Unit Cell Changes Research Articles

Crystal structure and morphology of biaxially oriented polyamide 12 films

AbstractThe structural and morphological characteristics of biaxially oriented polyamide 12 films are described on the basis of the results from differential scanning calorimetry, wide‐angle X‐ray diffraction (WAXD), polarized FT‐IR spectroscopy, and small angle X‐ray scattering (SAXS). The WAXD patterns of the oriented polyamide 12 films indicated only the monoclinic γ crystal with little dimensional changes of its unit cell depending on the stretching conditions. The crystallographic angles (α = γ = 90°, β = 121°) that were determined via the WAXD patterns confirmed the monoclinic symmetry of the γ crystal. Annealing the films stretched at 115 °C in boiling 20% formic acid solution did not result in structural changes of the crystalline unit cell. The chain‐axis repeat distance of 31.9 Å for the γ crystal was experimentally obtained with (0 26 0) planes. It was shortened as compared with that of all‐trans conformation. For films having primary orientation to MD, normals to the basal plane of folded‐chain lamellae were parallel to MD (primary stretch direction) resulting in two‐point SAXS patterns. Growth in long spacing with an increase of stretch temperature was discovered. Annealing the films induced further elongation in long spacing. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1189–1200, 2002

Characterization of micro- to macroscopic response of polymers containing voids under macroscopically uniform deformation

Abstract For the characterization of mechanical characteristics of polymers containing voids (cavitated rubbery particles), a homogenization method has been developed, which can handle large deformation problems including the onset and propagation of instability and can reproduce the micro- to macroscopic deformation behavior of polymers with periodically distributed voids under macroscopically inhomogeneous loading conditions. A parametric study has clarified the characteristic features of polymers containing periodically distributed heterogeneous cylindrical voids of different volume fractions subjected to macroscopically homogeneous stress in different directions. Different types of shear bands are observed, depending on the tensile direction with respect to the unit cell. The shear band connecting the voids that appear in the early stages of deformation is responsible for the macroscopic yielding. Increasing the volume fraction of the voids results in a decrease in the macroscopic yield stress and in the directional dependence of the macroscopic stress–strain relationship. The moderate heterogeneity of voids causes complicated and different types of shear bands; however, due to the high stress concentration caused by the presence of small voids, the onset and propagation of the shear bands are promoted, and the macroscopic stress–strain relationships become essentially isotropic. Nevertheless, the volumetric strain of unit cell changes with volume fraction and heterogeneity of the voids and tension directions.

Multispectral imaging microscope with millisecond time resolution.

A new multispectral imaging microscope with micrometer spatial resolution and millisecond temporal resolution has been developed. The imaging microscope is based on the use of an acousto-optic tunable filter (AOTF) for spectral tuning and a progressive scan camera capable of snapshot operation for recording. It can operate in two modes: images are recorded as a function of time or wavelength. When operated as a function of time, the microscope is configured so that as many images as possible are recorded, grabbed, and stored per one wavelength. Upon completion, the AOTF is scanned to a new wavelength, and a new set of images are recorded. Up to 33 images/ second (i.e., 30 ms/image) can be recorded in this mode. In the other configuration, the recording wavelength is rapidly scanned (by means of the AOTF) and only one image is rapidly recorded, grabbed, and stored for each wavelength. Because additional time is needed to scan the AOTF, the maximum number of images can be grabbed in this case is 16 frames/s. Preliminary applications of the imaging microscope include measurements of photoinduced changes of a single unit cell in temperature-sensitive cholesteric liquid crystals as a function of time and wavelength. The changes were found to be varied with time and wavelength. Interestingly, the photoinduced changes of unit cells in the liquid crystal are not the same but different from cell to cell. This imaging microscope is particularly useful for measurements of small-size samples that undergo rapid chemical or biochemical reactions, e.g., activities of a single biological cell.

Structure and related properties of (La,Ce,Nd,Pr)Ni 5 alloys

The effects of substitution of La by Ce, Nd and Pr in the LaNi 5-based alloys were systematically studied, with the use of factorial analysis. The changes of equilibrium pressure, hydrogen capacity and hysteresis were analyzed in relation to the structure changes of the hexagonal unit cell of (La,Ce,Nd,Pr)Ni 5. Both individual effects of each alloy component, and their interaction in the alloy were studied. It has been shown that variations in the basal plane parameter ( a) can be used as an indication for the plateau pressure changes resulting from substitution, giving better fits than variations in the unit cell volume. Structural anisotropy is discussed as a possible tool for the prediction of the decrepitation resistance of the alloy. Methods for optimization of the electrode alloys (including reduction of the cobalt content) are proposed.

Structural study of the high-temperature phases of the hexagonal perovskite KNiCl 3

Between room temperature and its melting point at 645°C, KNiCl 3 shows two structural phase transitions. The β→α transition at 287°C has a second-order character. The unit cell changes in size from a√3× a√3× c to a× a× c. A first-order-type phase transition, α→α′, takes place at 490°C. Structure determination of KNiCl 3, from powder neutron diffraction data taken at 350°C and 550°C, shows that the difference between the α and α′ phases is related to the total disorder of the K + ion in the α′ phase.

207Pb NMR of PbZrO3 and PbZr1-xTixO3 Solid Solutions

ABSTRACTPbZrO3 undergoes a phase transition to a paraelectric phase at 230°C. During this phase transition the unit cell changes from orthorhombic to cubic. Structural changes of PbZrO3 have been monitored using solid state NMR by measuring the variation in the 207Pb chemical shielding tensor as a function of temperature. The two distinct lead sites show rather different behavior as a function of temperature. The less shielded lead maintains an almost constant asymmetry parameter η = 0.2 from 0°C to 200°C while the more shielded lead resonance becomes more axially symmetric as the temperature is raised going from η = 0.24 at 0°C to [.eta] = 0.08 at 200°C. Powder pattern singularities become less distinct near the phase transitions, but the temperature dependence of the chemical shift tensor principal values remains continuous, and there is no evidence of an intervening higher-symmetry phase. At the phase transition, both resonances collapse into a single narrow line characteristic of a nucleus at a high-symmetry site. Results of a preliminary study of the PbZr1−xTixO3 solid solutions by 207Pb solid state NMR are also presented.

The Structure of an Open State of β-Actin at 2.65 Å Resolution

The structure of an “open state” of crystalline profilin:β-actin has been solved to 2.65 Å by X-ray crystallography. The open-state crystals, in 1.8 M potassium phosphate, have an expanded unit cell dimension in thecdirection of 185.7 Å compared with 171.9 Å in the previously solved ammonium sulphate-stabilized “tight-state” structure. The unit cell change between the open and the tight states is accompanied by large subdomain movements in actin. Furthermore, the nucleotide in the open state is significantly more exposed to solvent, and local conformational changes in the hydrophobic pocket surrounding cysteine 374 occur during the transition to the tight state. Significant changes were observed at the N terminus and in the DNase-I binding loop. Neither the structure of profilin nor its contact with β-actin are affected by the changes in the unit cell. Applying osmotic pressure to profilin:β-actin crystals brings about a collapse of the unit cell comparable with that seen in the open to tight-state transition, enabling an estimate of the work required to cause this transformation of β-actin in the crystals. The slight difference in energy between the open and collapsed states explains the extreme sensitivity of profilin:β-actin crystals to changes in chemical and thermal environment.

Application of moderate hydrostatic pressure induces unit-cell changes in rhombohedral insulin.

2Co(2+)-insulin crystals were subjected to hydrostatic pressures of up to 30 bar in a nitrogen gas cell. Changes in the diffraction pattern occurred at pressures as low as 5 bar. Analysis with standard image-processing software showed unit-cell dimension changes resulting in reductions in volume of up to 2.6%.

Structural Phase Transitions in Zinc(II) Perchlorate Hexahydrate

AbstractPhase transitions in zinc perchlorate hexahydrate are studied by X‐ray powder diffraction method. Structural changes in the vicinity of transitions detected at 364.5 K and 256.3 K are characterized. At the former the orthorhombic Pmn21 phase transforms into the Pmna one (a = 7.812(1), b = 13.518(3) and c = 5.279(1) Å at 348.6 K). The latter involves changes of the orthorhombic unit cell (a = 7.7922(9), b = 13.537(2) and c = 5.0588(6) Å at 251.0 K). Temperature dependence of lattice parameters in the range of 180 to 363.3 K are presented.

Grazing-incidence x-ray-scattering study of (001)-oriented high-quality epitaxial Co/Cr superlattices.

The structural properties of Co/Cr(001) superlattices grown by molecular-beam epitaxy have been studied by means of grazing-incidence x-ray-scattering techniques. From the in-plane diffraction pattern, we find that hcp-Co(11.0) is epitaxially grown on Cr(001) with the Co[00.1] axis parallel to Cr[110]. This epitaxial relation is equivalent to the Pitsch-Schrader orientational relationship. The Co layers exhibit a precursive structural phase transition from hcp to bcc with decreasing Co layer thickness ${\mathit{t}}_{\mathrm{Co}}$. First, the Co in-plane unit cell changes its shape from rectangular (hcp) to square (bcc), due to the epitaxial strain. Second, the intensity I of the hcp-Co{1\ifmmode\bar\else\textasciimacron\fi{}1.1} spots shows a continuous decrease with decreasing ${\mathit{t}}_{\mathrm{Co}}$. The intensity I is proportional to the square of the displacement \ensuremath{\delta} of the Co atoms from the bcc center sites, which is the order parameter for this structural transition. This intensity decrease therefore implies that the Co lattice continuously changes its symmetry from hcp to bcc. The experimentally determined ${\mathit{t}}_{\mathrm{Co}}$ dependence of \ensuremath{\delta} is in good agreement with a model calculation describing the competition of the surface potential on Cr(001) and the bulk hcp potential of Co. Finally, we find that the observed anomalous out-of-plane expansion of the Co layers is a precursor of the structural transition. The non-Poisson-like behavior can be explained by a rigid-atom model which takes into account the atomic displacement during the structural transition.

Using CPS120 (curved position-sensitive detector covering 120°) powder diffraction data in Rietveld analysis. The dehydration process in the zeolite thomsonite

A powder diffractometer (CPS120 by INEL) equipped with a position-sensitive detector covering 120° in 2θ has been tested and the data have been successfully used with the Rietveld method. The detector was found to have a characteristic variation in linearity of ±1.5%. A calibration compound with well known 2θ values was used to determine a cubic spline calibration function to describe the 2θ versus channel-number function, which was then incorporated in the Rietveld analysis program. A procedure is also described to correct for air and capillary scattering, taking sample absorption into account. The methods have been applied to a structure refinement of quartz and to a study of the dehydration process of the zeolite thomsonite. Quartz: Rp = 11.0%, Rw = 14.0% and RB = 7.9%, based on 10 < 2θ < 116.3° in 3470 steps, λ = 1.540598 Å. Space group P312, Z = 3, a = 4.91201 (7), c = 5.40301 (5) Å, x(Si) = 0.4702 (3), B ave(Si) = 0.77 (6) Å2, x(O) = 0.4112 (6), y(O) = 0.2712 (5), z(O) = −0.2811 (4) and B ave(O) = 1.0 (1) Å2. Thomsonite: Rp = 8.18%, Rw = 11.04% and RB = 3.33%, based on 10 < 2θ < 110° in 3270 steps. Space group Pncn, Z = 4, a = 13.0778 (3), b = 13.0398 (3) and c = 13.1970 (3) Å. The unit cell changes continuously with increasing dehydration temperatures up to 523 K, at which a = 12.9725 (3), b = 12.9537 (3) and c = 13.2288 (3) Å. In this first dehydration step, the W3 and W4 molecules coordinated to the Ca/Na site are lost and the Ca/Na and W1 sites are shifted approximately 0.3 Å from their original positions towards the framework O atoms. After dehydration at 573 K, the mode of unit-cell variations is changed: a = 12.908 (3), b = 13.039 (3) and c = 13.034 (3) Å. In this dehydration step, W1 is lost. The W2 site appears unaffected by the dehydration below 573 K. On dehydration above 573 K, the diffraction intensities decrease as the compound becomes amorphous.

Structure and properties of a new κ-phase organic metal: (BEDT-TTF) 2Cu 2(CN) 3

The electro-synthesis, structure and conductivity of a new κ-phase organic metal (BEDT-TTF) 2Cu 2(CN) 3 are described [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene C 10H 8S 8]. Partially oxidized BEDT-TTF cations form a structure similar to that found in the κ-phase superconductors, while Cu 2(CN) 3 −1 anions form infinite two-dimensional sheets which alternate with layers of BEDT-TTF cations along the crystallographic a axis. The morphology and conductivity of the crystals are dependent on the electrocrystallization conditions. Samples prepared under certain conditions show a semiconductor-to-metal transition in the 10–30K range, while others have an immeasurably large resistance in this temperature range, even though the room-temperature crystal structures are identical, and EDX shows all samples to have the same composition. However, application of a thin layer of epoxy induces the transition in all samples, and enhances the low-temperature conductivity of some samples by a factor of about 100. Low temperature crystallographic measurements of the unit cell change on cooling indicate the epoxy-coating to reduce the thermal contraction of the samples, in particular in the direction of the crystallographic a-axis.

Phonon characterization of YBa2(Cu1-xAlx)3O7- delta by Raman spectroscopy.

We present new Raman scattering data on high-temperature superconducting materials with composition $\mathrm{Y}{\mathrm{Ba}}_{2}{({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x})}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. In $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ we observe new high-frequency peaks greater than 700 ${\mathrm{cm}}^{\ensuremath{-}1}$ that we associate with two-phonon interactions of the first-order Cu---O Raman vibrations that occur at 337, (450\ifmmode\pm\else\textpm\fi{}10), and 502 ${\mathrm{cm}}^{\ensuremath{-}1}$. The intensity of these new peaks suggests that anomalous phonon interactions are taking place. Results on Al substitution are consistent with a unit-cell change from orthorhombic to tetragonal. The observation of superconductivity in the tetragonal structure means that it is not possible to simply assign superconductivity to the orthorhombic structure.

X-Ray Investigations of the Low-Temperature Phases of the Organic Metals α- and β-(BEDT-TTF)2I3

The structure of single crystals of the organic metals α- and β-(BEDT-TTF)2I3* was determined at 100 K, well below the phase transitions indicated by resistivity and thermopower measurements as well as by differential thermal analysis. In the α-phase no unusual change of the room temperature unit cell but a slight variation in the triiodide network and especially a more pronounced dimerization in one of the two donor stacks have been found. The β-phase develops a superstructure with a unit cell volume three times as large as that at room temperature and with pronounced distortions of the I3 --ions.

Order-disorder phase transition in BaCl2.2H2O crystals

Crystals of BaCl2.2H2O were studied by Raman spectroscopy and dielectric constant measurements, from 12K to room temperature. Both techniques gave evidence of an order-disorder phase transition occurring around 200K. The conclusion drawn is that half of the water molecules in the unit cell change their site symmetry during the transition.

Lattice expansion by water absorption and side‐chain interactions in synthetic polypeptides having a urethane group in the side chains

AbstractTwo polypeptides with a urethane group in the side chains, poly(δ‐N‐methoxycarbonyl‐L‐ornithine) (PMCO) and poly(ε‐N‐methoxycarbonyl‐L‐lysine) (PMCL), were synthesized. Their crystal structures were investigated from X‐ray diffraction. In both polymers, the molecules take the α‐helical conformation, and helices are packed in a (pseudo)hexagonal array. The lateral size of the unit cell changes reversibly with relative humidity (rh), while the axial dimension is constant irrespective of water absorption. The expansion of unit‐cell volume for rh = 100% with reference to rh = 0% was 5,8% for PMCO and 7,1% for PMCL at 20°C. In the dry state, side chains are partly associated via hydrogen bonds. The dielectric behavior of these films is discussed in terms of side‐chain interactions.

Interactive graphics for rapid indexing of oscillation films from large unit cells

An interactive graphics program for the Evans & Sutherland PS330 has been written to display and index screenless X-ray oscillation films from macromolecular crystals with unit-cell edges of at least 500 A. ALIGN, a Fortran program which works in conjunction with M. G. Rossmann's program OSCMGR, accurately displays densitometered reflection intensities, provides the required transformation between predicted and observed reflection coordinates, and is useful in determining crystal mosaicity and detecting unit-cell changes.

Stoichiometry and physical properties of ternary molybdenum chalcogenides MxMo 6X8 ( X = S, Se; M = Li, Sn, Pb)

Chemical and electrochemical insertion of Li at room temperature, as well as insertion of lead and tin at moderate temperatures (500°C), into the binary phase Mo 6 X 8 forms ternary molybdenum chalcogenides M x Mo 6 X 8 ( X = S, Se). Crystallographic parameters, superconducting properties, and magnetic susceptibility are reported. The stoichiometry x for lead and tin is shown not to exceed x = 1, while for Li, x can reach approximately 4.0. For the lead and tin sulfide series, the hexagonal lattice parameters and superconducting critical temperatures ( T c) are invariant to changes in the nominal composition of 0.8 < x < 1.2, while both an increase in T c and a small decrease in c h is observed for the selenides; a narrow homogeneity range exists near x = 1 below 500°C for both these sulfides and selenides, the single-phase region being somewhat larger in the selenides. In contrast, several single-phase regions and large unit cell changes are observed in Li x Mo 6 X 8 (0 < x < 3.2). Magnetic susceptibility measurements of the lithiated compounds at x ∼ 3.2 reveals a structural phase transition at 140 and 185 K for the sulfide and selenide, respectively; but neither superconducts down to 1.5 K. At lower lithium concentration near x ∼ 1.0, the T c of the sulfide is raised from that of Mo 6S 8 (1.8 K) to 5.2 K but the T c of Mo 6Se 8 (6.5 K) is depressed to 3.9 K.

Crystalline transitions in powders of nylon 66 crystallized from solution

AbstractPowders of nylon 66 were crystallized from solution in methanol and in other solvents. These powders exhibit a latent heat of about 4.5 cal/g at the Brill transition near 200°C where the unit cell changes from a triclinic to a pseudohexagonal form. The dimensions of the hydrogen‐bonded sheets are almost unchanged up to 240°C, but the separation between the sheets increases with increasing temperature. Above 245°C, the interchain separation increases rapidly, and permits the powder to be extruded in an essentially plastic manner to form coherent extrudates. As the temperature of treatment is increased above 245°C, the latent heat of the Brill transition is reduced toward zero, the heat of fusion is reduced from about 30 cal/g to about 20 cal/g, and an endotherm at 261°C is replaced by one at 267°C.

Electron paramagnetic resonance of triplet diphenylmethylene in single crystal benzophenone: Evidence for a low temperature phase transition

The angular variation of the field for resonance for diphenylmethylene in benzophenone at T?2 °K has been measured over the entire unit sphere. Both the values of the fine structure tensors T and g and the spatial relationships between the four substitutional sites in the unit cell change as the temperature is lowered from 77 °K to ∼2 °K. The geometrical changes are consistent with a benzophenone phase transition from P212121 to P21 symmetry.