The structure of wholly aromatic polyesters with bulky side chains: Poly(p-phenylene phenylthioterephthalate)

Poly(p-phenylene phenylthio-terephthalate) (PPTT) forms nematic melts and is highly crystalline in the solid state, despite the probable random 2- and 3-dispo-sition of the S-phenyl substituents. The X-ray pattern of melt-spun fibers of PPTT contains 24 Bragg reflections that are indexed by a monoclinic unit cell with dimensions a = 28.6 Å, b = 4.81 Å, c = 12.57 Å (fiber axis), and γ= 101.6°. The cell contains monomer units of four chains that are arranged in pairs with the thiophenyl side chains interdigitated; successive pairs of chains are staggered by about c/2. We used molecular mechanics modeling to simulate arrays of chains with random 2- and 3-disposition of the side chains on the terephthalic acid units and compared the results with those for an idealized structure in which all the substituents were at the 2-position. The refined model for random substitution is more distorted, but the average separations of the monomer units are within the experimental errors of the observed unit cell dimensions, and their standard deviations are very similar to those derived from the line-broadening data. The potential energy of the model with random substitution is only about 1.9 kcal/mol of monomer higher than that for the model with all-2-substitution, indicating random substitution is not a major problem to the formation of an ordered structure. Compared to the structure formed by the analogous polyester, poly(p-phenylene phenylterephthalate) (PPT), the additional flexibility due to the thioether linkage between the backbone and phenyl side groups in PPTT allows better chain packing both within and between the layers of stacked chains.     

Molecular Packing in Crystalline Domains of Unsymmetrical Poly(benzoxazole-imide): Investigation using Modeling of a Model Compound

To probe the molecular packing in crystalline domains of an unsymmetrical poly(benzoxazole-imide) (BPDA-BOA) introduced alternating phenylbenzoxazole and bisphthalimide units via the two-step polymerization of 5-amino-2-(4- aminophenyl)benzoxazole (BOA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), the crystal structure of its model compound, 5-phthalimido-2-(4-phthalimidophenyl)benzoxazole (PA-BOA), was investigated using powder wide-angle X-ray diffraction (WAXD) combined with molecular modeling using a Materials Studio program. Powder WAXD pattern of the model compound can be well indexed in terms of amonoclinic unit cell with parameters of a = 12.005 Å, b = 3.837 Å, c = 23.562 Å, β = 96.711°. There are two molecules in the unit cell with the space group of P2 (3). Based on the crystal structure of the model compound powders and the WAXD analysis of poly(benzoxazole-imide) film, it can be concluded that the projection of the monomer repeat length of the poly(benzoxazole-imide) chains in the chain directionis around 19.0 Å, and the interchain side-by-side distance and face-to-face distance between the centroids of two neighboring BPDA-BOA chains are around 6.10 and 4.01 Å, respectively. The difference of molecular packing between the poly(benzoxazole-imide) chains in their crystalline domains and the model compounds in their crystals are also presented. Both the side-by-side and face-to-face π-π stacking distances between two neighboring polymer chains in the poly(benzoxazole-imide) crystalline domains are significantly larger than the corresponding values between two neighboring molecules in the model compound crystals due to a more kinked and twisted conformation.     

AgTh2(PO4)3 — A new case of the non-hydrogen bonded phosphate ferroelectric

The basic crystallographic data of AgTh₂(PO₄)₃ single crystals have been determined for the first time by means of X-ray diffraction methods. The crystals are monoclinic, space group isCc with four formula units per unit cell. The dimensions of the unit cell are:a=17·385 Å,b=6·815 Å,c=8·148 Å,β=101·10°. Using the Sawyer and Tower method it has been proved that the crystals possess ferroelectric properties. Performing the measurements at room temperature the values of spontaneous polarization and coercive field in the direction normal to (100) face have been determined.     

Reactivity of 2‐aryl‐1,3‐dithiane anions towards neopentyl,neophyl and phenyl iodides. New evidence for an SRN1 mechanism

The reactions of 2‐(4‐Z‐phenyl)‐1,3‐dithiane anions (Z = H, OMe, Cl, CN) with neopentyl, neophyl and phenyl iodides were studied in DMSO, taking into consideration the effect of the Z substituent on the dithiane anions reactivity as well as on the product distribution. These substitution reactions proceed by an S_RN1 mechanism with radicals and radical anions as intermediates. Two competitive pathways are possible for the radical anion of the substitution product, namely electron transfer (ET) to the substrate giving the substitution product and C–S bond fragmentation to yield a distonic radical anion. ET is the main pathway for the reactions between dithiane anions bearing electron‐donor substituents and neopentyl or its analogue iodides affording the substitution products in moderate yields (41–53%). Copyright © 2011 John Wiley & Sons, Ltd.     

Tetragonal rare earth (R) Iron borides,R1+εFe4B4 (ε≅0.1), with incommensurate rare earth and iron substructures

Crystallographic and magnetic properties of a new structural series of ternary borides with composition R_1+εFe₄B₄ (R = Ce, Pr, Nd, Sm, Gd, Tb, 0.11(Pr) ≤ε≤ 0.15(Tb) are reported. The compounds are built of incommensurate substructures of rare earth atoms (linear strings ?? c?), iron atoms (chains of edge sharing tetrahedra ?? c?),and boron atom pairs. A single crystal X-ray diffraction study of one representative (Sm_1.13Fe₄B₄) based on a commensurate structure model (composition : Sm₁₇(Fe₄B₄)₁₅, a = 7.07 Å, c ≡ 17c_Sm ≡ 15c_Fe= 58.69 Å, space group P4₂/n) revealed a periodic twist modulation of the Fe tetrahedra chains around c?. Magnetic susceptibility measurements on single crystals of another representative (Nd_1.11Fe₄B₄) revealed ferromagnetic ordering at T_c = 13 K. Above this temperature the magnetic properties are dominated by ferromagnetic inclusions (Fe₂B, Nd₂Fe₁₄B).     

The ferroelectric phase transition in tridymite type BaAL2O4 studied by electron microscopy

The paraelectric-ferroelectric (PE-FE) phase transition in stuffed tridymite BaAl₂O₄ was studied in situ by transmission electron microscopy. Electron diffraction revealed that the PE and FE phases have hexagonal symmetry. The PE-FE phase transition is accompanied by a doubling of the cell dimensions in the a-b plane. The transition is reversible, takes place over a wide temperature range (400–670 K.) and the interfaces related to the transition have a fluctuating character. The crystal structure of the high temperature PE phase was determined by high resolution electron microscopy. The structures of the PE phase (space group P6₃22, a≈ 5.22 Å, c ≈ 8.8 Å) and of the FE phase (space group P6₃, a= 10.4469(1)Å, c = 8.7927(1)Å) differ mainly by the configuration of the Al-O strings oriented along the c-axis. In the PE phase all the strings are equivalent whereas straight and corrugated strings alternate in an ordered manner in the FE phase resulting in doubling of the a and b cell parameters. Translation and orientation domains due to the decrease of the translation and point symmetry were frequently observed.     

Preface

Phenylethyl ammonium trichloromercurate exhibits a structural phase transition at 402K Phase I—(402K)→ Phase II

This transition has been characterized by differential scanning calorimetry, dielectric measurements and X-ray diffraction. The space groups and the cell parameters of both phases were determined by X-ray diffraction from single crystals and powder samples. Phase I has space group I2 or I2/m, a = 25.88(2) Å, b = 7.792(3) Å; c = 5.971(4) Å; β = 96.14(1)°. Phase II has orthorhombic symmetry, space group Cmm2 or C222 with a = 25.91(1) Å; b = 7.836(5) Å; c = 6.116(4) Å.     

The polymorphism of poly(vinylidene fluoride). I. The effect of head-to-head structure

The phenomenon of polymorphism in poly(vinylidene fluoride) has been observed recently by several authors. It has also been reported that high-resolution NMR measurements demonstrate the presence in this polymer of head-to-head linkages, resulting from the “backward” addition of from 5-6% of the monomer units. Since the van der Waals radii of fluorine (1.35 Å) and hydrogen (1.1-1.2 Å) are similar, the cocrystallization in a polymer chain of units that differ only by the substitution of fluorine atoms for hydrogen atoms is not unexpected. The two polymorphic forms of poly(vinylidene fluoride), examined in this investigation, have different chain conformations. Chains in phase I have a planar zigzag conformation, while chains in phase II are assumed to exhibit a 2₁ helical conformation. The incorporation into the polymer chain of small amounts of tetrafluoroethylene or trifluoroethylene comonomer favored the crystallization of phase I. This is in accord with the relative abilities, deduced from consideration of atomic size, of these comonomers to cocrystallize with vinylidene fluoride units in the two indicated chain conformations of the polymer. Since tetrafluoroethylene units are present in the head-to-head structure in the homopolymer, it can be concluded that the elimination of the head-to-head structure will eliminate or restrict crystallization in phase I.     

Dielectric permittivity and AC conductivity investigations of the structural phase transitions in (NH3)2(CH2)7CdCl2Br2

The AC dielectric permittivity as a function of temperature (100–400 K) at different frequencies (between 80 Hz and 20 kHz) for the layered alkylene diammonium insulator containing Cd, namely, [(NH₃)₂(CH2)₇CdCl₂Br₂] has been measured. The formation of the compound was confirmed by microchemical analysis and IR absorption spectrometry. X-ray powder diffraction indicates an orthorhombic unit cell of dimensions: a = 10.219(2) Å, b = 9.168(2) Å and c = 38.694(4) Å. The AC conductivity ([sgrave]) is presented as a function of temperature and frequency. The conductivity and permittivity results indicate the presence of first-order phase transitions at 317 and 345 K. This has been confirmed by thermal analysis techniques. The activation energies were of values ranging between 0.15 and 0.62 eV depending upon the temperature range and the applied frequencies.     

Effects of neutral and charged substituents on the infrared carbonyl stretching frequencies in phenyl and alkyl benzoates in DMSO

The carbonyl infrared stretching frequencies for 57 meta‐, para‐ and ortho‐substituted phenyl benzoates, C₆H₅CO₂C₆H₄‐X and alkylbenzoates, C₆H₅CO₂R, containing besides neutral substituents the charged substituents in phenoxy and alkoxy part in dimethyl sulfoxide (DMSO) have been recorded. The carbonyl stretching frequencies, ν_CO, for meta‐ and para‐substituted phenyl esters of benzoic acids in the case of neutral substituents were found to correlate well with the substituent constants, σ°. The ν_CO values for ortho derivatives correlated with the inductive substituent constants, σ_I, only. The values of constants for charged substituents, σ°_±, calculated on the basis of the ν_CO and the ¹³C NMR chemical shifts, δ_CO, in DMSO agree well with the σ°_± values for the corresponding ion pairs reported by Hoefnagel and Wepster and those determined from the log k values of the alkaline hydrolysis in 4.4 M NaCl solution at 50 °C. Thus, the values of substituent constants for ion pairs of charged substituents estimated on the basis of aqueous data could be successfully used in non‐aqueous solution (DMSO) simultaneously with neutral substituents in case the charged substituents were not completely ionized and are in ion pair form. Copyright © 2016 John Wiley & Sons, Ltd.     

Potts model,Dirac propagator,and conformational statistics of semiflexible polymers

A new discretized version of the Dirac propagator ind space and one time dimensions is obtained with the help of the 2d-state, one-dimensional Potts model. The Euclidean version of this propagator describes all conformational properties of semiflexible polymers. It also describes all properties of fully directed self-avoiding walks. The case of semiflexible copolymers composed of a random sequence of fully flexible and semirigid monomer units is also considered. As a by-product, some new results for disordered one-dimensional Ising and Potts models are obtained. In the case of the Potts model the nontrivial extension of the results to higher dimensions is discussed briefly.     

Phase transition in a chloro-elpasolite,Cs2KInCl6

Using high-purity starting materials, we synthesized a new room-temperature Cs₂KInCl₆ phase (I): monoclinic (C2/c), a = 25.484(11), b = 7.699(2) and c = 13.225(3) Å, β = 100.69(3)°. A ferroelasticparaelastic phase transition is noted at T_c = 100°C (by Thermal Analysis and X-ray, Raman-scattering, electrical permittivity versus temperature) leading to the prototype Fm3m phase(II) with a = 10.870(5) Å, quenchable when very slightly spoiled by impurities.     

A new acentric borate of K2Ba[B4O5(OH)4]2·10H2O: synthesis,structure and nonlinear optical property

A new acentric mixed metal borate of composition K₂Ba[B₄O₅(OH)₄]₂·10H₂O, has been successfully obtained by slow evaporation solution method. The compound crystallizes in the Orthorhombic space group Pna21 (No.33) with a = 16.8668(7) Å, b = 13.0903(5) Å, c = 11.5529(5) Å and Z = 4. [B₄O₅(OH)₄]^2? clusters serve as fundamental building unit linking with BaO₈, K1O₆, K2O₇ by common O atoms to form three-dimensional layer structure. The second harmonic generation measurements in the powder samples reveal that the compound exhibits approximately 0.5 times that of KH₂PO₄ (KDP) and phase matching.     

Synthesis,crystal structure and electrical properties of (C5H13NCl)2 SnCl6

In this work, a novel compound Bis(2-chloropropyl-N,N-dimethyl-1-ammonium) hexachloridostannate(IV) was synthesized and characterized by; single X-ray diffraction, Hirshfeld surface analysis, differential scanning calorimetric and dielectric measurement. The crystal structure refinement at room temperature reveled that this later belongs to the monoclinic compound with P2₁/n space group with the following unit cell parameters a = 7.2894(7) Å, b = 12.9351(12) Å, c = 12.2302(13) Å and β = 93.423 (6) °. The structure consists of isolated (SnCl₆)^2? octahedral anions connected together into layers via hydrogen bonds N–H….Cl between the chlorine atoms of the anions and the hydrogen atoms of the NH groups of the [C₅H₁₃NCl]⁺ cations. Hirschfeld surface analysis has been performed to gain insight into the behavior of these interactions. The differential scanning calorimetry spectrum discloses phase transitions at 367 and 376.7 K. The electrical properties of this compound have been measured in the temperature range 300–420 K and the frequency range 209 Hz–5 MHz. The Cole–Cole (Z′ versus Z″) plots are well fitted to an equivalent circuit model. The transition phase observed in the calorimetric study is confirmed by the change as function of temperature of electrical parameter such as the conductivity of grain (σ_g) and the σ_dc.     

Pressure-dependent phase transition in the ordered BaBi0.7Nb0.3O3 perovskite

BaBi_0.7Nb_0.3O₃, an ordered perovskite, crystallizes in a centrosymmetric rhombohedral structure with the space group R3¯. The refined cell parameters obtained from synchrotron powder X-ray diffraction data for the rhombohedral phase at ambient pressure are a=6.109 (2) Å and α=60.3 (1)°. The pressure-dependent synchrotron powder X-ray diffraction studies show a phase transition around 8.44±1 GPa, where it transforms from rhombohedral structure to a monoclinic structure. The lattice parameters obtained for the monoclinic phase at a pressure of 15±1 GPa are a=5.91 (2) Å, b=6.25 (3) Å and c=8.22 (1) Å with monoclinic angle, β=88 (1)°.     

Dielectric measurements,X-ray diffraction and raman studies of mixed caesium-ammonium mercury chloride: Cs0.7(NH4)0.3HgCl3

Measurements of the dielectric constants have revealed a transition at T=449K in Cs_0.7(NH₄)_0.3HgCl₃. This transition was confirmed by X-ray diffraction and Raman scattering on polycrystalline samples. The room-temperature phase is ordered and exhibits trigonal symmetry (space group P3₂ with the unit cell dimensions a = 13.295(11) Å; c = 9.419(8) Å). Transport properties in this material appear to be due to the high mobility of NH⁺ ₄ andCs⁺.     

Structure determination of MnO2 films grown on single crystal α-Al2O3 substrates

Manganese oxide films have been grown by atomic layer deposition and investigated using electron diffraction and high-resolution electron microscopy (HREM). The films were deposited on the (001) surface of monocrystalline α-Al₂O₃. The films were found to consist of an ordered version of the hexagonal ε-MnO₂ (Akhtenskite) type. Using X-ray diffraction, the cell parameters were determined to be a?=?2.75(2)?Å and c?=?4.302(5)?Å. The films are epitaxial with a specific orientation relative to the Al₂O₃ substrate. The [210] and [001] axes of ε-MnO₂ are parallel to the [110] and [001] axes of α-Al₂O₃, respectively. Evidence of cation ordering was found by parallel beam electron diffraction. The ordered domains are needle shaped with widths of 2–10?nm. The unit cell of the ordered structure was found to be orthorhombic with cell dimensions a?=?2.75, b?=?4.76, c?=?4.302?Å and space group Pmnn (No. 58).     

A new crystalline HMX polymorph: ɛ-HMX

A new crystalline HMX polymorph, ?-HMX, was obtained. ?-HMX crystals were studied by X-ray structure analysis, optical microscopy, and differential scanning calorimetry. Their space group is P2₁/c. The unit cell parameters are a = 21.799(3) Å, b = 10.913(2) Å, c = 10.819(2) Å, and β = 97.43(2)°, V = 2552.15 ų, Z = 4. ?-HMX molecules are not equivalent in crystals and have chair conformations. The heat of the polymorphic transition of ?-HMX into the δ-polymorph was measured. The transition occurred with the intermediate formation of β-HMX. The dependence between the heats of polymorphic transitions and the densities of crystals of various HMX polymorphs was demonstrated. The character of this dependence was to a substantial extent determined by the type of HMX molecule conformation.     

Synthesis,structural study and thermal behaviour of a new compound: trirubidium bis(hydrogen sulphate) dihydrogen arsenate Rb3(HSO4)2.5(H2AsO4)0.5

Mixed crystals Rb₃(HSO₄)_2.5(H₂AsO₄)_0.5 have been prepared by slow evaporation from aqueous solution at room temperature. The crystals were characterized by X-ray single analysis, which revealed that Rb₃(HSO₄)_2.5(H₂AsO₄)_0.5 crystallizes in the space group P with lattice parameters: a = 7.471(3) Å; b = 7.636(1) Å; c = 12.193(2) Å; α = 71.91(1)°; β = 73.04(6)° and γ = 88.77(2)°. In this structure, the ordered S(1)O₄ and the disordered S(3)/AsO₄ tetrahedra are connected by O–H..O hydrogen bonds, to a zigzag chains running in the b-direction. These chains are, in turn, bonded to one another by disordered hydrogen bridges O–H..H–O, to give a planar structure, with hydrogen-bonded sheets, laying parallel to (1 0 0). Each disordered tetrahedron is linked to a tetrahedron neighbouring S(2)O₄ by ordered hydrogen bonds. Broader peaks in IR spectrum of the title material support the assumption of disordered structure. Thermal analysis of the superprotonic transition in Rb₃(HSO₄)_2.5(H₂AsO₄)_0.5 showed that the transformation to the high-temperature phase occurs by one-step process at 404 K. Thermal decomposition of this compound takes place at much higher temperatures, with an onset of approximately 473 K.     

Structural phase transitions in Cu2–x Te crystals (x = 0.00, 0.10, 0.15, 0.20, 0.25)

Structural phase transitions in Cu_2–x Te crystals have been investigated by high-temperature X-ray diffraction.

At room temperature Cu₂Te and Cu_1.90Te specimens are two-phased, i.e. they consist of an orthohombic phase with a = 7.319, b = 22.236, c = 36.458 Å and a hexagonal phase with a = 4.150, c = 7.188 Å. The changes in both compounds take place generally in the hexagonal phase with increasing temperature. At 821, 873 K they transform to a FCC phase. Monocrystals of the other compounds at room temperature crystallize in the hexagonal system and at 673, 773, 723 K, respectively, they transform to a FCC phase. It is determined that as the cation deficiency increases the crystal quality becomes better.