°C) in a small temperature range (4·1°C). The phase changes across the S_A-I transition, and for the first time across S_F-S_A transition, are carried out by volumetric studies. The detailed inferences of these are also presented.     

Phase transition studies in 9O.6 and 9O.8

The variation of density with temperature across different phases for two homologues of the N(4-n-alkyloxybenzylidene)-4'-n-alkylanilines (9O.6 and 9O.8) exhibiting the S_AS_CS_FS_G phase variant is determined. The influence of pretransitional effects in the fluctuation dominated non-linear region in the vicinity of the isotropic-smectic A transition and the nucleation growth analogy of this transition are discussed.     

Laser Raman studies on compounds 7.O4O.7 and 7.O5O.7

Laser Raman studies on two compounds of the α,ω-bis(4-alkylanilinebenzylidene -4′-oxy)alkane series were carried out in the spectral regions 1140–1220 and 1550–1650 cm^-1 as a function of temperature. Compounds 7.O4O.7 and 7.O5O.7, exhibit SmA and SmF phases. The structural differences between these liquid crystal dimers, having either an odd or an even number of carbon atoms in the spacer, are remarkably existent in the room temperature Raman spectra. The results are rationalized on the basis of the tendency of these compounds to exhibit bent shapes, and how this manifests in the odd-even effect at the molecular level assuming a semi-rigid core region of the dimeric molecule. It is found that the behaviour of the odd spacer dimer (7.O5O.7) agrees with the molecular model, whereas the even spacer dimer (7.O4O.7) behaves in a similar fashion to monomeric compounds such as the nO.m. compounds. In both cases, dynamic changes around the C=N bond have a profound effect both on the molecular shape in the different phases and on phase behaviour.     

Neutron diffraction studies of U4O9: comparison with EXAFS results

Conradson et al. have analyzed X-ray absorption fine-structure spectra of the UO2-U4O9 system and concluded that oxygen atoms are incorporated in U4O9 as oxo groups with U-O distances in the range 1.72-1.76 A. They also found that the uranium sublattice consists of an ordered portion and an additional 'spectroscopically silent' glassy portion. We have carried out studies of powdered U4O9 by neutron diffraction which contradict these conclusions from EXAFS measurements. Our analysis shows that there are no U-O bonds shorter than 2.2 A and that U4O9 is crystallographically ordered with no evidence of a glassy structure.     

Structural and Raman Spectroscopic Investigations of K4BaSi3O9 and K4CaSi3O9

The crystal structures of K₄BaSi₃O₉ and K₄CaSi₃O₉ have been characterized by X‐ray diffraction techniques as well as Raman spectroscopy. The structure of K₄CaSi₃O₉ has been refined from powder diffraction data via the Rietveld method using polycrystalline material prepared from solid state reactions. The compound is isostructural with form I of K₄SrSi₃O₉. It crystallizes with 16 formula units in a cubic primitive cell (a = 15.94014(3) Å, V = 4050.20(1) ų) and adopts space group . K₄CaSi₃O₉ belongs to the group of cyclosilicates and contains highly puckered twelve‐membered [Si₁₂O₃₆]‐rings centered on the . Five of the seven crystallographically independent alkaline and alkaline earth cations are surrounded by six oxygen ligands in the form of distorted octahedra, which share opposite triangular faces and form non‐intersecting columns parallel to the body diagonals of the cubic unit cell. This arrangement corresponds to one of the cubic cylinder or rod packings. The two remaining sites have more irregular coordination environments with eight next oxygen neighbors. High temperature X‐ray powder diffraction data have been collected to determine the thermal expansion of this material: between room temperature and 700 °C the coefficient of thermal expansion has a value of α = 12.9(2) × 10^?6 [°C^?1]. Single crystals of K₄BaSi₃O₉ have been obtained from the devitrification of a glass with the same composition. The structure was determined from a single crystal diffraction data set collected at ?100 °C and refined to a final R index of 0.0298 for 1288 observed reflections (I > 2σ(>I)). The compound is isostructural with modification II of K₄SrSi₃O₉. Basic crystallographic data are as follows: space group Ama2, a = 11.0695(15) Å, b = 8.0708(10) Å, c = 11.905(2) Å, V = 1063.6(3) ų, Z = 4. With respect to the silicate anions the material can be classified as a sechser single chain silicate. The crankshaft‐like chains run parallel to [100] and are linked by K and Ba cations, which are distributed among five crystallographically independent sites. The coordination polyhedra of two of the non‐tetrahedral cations can be described by distorted octahedra sharing opposite triangular faces. They build non‐intersecting columns parallel to [011] and [0‐11], respectively. The other sites exhibit more irregular coordination spheres with 7‐9 neighbours.     

Mixed crystal formation and structural studies in the mullite-type system Bi2Fe4O9–Bi2Mn4O10

The limits of metal cation substitution and distribution in the sequence Bi₂Fe₄O₉–Bi₂Mn₄O₁₀ have been investigated by solid state synthesis, X-ray powder diffraction, and Mössbauer spectroscopy. Rietveld refinements conducted for the entire range along the join indicate the structures are orthorhombic with space group Pbam, with partial transition-metal site disorder confirmed and detailed by Mössbauer spectroscopy. Single-phase regions are found near each end-member and a two-phase region is observed at intermediate compositions, extending from about x=1 to 3, according to the general formula of the mixed crystals Bi₂Fe_4?xMn_xO_10?δ. An incorporation of Mn at octahedral sites replacing Fe is taken into account for the Bi₂Fe₄O₉-related side of the system. Charge compensation is believed to be effected by addition of O, which gives rise to the formation of FeO₅ pyramids. At the Bi₂Mn₄O₁₀-related side of the system, substitution of pyramidal Mn³⁺ by Fe³⁺ is envisaged.     

Reactions of alkoxy radicals with O2. III. i-C4H9O radicals

i-C₄H₉ONO was photolyzed with 366-nm radiation at ?8, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yield of i-C₃H₇CHO, Φ{i-C₃H₇CHO}, was measured as a function of reaction of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?₁ = 0.24 ± 0.02 independent of temperature. The i-C₄H₉O radicals can react with NO by two routes The i-C₄H₉O radical can decompose via or react with O₂ via Values of k₄/k₂ ? k_4b/k₂ were determined to be (2.8 ± 0.6) × 10¹⁴, (1.7 ± 0.2) × 10¹⁵, and (3.5 ± 1.3) × 10¹⁵ molec/cm³ at 23 55, and 88°C, respectively, at 150-torr total pressure of N₂. Values of k₆/k₂ were determined from ?8 to 120°C. They fit the Arrhenius expression: For k₂ ? 4.4 × 10¹¹ cm³/s, k₆ becomes (3.2 ± 2.0) × 10^?13 exp{?(836 ± 159)/T} cm³/s. The reaction scheme also provides k_4b/k₆ = 3.59 × 10¹⁸ and 5.17 × 10¹⁸ molec/cm³ at 55 and 88°C, respectively, and k_8b/k₈ = 0.66 ± 0.12 independent of temperature, where      

Darstellung und kristallographische Daten von K2Si2O5, KHSi2O5I und K2Si4O9

K₂Si₂O₅ KHSi₂O₅I und K₂Si₄O₉ have, been synthesized partly by annealing and partly by hydrothermal treatment of glasses of suitable chemical composition. From powder and single crystal x-ray diagrams the lattice dimensions have been determined (see: ?Inhaltsübersicht”?).     

Phase transition in V5O9

Electrical resistivity, magnetic susceptibility, and heat capacity of V₅O₉ have been measured through its phase transition temperature (T_t) around 129 K. The associated changes in enthalpy and entropy were found to be 2095 J/mole and 19.24 J/mole · deg., respectively. A qualitative thermodynamic analysis has been attempted to correlate the crystal symmetry, electrical, magnetic, and heat capacity behavior at T_t. The metal-semiconductor transition appears as a consequence of the crystallographic order-disorder process, since the electrical and magnetic contributions to configurational entropy change are relatively small.     

Über die Silbergermanate Ag4Ge9O20 und Ag2Ge4O9

Zusammenfassung Der Germanat-Zeolith Ag₃HGe₇O₁₆·4 H₂O wird durch Ionenaustausch aus dem isotypen Ammonium-Zeolith hergestellt. Nach Dehydratation des Ag-Zeoliths bildet sich bei 460°C das zuMe ₄Ge₉O₂₀ (Me=Na, K) isotype Ag₄Ge₉O₂₀. Durch Erhitzen auf 650°C entsteht das zuMe ₂Ge₄O₉ (Me=Na, K, Rb, Tl) isotype Ag₂Ge₄O₉. Oberhalb 760°C erfolgt unter Dissoziation die Bildung von metallischem Silber und GeO₂ (Rutilform).

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The zeolithic germanate Ag₃HGe₇O₁₆·4 H₂O has been prepared by ion-exchange from the analogous ammonium compound. After dehydration at 460°C Ag₄Ge₉O₂₀ is formed, which is isostructural withMe ₄Ge₉O₂₀ (Me=Na, K). By heating up to 650°C Ag₂Ge₄O₉ is obtained, having the structure ofMe ₂Ge₄O₉ (Me=Na, K, Rb, Tl). Above 760°C the compound decomposes into silver, oxygen and GeO₂ (rutile).

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Ab initio studies of structural and electronic properties of Li4Ti5O12 spinel

Structural and electronic properties of Li₄Ti₅O₁₂ spinel are studied from density functional theory based first principles calculations. Differences on these properties between delithiated state Li₄Ti₅O₁₂ and lithiated state Li₇Ti₅O₁₂ are compared. The optimized lattice constant of Li₄Ti₅O₁₂ is 8.619 Å, which is even a little larger (0.2%) than 8.604 Å of the lithiated state Li₇Ti₅O₁₂. The arrangement of the Li and Ti atoms at the 16d sites of the spinel structure is also investigated in a cubic unit cell. Large 1 × 1 × 3 supercell models are constructed and used to calculate the total energy and electronic structure. The average intercalation potential is also calculated, with metallic lithium as reference.     

ChemInform Abstract: Preliminary Crystal Structure of Mixed-Valency Sr4Ni3O9, the Actual Formula of the so-called Sr5Ni4O11.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Perovskite related phases in the Sr5Nb4O15 - SrTiO3 - Sr4Nb2O9 system

Phase relations have been investigated within the Sr₅Nb₄O₁₅−SrTiO₃−Sr₄Nb₂O₉ region of the SrO−Nb₂O₅−TiO₂ system with a view to clarifying the occurrence of fully oxidised perovskite related phases. Overall phase analysis was carried out by powder X-ray diffraction and microstructures were clarified by transmission electron microscopy. There is only one main composition triangle in this area at 1350°C. The tie-line between Sr₅Nb₄O₁₅ and SrTiO₃ contains a homologous series of hexagonal layered perovskite phases including Sr₆Nb₄TiO₁₈ and Sr₇Nb₄Ti₂O₂₁. The phase Sr₄Nb₂O₉ is a nonstoichiometric phase with a disordered perovskite structure. There is some extension of this phase along the Sr₄Nb₂O₉−SrTiO₃ tie-line, but SrTiO₃ does not show a significant composition range. Samples with a composition Sr₄Nb₂O₉, when heated at 900°C show several ordered modifications. Samples along the Sr₄Nb₂O₉−SrTiO₃ tie-line which are annealed at 900°C contain these ordered materials together with samples showing considerable short range order which increases as the Ti content increases.