Anisotropic and heterogeneous thermal expansion of polyethylene foam blocks: Effect of thermal treatments

Crosslinked closed cell polyethylene foams produced in blocks by compression moulding present an anisotropic and heterogeneous thermal expansion behaviour when the temperature is increased. This paper analyses the main reason for this particular behaviour and presents a way to reduce it by using thermal treatments.In order to perform this analysis, an experimental study on the cellular structure, lamellar distribution and thermal expansion is presented as a function of two kinds of thermal treatments. The experimental results have showed that the main factor controlling the foams thermal expansion is an anisotropic and heterogeneous cellular structure of the original foams. It has been also proved that an adequate thermal treatment allows homogenising the foams thermal expansion.     

Chiolite-like Ca5Te3O14: An X-ray and neutron diffraction study

The crystal structure of Ca₅Te₃O₁₄ at room temperature was studied by the Rietveld method using combined X-ray and neutron powder diffraction data. The compound crystallizes in the space group Cmca with the lattice parameters a=10.4268(2) Å, b=10.3908(2) Å and c=10.4702(2) Å. The structure of Ca₅Te₃O₁₄ is chiolite-like and consists of a framework of corner-linked TeO₆ octahedral layers in which a linear TeO₂ group of every fourth octahedron is substituted by a Ca atom. This type of structure was previously observed in BaSr₄U₃O₁₄. The relationship between the chiolite-like structure and the fluorite structure is discussed.     

Crystal structure of Mg0.65Sc0.35Dx deuterides studied by X-ray and neutron powder diffraction

The structural properties of the Mg_0.65Sc_0.35D_x deuterides have been investigated by X-ray and neutron powder diffraction at different deuterium content (0?x?2.2 D/f.u.). The metallic phase adopts a pseudo-CsCl structure (Pm-3m space group (SG); a=3.5921(2) Å) that transforms upon hydrogenation into a face centered cubic (FCC) phase involving an elongation of the c-axis, a shrinkage of the a-axis and a re-ordering of the metallic atoms. The fully hydrided compound (2.2 D/f.u.) adopts a cubic structure (Fm-3m SG; a=4.8087(7) Å) and deuterium is located in fully occupied tetrahedral sites and partially filled (24%) octahedral sites. Upon desorption, a two-phase domain appears with coexistence of a hydrogen-rich (1.55 D/f.u.) and a hydrogen-poor (0.85 D/f.u.) phase (Fm-3m SG; a=4.7598(3) and 4.6936(3) Å, respectively). All deuterium atoms are located in the tetrahedral sites with different occupancy factors: 77% for the H-rich phase and 43% for the H-poor phase. The appearance of a plateau in the pressure-composition-isotherm curve measured at 573 K confirms this two-phase behavior. The structural properties of the Mg_0.65Sc_0.35D_x system are discussed and compared with other body centered cubic (BCC) alloys adopting the same structure.     

Structural study of polymorphism and thermal behavior of CaZr(PO4)2

The crystal structure of CaZr(PO₄)₂ has been revised by ab initio Rietveld analysis of X-ray powder diffraction data. At room temperature, CaZr(PO₄)₂ crystallizes in the orthorhombic space group Pna2₁ (Z = 4). Differential thermal analysis suggests a reversible second order transition at 1000 °C confirmed by high temperature XRD analysis that brings out the existence of a high temperature form, very similar to the room temperature one, but more symmetrical (Pnma, Z = 4). Analysis of the crystal parameters evolution during heating reveals that CaZr(PO₄)₂ exhibits a quite low thermal expansion coefficient of 6.11·10⁻⁶ K⁻¹. This value stems from a combination of several mechanisms, including Coulombic repulsion and bridging oxygen rocking motion.     

Unique trifurcated hydrogen bonding in a pseudopolymorph of tricyclohexane triperoxide (TCTP) and its thermal studies

Tricyclohexane triperoxide (TCTP) was synthesized as a main by-product of tetraoxane synthesis and was characterized by spectroscopic techniques viz. ¹H NMR, ¹³C NMR, FT-IR and Raman. The single crystal X-ray structure revealed the inclusion of solvent in 1:1 stoichiometric ratio involving a unique trifurcated C(sp³)-H?O hydrogen bonding imparting remarkable symmetry to the molecule. The thermal stability of single crystal was determined by TG-DTA and DSC.     

Long-range magnetic ordering in Ba2CoS3: A neutron diffraction study

Neutron powder diffraction has been used to determine the magnetic structure of the quasi-one-dimensional compound Ba₂CoS₃, which contains linear [001] chains of vertex-sharing CoS₄ tetrahedra, spaced apart by Ba²⁺ cations. At 1.5 K the Co²⁺ cations in the chains are antiferromagnetically ordered with an ordered magnetic moment of 1.97(4) μ_B per cation aligned along [100]. Each Co²⁺ cation is ferromagnetically aligned with four cation in neighbouring chains and antiferromagnetically aligned with two others.     

Directional metal-hydrogen bonding in interstitial hydrides. III. Structural study of ErCo3Dx (0?x?4.3)

The ErCo₃-D₂ system has been studied by in situ neutron powder diffraction (NPD) at 60 °C and 0-16 bar deuterium pressure. Two deuteride phases were identified, β-ErCo₃D_1.07−1.38 and γ-ErCo₃D_3.7−4.3. They were structurally characterized at the compositions β-ErCo₃D_1.37 and γ-ErCo₃D_3.7 by high-resolution neutron and synchrotron powder diffraction. In contrast to the analogous nickel systems RNi₃-D₂ (R=Er, Ho; see part I, J. Alloys and Compds. 404-406 (2005) 89-94, and part II, J. Alloys and Compds. 2005, in press), their structures preserve the symmetry of the parent alloy (PuNi₃-type, space group R-3m). Deuterium occupies mainly AB₂ building blocks in the β-phase, and AB₂ and AB₅ building blocks in the γ-phase. In the AB₂ building blocks cobalt is surrounded by an average of 3.8 (β-ErCo₃D_1.37) and 4.4 D-atoms (γ-ErCo₃D_3.7) in disordered distorted octahedral configurations (point symmetry −3), in contrast to nickel that is surrounded by ∼3 (β₁- and β₂-RNi₃D_x, R=Er, Ho) and ∼4 (γ-ErNi₃D_3.7) D-atoms in disordered trigonal (pyramidal) and tetrahedral configurations, respectively (point symmetry 3). These results indicate that the D-atom distributions in this homologous series depend on the nature of the transition element rather than on geometric factors, and that directional bonding effects similar to those in non-metallic complex transition metal hydrides also prevail in metallic interstitial metal hydrides.     

X-ray and neutron powder diffraction study of the double perovskites Ba2LnSbO6 (Ln=La, Pr, Nd and Sm)

The crystal structures of Ba₂LnSbO₆ (Ln=La, Pr, Nd and Sm) at room temperature have been investigated by profile analysis of the Rietveld method using either combined X-ray and neutron powder diffraction data or X-ray powder diffraction data. It has been shown that the structure of Ba₂LnSbO₆ with Ln =La, Pr and Nd are neither monoclinic nor cubic as were previously reported. They are rhombohedral with the space group . The distortion from cubic symmetry is due to the rotation of the LnO₆/SbO₆ octahedra about the primitive cubic [111]_p-axis. On the other hand, the structure of Ba₂SmSbO₆ is found to be cubic. All compounds contain an ordered arrangement of LnO₆ and SbO₆ octahedra.     

Crystal structure and thermal expansion of the low- and high-temperature forms of BaM(PO4)2 compounds (M=Ti, Zr, Hf and Sn)

The crystal structure of β-BaZr(PO₄)₂, archetype of the high-temperature forms of BaM(PO₄)₂ phosphates (with M=Ti, Zr, Hf and Sn), has been solved ab initio by Rietveld analysis from synchrotron X-ray powder diffraction data. The phase transition appears as a topotactic modification of the monoclinic (S.G. C2/m) lamellar α-structure into a trigonal one (S.G. ) through a simple mechanism involving the unfolding of the layers. The thermal expansion is very anisotropic (e.g., −4.1<α_i<34.0×10⁻⁶ K⁻¹ in the case of α-BaZr(PO₄)₂) and quite different in the two forms, as a consequence of symmetry. It stems from a complex combination of several mechanisms, involving bridging oxygen rocking in M-O-P linkages, and “bond thermal expansion”.     

In situ neutron diffraction study on Pd-doped Mg0.65Sc0.35 electrode material

The behavior of a negative composite electrode made of Pd-doped Mg_0.65Sc_0.35 active material has been studied dynamically by in situ neutron diffraction during a complete charge-discharge electrochemical cycle starting from a virgin alloy. From the analysis of the collected diffraction patterns, phase identification, phase amounts, structural changes and cell volume evolutions have been determined as a function of the electrochemical state of (dis)charge. For the first charge, the active material shows a structural transformation from body-centered cubic (bcc) to face-centered cubic (fcc). During the following discharge, a two-phase behavior with equilibrium between a hydrogen-poor fcc phase and a hydrogen-rich one is observed. Complete discharge beyond the hydrogen-poor phase was not reached and a good electrochemical reversibility is expected between the hydrogen-poor and the hydrogen-rich fcc phases for this electrode material.     

Miscible blends through hydrogen bonding: effects on polymer properties

Miscible blends through hydrogen bonding have been intensively studied. The effects of a variety of miscible hydrogen bonded polymer blends on properties such as thermal and thermal oxidative stability, moisture sensitivity, modulus and glass transition temperature are discussed. In addition, the preparation of semi-interpenetrating polymer networks (IPNs) and studies of the effect of crosslinking on the miscibility in hydrogen bonded polymer blends are reviewed.     

Refinement of two different crystal structures simultaneously from a multiple-crystal sample: neutron study of 3-deazauracil and lead hydrogen arsenate at 100 K

The structures of 3-deazauracil and lead hydrogen arsenate have been studied by neutron single crystal diffraction at 100 K, using a sample containing one crystal of each material. The experiment on the composite sample was entirely routine, using recently developed multiple-crystal techniques. The structures themselves represent the best low temperature neutron determinations on each of these materials and are discussed.     

The effect of hydrogen bonding on oligoleucine structure in water: A molecular dynamics simulation study

Molecular dynamics simulations were conducted in order to improve our understanding of the forces that determine polyleucine chains conformations and govern polyleucine self-assembly in aqueous solutions. Simulations of 10 repeat unit oligoleucine in aqueous solution were performed using the optimized potential for liquid simulations (OPLS) - all atom force field using the canonical ensemble for a minimum of 1.3 ns. These simulations provided information on conformations, chain collapse and intermolecular aggregation. Simulations indicate that single isotactic oligoleucine chains in dilute solution assume tightly packed, regular hairpin conformations while atactic oligoleucine assumes a much less regular and less compact structure. The regular, compact collapsed isotactic chain exhibited a greater degree of intramolecular hydrogen bonding and an increased level of hydrophobic t-butyl functional group aggregation compared to the atactic chain. This occurs at the expense of reduced leucine-water hydrogen bonding.     

Order and disorder in Ca2ND0.90H0.10-A structural and thermal study

The structure of calcium nitride hydride and its deuterided form has been re-examined at room temperature and studied at high temperature using neutron powder diffraction and thermal analysis. When synthesised at 600 °C, a mixture of both ordered and disordered Ca₂ND_0.90H_0.10 phases results. The disordered phase is the minor component and has a primitive rocksalt structure (spacegroup Fm3m) with no ordering of D/N on the anion sites and the ordered phase is best described using the rhombohedral spacegroup R-3m with D and N arranged in alternate layers in (111) planes. This mixture of ordered and disordered phases exists up to 580 °C, at which the loss of deuterium yields Ca₂ND_0.85 with the disappearance of the disordered phase. In the new ordered phase there exists a similar content of vacancies on both anion sites; to achieve this balance, a little N transfers onto the D site, whereas there is no indication of D transferring onto the N-sites. These observations are thought to indicate that the D/N ordering is difficult to achieve with fully occupied anion sites. It has previously been reported that Ca₂ND has an ordered cubic cell with alternating D and N sites in the [100] directions [1]; however, for the samples studied herein, there were clearly two coexisting phases with apparent broadening/splitting of the primitive peaks but not for the ordered peaks. The rhombohedral phase was in fact metrically cubic; however, all the observed peaks were consistent with the rhombohedral unit cell with no peaks requiring the larger ordered cubic unit cell to be utilised. Furthermore this rhombohedral cell displays the same form of N-D ordering as the Sr and Ba analogues, which are metrically rhombohedral.     

Structural properties of two deuterides LaY2Ni9D12.8 and CeY2Ni9D7.7 determined by neutron powder diffraction and X-ray absorption spectroscopy

The crystal structure of two deuterides RY₂Ni₉D_x (R=La; x=12.8 and R=Ce; x=7.7) have been investigated by means of neutron powder diffraction and X-ray absorption spectroscopy. The structures are best described in the space group . The deuterium location has been determined for both compounds. The nature and the occupancy factors of the different D sites are presented. Comparisons are made between the La-based deuteride and the Ce-one in relation with the crystal structure of the intermetallic compounds. Differences in site occupancies within the RM₂ and RM₅ building units of the PuNi₃-type structure are discussed and heterogeneous mixed valence state is reported for the cerium compound.     

A high-temperature neutron diffraction study of pure and scandia-stabilized zirconia

A high-temperature neutron diffraction apparatus has been used to study a section of the zirconia-scandia system; the purpose was to determine whether this technique can be generally applied to phase equilibria studies. The structure of the tetragonal form of zirconia at 1200°C has been confirmed, and the parameters obtained were z(4d) = 0.188 ± 0.002, B₀ = 2.50Ų, B_Zr = 0.80 Ų, and R = 0.045. The effect of substituting scandia into the tetragonal zirconia structure was studied and the transformation of the ordered low-temperature β-phase to a grossly nonstoichiometric fluorite phase was also observed.     

The ferroelectric phase of CdTiO3: A powder neutron diffraction study

The synthesis of bulk samples of polycrystalline CdTiO₃ in both the rhombohedral ilmenite and orthorhombic perovskite forms is described and the structures of these have been refined using powder neutron diffraction data. This involved the preparation of samples enriched in ¹¹⁴Cd. Cooling perovskite-type CdTiO₃ to 4 K induces a ferroelectric phase transition, with the neutron data suggesting the low temperature structure is in Pna2₁. Mode analysis shows the polar mode to be dominant at low temperatures. The ilmenite-structure of CdTiO₃ is compared with that of ZnTiO₃. The refined scattering length of the ¹¹⁴Cd is estimated to be 5.56 fm. Attempts to dope CdTiO₃ with Ca and Sr are described.     

Structure and hydrogen bonding in polyhydrated complexes of guanine

Abstract Molecular structure of complexes of guanine with 12, 13, 16, and 17 water molecules were calculated using B3LYP/6-311G(d,p) level of theory. Interaction with water results in some deformation of geometrical parameters of guanine, which can be described as contribution of zwitter-ionic resonant form into the structure of DNA base. Saturation of water binding sites within guanine creates possibilities for the formation of the N···H–O hydrogen bond where the nitrogen atom of amino group acts as proton acceptor. The NBO analysis of guanine–water interactions reveals that hydrogen bonds involving the N(3) and N(7) atoms of guanine represent a case of mixed N···H–O/π···H–O hydrogen bonds where contribution of π-system into total energy of interaction varies from 3% to 41%. This contribution significantly depends on orientation of the hydrogen atom of water molecule with respect to plane of purine bicycle and influence of neighboring water molecules. Graphical Abstract      

High-temperature neutron diffraction study of the cation ordered perovskites TbBaMn2O5+x and TbBaMn2O5.5−y

The miscibility of TbBaMn₂O_5+x and TbBaMn₂O_5.5−y has been investigated at 100-600 °C using in situ powder neutron diffraction. No miscibility is observed, and the two phases remain oxygen stoichiometric (x,y=0) at 600 °C. Structure refinement results show that neither material undergoes a phase transition in this temperature range. TbBaMn₂O₅ is Mn²⁺/Mn³⁺ charge ordered and any charge melting transition is >600 °C. This symmetry-broken charge ordering is remarkably robust in comparison to that in other oxides.     

Synthesis and characterization of trichlorogermyl dioic acids: crystal structures and complementary hydrogen bonding motifs in 3-(trichlorogermyl) pentanedioic acid and 2-[(trichlorogermyl)methyl]butanedioic acid

A series of trichlorogermyl-substituted dicarboxylic acids of general formula HOOC–R′–COOH where R′=–CH₂CH(GeCl₃)CH₂– 1, –CH(CH₂GeCl₃)CH₂– 2, –CH(GeCl₃)CH₂– 3 and –CH(CH₃)CH(GeCl₃)– 4 were synthesized by the hydrogermylation reaction of unsaturated acids, such as trans-glutaconic (2-pentenedioic acid), itaconic (methylenebutanedioic acid), fumaric (2-butenedioic acid), and citraconic (2-methyl-2-butenedioic acid) acids with HGeCl₃, which was produced in situ by the reaction of GeO₂ with 37% HCl in presence of NaH₂PO₂ · H₂O. All these compounds were characterized by melting point, CHN analysis, FTIR, and multinuclear NMR (¹H; ¹³C; H,H-COSY). X-Ray crystal structures of 1 and 2 were analyzed to show supramolecular structures in which central Ge atom in each of these structures is four-coordinated with a slightly distorted tetrahedral geometry. Structurally, both compounds adopt supramolecular forms via strong intermolecular O–H–O interactions through 8-membered and 22-membered hydrogen bonded rings. Supplementary material to this paper is available in electronic form at Correspondence: Muhammad Mazhar, Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan.