Temperature-dependent diffraction studies on the phase evolution of tetraindium heptabromide

The mixed-valent compound In₄Br₇ undergoes a higher-order phase transition below which leads to a decrease in symmetry from the trigonal to the monoclinic (C2/c) system via . The phase transition has been monitored by X-ray powder diffraction using a linear position-sensitive detector between 15 and , and the crystal structures at room temperature and at 90 K have been refined by means of time-of-flight neutron powder-diffraction data; at , the lattice parameters are , , , and β=98.20(1)°; the new unit cell contains 88 atoms (Z=8) of which 12 are symmetry-independent. Due to their electronic instability because of a second-order Jahn-Teller effect, two of the three crystallographically independent monovalent indium cations are severely affected by the phase transition with respect to their coordination spheres; bond-valence calculations reveal significant strengthening of In⁺-Br⁻ bonding upon symmetry reduction. Structural changes and group-subgroup relationships as well as possible intermediate phases are discussed.     

Synthesis of poly(ethylene adipate) and poly(ethylene adipate-co-terephthalate) via ring-opening polymerization

Syntheses of poly(ethylene adipate) (ROP-PEA) and poly(ethylene adipate-co-terephthalate) (ROP-PEA-co-PET) were achieved via ring-opening polymerization of corresponding cyclic oligoesters. In case of ROP-PEA, cyclic oligo(ethylene adipate) (C-OEA) was equilibrated in the presence of di-n-butyltin oxide as a catalyst under high-concentration conditions at 180 and 200 °C for 1-24 h. The polymer products were obtained in yields up to 100% with the and in the ranges of 3000-23 000 g/mol and 5000-60 000 g/mol, respectively. The ROP-PEA-co-PET was prepared by equilibrating an equimolar amount of C-OEA and cyclic oligo(ethylene terephthalate) (C-OET) using di-n-butyltin oxide catalyst under high-concentration conditions at 250 °C for 24 h. The copolyester produced was obtained in yield of 97% with the and of 18 000 and 46 000 g/mol, respectively. ¹H NMR spectrum of ROP-PEA-co-PET showed two new proton signals of ethylene unit representing the existence of heterolinkage with different chemical environment in the copolymer. This indicated the random transesterification of C-OEA and C-OET resulting in random structure in copolyester. In addition, the result of ROP-PEA-co-PET from DSC showed the glass transition temperature in the values of −8 °C with no melting temperature indicating thermoplastic elastomeric behavior.     

Synthesis and characterizations of well-defined dendrimer-like copolymers with the second and third generation based on polystyrene and poly(l-lactide)

Dendrimer-like copolymers with two and three generations, (polystyrene)₃-b-(poly(l-lactide))₆ (PSt₃-b-PLLA₆) and PSt₃-b-PLLA₆-b-PSt₁₂ have been successfully prepared using core-first method. The first step of this synthesis is the preparation of three-armed PSt by atom transfer radical polymerization (ATRP) of St using 1,1,1-tri(methylene-α-bromoisobutyryl)propane as initiator. Terminal divergence of the polymers obtained was achieved by the reaction of terminal bromines with branching agent, 2,2-dimethyl-1,3-dioxolane-4-methanol. After deprotection, the polymer with six terminal hydroxyl groups was used in the ring-opening polymerization of LLA. The dendrimer-like copolymer with PLLA as a second generation diverged continuously by the reaction of 6 terminal hydroxyl groups with branching agent, 2,2-bis(methylene-α-bromoisobutyryl)propionyl chloride. The resultant polymer with 12 terminal bromines was used as macroinitiator in the ATRP of St to produce the target dendrimer-like copolymer, PSt₃-b-PLLA₆-b-PSt₁₂. The structures of polymers obtained from each step were confirmed by their ¹H NMR spectra and GPC measurements. DSC results show one for the three-armed PSt, , and for the dendrimer-like copolymer with two generations, C(PSt(PLLA)₂)₃, and , and for the copolymer with three generations, C(PSt(PLLA(PSt)₂)₂)₃.     

On the symmetry and crystal structures of Ba2LaIrO6

Accurate profile analysis of X-ray diffraction data was carried out to settle recent dispute on the symmetry and crystal structures of the double perovskite Ba₂LaIrO₆. Even through careful comparison of the full-width at half-maximum values, we found no evidence for Ba₂LaIrO₆ adopting either monoclinic (I2/m) or mixed rhombohedral and monoclinic (I2/m) structures at room temperature, becoming triclinic at below about 200 K. The correct space group is just at temperatures between 82 and 653 K. Furthermore, the phase transition does occur in Ba₂LaIrO₆, but the transition temperature is found to be much higher than the reported value.     

Formation and morphology of “shish-like” fibril crystals of aliphatic polyesters from the sheared melt

We found the formation of “shish-like” fibril crystals of aliphatic polyesters such as poly(l-lactic acid) (PLLA), poly(ε-caprolactone) (PCL), poly(12-hydroxydodecanoic acid) (PHDA) and poly(16-hydroxyhexadecanoic acid) (PHHA) from the sheared melt with shear rate  = 5 s⁻¹ observed by polarizing optical microscope (POM). The melting temperature T_ms of obtained fibril crystals of PLLA and PCL were higher than those of spherulites and were close to the equilibrium melting temperature . The small angle X-ray scattering (SAXS) patterns from the bulk sample including fibril crystals, small amount of unoriented small crystals and amorphous showed no peaks arose from the existence of long periods in fibril crystals. These are the evidence that the observed fibril crystals consist of assemblies of a lot of extended chain crystals (ECCs). We observed the morphology of moderately extracted single strand of fibril crystals at the magnification of POM by means of scanning electron microscope. We found that macroscopic fibril crystals of PLLA with diameter d = 10 μm consist of the bundle structure of microscopic fibril crystals with d = 2 μm. From POM observation of the formation of fibril crystals of PLLA and PCL, we showed phase diagrams of molecular weight M and crystallization temperature T_c for the formation of fibril crystals. From these phase diagrams, we evaluated a critical M and T_c for the formation of fibril crystals. Moreover, from the sequential melting and crystallization experiments, it was implied that the entanglement and transesterification play an important role on the formation of fibril crystals of aliphatic polyesters.     

Preparation and characterization of poly(ether sulfone)/sulfonated poly(ether ether ketone) blend membranes

Polymer blends of sulfonated poly(ether ether ketone) (SPEEK) and poly(ether sulfone) (PES) in N-methyl-2-pyrrolidinone (NMP) were prepared by solution casting. The investigation on water uptake, methanol uptake, permeability and proton conductivity has been conducted. The spin-lattice relaxation time in the rotating frame of PES/SPEEK blend was obtained from the results of cross-polarization magic angle spinning (CP/MAS) solid state ¹³C NMR. SPEEK blended with PES resulted in increasing , indicating the molecular motion of polymer chain was reduced. The glass transition temperature of the PES/SPEEK blend membranes were predicted by the Kwei equation. PES plays an important role in the decreasing water uptake, methanol uptake and methanol permeability while enhancing the thermal stability of the blend membrane, which shows the feasibility for direct methanol fuel cell.     

Tetragonal low-temperature structure of LiAl

The cubic face-centered structure of LiAl (, at ) transforms into a tetragonal body-centered structure (I4₁/amd, , at ). This first-order phase transition at about during heating is probably the reason for the so-called “ anomalies” in some physical properties like specific heat, electrical resistivity and nuclear-spin lattice relaxation. This transition seems to be correlated with the composition Li:Al of the alloy and the amount of Li vacancies.     

The low-temperature form of Rb2KCrF6 and Rb2KGaF6: The first example of an elpasolite-derived structure with pentagonal bipyramid in the B-sublattice

The crystal structure of the low-temperature forms of Rb₂KCrF₆ and Rb₂KGaF₆ has been solved on single crystal. The symmetry is tetragonal with F4/m space group; the unit cell parameters are: , for Rb₂KCrF₆ at and , for Rb₂KGaF₆ at . The relationships between the parameters of the prototype cubic elpasolite, which is stable at high temperature, and the tetragonal superlattice of the low temperature form have been established. Considering the general formulation A₂BB′F₆, the cationic positions in the A and (B,B′) sublattices remain identical in the two allotropic varieties. The main originality of the structure concerns the environment of 4/5 of the potassium atoms (B sublattice) which is transformed from octahedra into pentagonal bipyramids sharing edges with adjacent B′F₆ octahedra containing Cr or Ga. The displacive phase transition is simply explained by the rotation of 45° in the (a,b) plane of 1/5 of the B′F₆ (B′=Cr, Ga) octahedra. The similarity of this phase transition and the transformation of perovskite into tetragonal tungsten bronze (TTB) will be discussed.     

Effect of temperature on the volumetric properties of (cyclohexanone + an aromatic hydrocarbon)

The excess molar volume of (cyclohexanone + benzene, or toluene, or ethylbenzene, or styrene) were obtained from the densities measured by means of a vibrating-tube densimeter over the whole composition range at temperatures (293.15, 303.15, 313.15, 323.15, 333.15, 343.15, 353.15) K and atmospheric pressure. The excess molar volume provide the temperature dependence of in the temperature range of (293 to 353) K. The results were correlated using the fourth-order Redlich-Kister equation, with the maximum likelihood principle being applied for the determination of the adjustable parameters. It was found that the in the systems studied increase with rising temperature.     

Structural characterization and ferroelectric ordering in (C3N2H5)5Sb2Br11

A ferroelectric crystal (C₃N₂H₅)₅Sb₂Br₁₁ has been synthesized. The single crystal X-ray diffraction studies (at 300, 155, 138 and 121 K) show that it is built up of discrete corner-sharing bioctahedra and highly disordered imidazolium cations. The room temperature crystal structure has been determined as monoclinic, space group, P2₁/n with: , and and β=96.19^°. The crystal undergoes three solid-solid phase transitions: ) discontinuous, continuous and discontinuous. The dielectric and pyroelectric measurements allow us to characterize the low temperature phases III and IV as ferroelectric with the Curie point at 145 K and the saturated spontaneous polarization value of the order of along the a-axis (135 K). The ferroelectric phase transition mechanism at 145 K is due to the dynamics of imidazolium cations.     

Isomorphy of structural phase transitions in LiTaOSiO4, LiTaOGeO4 and titanite, CaTiOSiO4

Structural phase transitions in LiTaOGeO₄ (LTGO) and LiTaOSiO₄ (LTSO) have been observed using differential scanning calorimetry, X-ray diffraction and MAS NMR spectroscopy. LTGO transforms from P2₁/c to C2/c space group symmetry at , while the isomorphic transition occurs at in LTSO. An analogous phase transition is known to occur in the structurally related mineral titanite, CaTiOSiO₄. Spontaneous strain accompanying this phase transition in LTSO is significantly stronger than in titanite. As in titanite non-vanishing strain components are observable for T_c<T<T_i, with a similar ratio T_i/T_c. MAS NMR spectroscopy in combination with computation of the electric field gradient by first principle methods confirms that the tetrahedral Li coordination environment is retained during the phase transitions in LTGO and in LTSO. In LTSO substantial motional narrowing is observed, indicating increased mobility of the Li cation above . The narrowing of the spinning sidebands is significantly modified immediately above and below the critical temperature.     

Structural investigation of trimethylammonium tetrachloromercurate

The temperature dependence of the crystal structure of TrMA₂HgCl₄ (TrMA=trimethylammonium) is studied by single-crystal and powder X-ray diffraction between 161 and . Below room temperature, we did not find significant changes of the crystal structure. The phase transition at is described by a rotation of one of the two independent TrMA⁺ cations by 40° around the N-H axis, resulting in major rearrangements in the network of hydrogen bonds. A large change of the volume of the unit cell is found at the phase transition at , resulting in damage to the single crystals. Powder diffraction then shows that this transition can be described as a α-β transition between the α and β-K₂SeO₄ structure types. Structure refinements employing alternatively restrictions by the space group and additional restrictions by the C₃ non-crystallographic site symmetry for the TrMA⁺ cations show that significant deviations from C₃ symmetry of the TrMA⁺ cations do not exist at any temperature. The molecular dynamics and orientational disorder were studied by the maximum entropy method applied to the X-ray data. The diffraction results are corroborated by temperature dependent NMR experiments.     

Effect of pressure and temperature on structural stability of potential hydrogen storage compound Li3AlH6

The crystal structure of Li₃AlH₆ was investigated at high pressures upto 27 GPa using a diamond anvil cell with synchrotron radiation in addition to high temperature X-ray diffraction. Density functional theoretical (DFT) calculations were performed simultaneously. While the structure of Li₃AlH₆ is stable on increasing temperature, the results of high pressure experiments show a pressure induced phase transition from the ambient phase to a high pressure cubic phase around 10.6 GPa. The transition pressure of 10.6 GPa and the bulk modulus value B₀ = 32(2) GPa for the phase obtained are in good agreement with the theoretical results.     

Large magnetoresistance in a ferromagnetic cobaltite: The 12H Ba0.9CoO2.6

The magnetic and transport properties of a hexagonal cobaltite related to the perovskite structure have been studied. By combining transmission electron microscopy, X-ray powder diffraction and iodometric titration, it is found that Ba_0.9CoO_2.6 crystallizes in the 12H structure [P6₃/mmc, a=5.6612 (1) Å and c=28.4627(8) Å]. Interestingly, this compound is a ferromagnet with a Curie temperature T_C=50 K and a saturation magnetization . This value is smaller than expected from the effective paramagnetic moment, μ_eff=3.7μ_B/Co, corresponding to an average spin per Co, from which one would expect . This suggests either a canted structure or a strong local magnetic anisotropy related to the crystal field of the CoO_n polyhedra. A clear transition in the electrical resistivity is found at T_C consistent with a spin scattering reduction as the sample becomes ferromagnetic. The spin-charge coupling is evidenced by the large negative magnetoresistance effect optimum near T_C=50 K, with .     

Copolymers from tert-butyl methacrylate and itaconic anhydride--reactivity ratios and polymer analogous reactions

Free radical copolymerisation of tert-butyl methacrylate (1) and itaconic anhydride (2) has been investigated and reactivity ratios and have been obtained. Peak temperature of the endotherm and the enthalpy of the cis-elimination of the tert-butyl ester show a linear dependence on the content of anhydride in the copolymer. Alcoholysis of the anhydride with methanol resulted in complete formation of monomethyl itaconate, whereas the reaction with isopropanol in the presence of N,N-dimethyl-aminopyridine as catalyst stopped at lower conversion. Re-formation of the anhydride takes place around 100 °C in case of the monoisopropyl itaconate and at 120 °C for the monomethyl ester. This offers the possibility to adjust polymer properties, e.g. glass transition temperatures and solubility by controlled polymer analogous reaction. The copolymers can be used in photoresist processing, where adhesion and solubility can be adjusted during processing by alcoholysis and baking steps.