Changes In Unit Cell Parameters Research Articles

Time-resolved simultaneous SAXS/WAXS studies of peek during isothermal crystallization, melting, and subsequent cooling

Time-resolved simultaneous small- and wide-angle x-ray scattering (SAXS and WAXS, respectively) of poly(aryl ether ether ketone) (PEEK) during isothermal crystallization, melting, and subsequent cooling were conducted using synchrotron radiation. Unit cell parameters were determined using WAXS measurements, and the dimensions of lamellar-level moieties were extracted from SAXS profiles. It was found that the changes in unit cell parameters were reversible on heating and cooling, and there were remarkable differences in the thermal expansion coefficients in different lattice directions. The long period L, the lamellar thickness lc , and the thickness of amorphous layers la were calculated from the correlation function, and the changes of these dimensions during the processes mentioned above were traced. Sharp decreases in L and lc in the initial stage of isothermal crystallization and cooling from melt were observed, and a corresponding continuous distributed model is proposed. When cooling from the melt, the invariant Q exhibited a maximum. The maximum is attributed to a change in the difference of the densities of crystalline and amorphous phases (ρc - ρa) with decreasing temperature.

Stoichiometric deviations along an ingot of CuGaSe2

The dependence of the structural parameters on compositional deviations of CuGaSe2 has been studied. These deviations have been induced along an ingot by a single fusion of the components at 1150 °C and subsequent slow cooling in a stationary ampoule in a vertical furnace. All along the sample a single chalcopyrite phase is present and a compositional gradient along the ingot was found by energy-dispersive analysis of X-rays (EDAX) measurements, the upper part being rich in Ga (series B) and the lower part in Cu (series A), with Cu/Ga ratios of 0·95 and 1·1 respectively. A hypothesis of the existence of two phases in the melt is proposed to explain these facts. The unit cell parameters, anion displacement and Cu and Ga occupation numbers in their sublattices were analysed by X-ray powder diffraction and Rietveld refinement methods. In series A the occupation numbers are near stoichiometry, while in series B a Cu defect appears. In both series, changes in unit cell parameter are related to changes in Cu content, suggesting the presence of a fraction of Cu ions either as interstitials or at Ga sites when Cu is in excess, or of Cu vacancies in its sublattice when there is a Cu deficiency. © 1998 John Wiley & Sons, Ltd.

Mechanical milling of auriferous arsenopyrite and pyrite

Abstract The behaviour of pyrite and arsenopyrite in the process of mechanical milling and its potential for exsolution of gold were examined. Both reactants and milled products were characterised by powder X-ray diffractometry and by scanning electron microscopy of polished sections. In the presence of pyrite, arsenic-rich arsenopyrite can readily be transformed to sulphur-rich arsenopyrite, a process which does not readily occur by heating methods. The transformation mechanism can be explained by relating it to similar structural relationships between pyrite, marcasite and arsenopyrite. Detailed structural refinement of powder XRD data by the Rietveld method confirmed changes in unit cell parameters of arsenopyrite as a result of changes in stoichiometry. While the milling process may possibly liberate gold from the sulphide structures, the fracturing and welding processes occurring during milling can also potentially lock the gold up in the large aggregate arsenopyrite.

Preparation of Y-α-sialon with glassy or crystalline phases at grain boundaries

The phase stability of α-sialon has been investigated on a Y-α-sialon-Y 2O 3Al 2O 3 plane. Y-α-sialon was stable with YAG liquid composition at 1750 °C which could be devitrified to YAG (3Y 2O 3·5Al 2O 3) by post heat treatment at 1500 °C for 4 h. With increase of Al 2O 3 in the liquid composition of YAG, the amount of β-sialon increased. On the other hand, with increase of Y 2O 3 in the liquid composition, the amount of melilite (Si 3N 4 · Y 2O 3) increased. The changes of unit-cell parameters and microstructures with respect to the compositions and amounts of the liquid phase were also discussed.

Ultrasound effect on Ni diffusion in YBa 2Cu 3O 7−x

Positron annihilation technique, X-ray structureal analysis and radioactive tracers have been used to study ultrasound treatment effect on diffusion of Ni atoms in YBa 2Cu 3O 7−x. Ultrasound treatment results in anion sublattice ordering and redistribution of the electronic density inside the unit cell. Electron density redistribution contributes to the changes of unit cell parameters. The effects mentioned above lead to acoustic stimulation of the diffusion processes in ceramics, the diffusion coefficient of Ni being increased by a factor of 10 in 473 to 673 K temperature interval.

Crystals of HIV-1 Reverse Transcriptase Diffracting to 2·2 Å Resolution

Reverse transcriptase (RT) from the human immunodeficiency virus type 1 has been crystallized in four closely related forms, the best of which diffract X-rays to 2·2 Å resolution. The RT was crystallized as a complex with a non-nucleoside inhibitor, either nevirapine or a nevirapine analogue. Crystals grew from 6% PEG 3400 buffered at pH 5. These were of space group P 212121 with unit cell parameters a = 147 Å, b = 112 Å, c = 79 Å (form A), with one RT heterodimer in the asymmetric unit. Changes in unit cell parameters and degree of crystalline order were observe on soaking pregrown crystals in various solutions, giving three further sets of unit cells. These were a = 143 Å, b = 117 Å, c = 79 Å (form B), a = 141 Å, b = 111 Å, c = 73 Å. (form C), a = 143 Å, b = 117 Å, c = 66·5 Å (form D). The last two forms diffract X-rays to 2·2 Å resolution. Structure determinations of these latter crystal forms of RT should give a detailed atomic model for this therapeutically important drug target.

Crystallinity and Unit Cell Variations in Linear High-Density Polyethylene

Abstract The degree of crystallinity and unit cell parameters have been determined using WAXS on a number of compression molded high-density polyethylene (HDPE) plaques processed at widely varying conditions of crystallization and annealing times and temperatures. Changes in unit cell parameters with variations in processing conditions can be explained in terms of increases in lamellar thickness of polyethylene crystals with increasing thermal treatments. Concomitant increases in the degree of crystallinity of these samples can also be explained in terms of lamellar thickening and other changes in polyethylene morphology. Crystallinity determinations using XRD data are also compared with values determined by differential scanning calorimetry (DSC) and solid-state 13C nuclear magnetic resonance (NMR). Comparisons of crystallinity values obtained by these three different techniques can reveal details of the morphology of HDPE including the presence of an interfacial zone in addition to the crystalline and amorphous components of the system.

Chemical and structural properties of the system Fe2O3-Cr2O3

Chemical and structural properties of the mixed metal oxides (1−x)Fe2O3+xCr2O3 were studied by different techniques. X-ray powder diffraction showed the existence of solid solutions, (Fe1−x Cr x )2O3, over the whole concentration region, 0⩽x⩽1. The gradual replacement of Fe3+ with Cr3+ ions in samples prepared at 900°C caused changes in unit-cell parameters; most of these changes took place in the region fromx≈0.3–0.9. The samples having the fraction of Cr2O3 in the region from ∼0.7–0.8, contained two closely related phases, with slightly different compositions. After an additional heat treatment at 1100°C, these samples contained only one phase.57Fe Mossbauer spectroscopy showed a gradual decrease of hyperfine magnetic field with increasing Cr2O3 content. The sample having the fraction of Cr2O3 of 0.7, and prepared at 900°C, exhibited two separated sextets at room temperature, in comparison with other compositions showing one sextet. It was shown that Fourier transform infrared (FT-IR) spectroscopy is a powerful method for the investigation of structural changes in these solid solutions. The increase in the Cr2O3 content resulted in shifts of the corresponding infrared bands. In addition, a gradual transition of the spectrum typical for α-Fe2O3 to the spectrum typical for Cr2O3 was shown. The transition effects observed in the FT-IR spectra were correlated with the X-ray powder diffraction and57Fe Mossbauer spectroscopic results.

Computation of low-energy crystalline arrangements of cellulose triacetate

Low-energy crystal packings of cellulose triacetate (CTA) have been computed, based on a two-dimers unit cell as suggested for CTA I. In addition to the different packing arrangements, parallel versus antiparallel, the effect of changes in the main-chain conformation on the crystal packing was investigated. Minor alterations of the main-chain torsional angles were sometimes found to induce significant changes of unit-cell parameters, although yielding structures with comparable energies. Since small changes of the main-chain conformation do not involve large energy barriers, it is therefore assumed that, in crystalline CTA, local transitions between unit cells with slightly different parameters are possible. Even highly ordered CTA samples might thus consist of a distribution of marginally differing crystalline domains. The resulting imperfections at the interfaces of slightly different unit cells, conceivable on the basis of the computed structures, might explain the problems encountered in the experimental elucidation of the crystal structures in both CTA morphologies, CTA I and CTA 11.

Thermal enhancement of the electrical conductivities of alkali metal-doped polyacetylene complexes

Alkali metal-doped polyacetylene exhibits higher thermal stability than either the parent polymer or acceptor-doped complexes. For the Na, K, Rb and Cs complexes in which a channel structure is indicated, this stability appears to be related to the packing arrangement, which interferes with interchain reactions. In contrast with the case for acceptor doping, the conductivities of the alkali metal-doped cis-polyacetylenes are lower than for correspondingly doped trans-polyacetylenes. However, thermal annealing of the K-doped or Rb-doped polyacetylene (obtained using the naphthalide in THF) results in dramatic enhancement of conductivity for the doped cis-polymer, but not for the doped trans-polymer. After about a six-fold increase in room-temperature conductivity, the conductivity of K-doped polyacetylene is stable for many hours at 200 °C. Cis-polyacetylene films doped in the same way with other alkali metals do not show this enhancement. Significant changes in unit cell parameters, ESR linewidths, electrical anisotropy and contact resistance also occur during the annealing of K-doped cis-polyacetylene. Results presented suggest that thermal annealing of the K-doped polyacetylene eliminates residual conformational defects in partially chemically-isomerized polymer chains, thereby increasing crystalline perfection, increasing the effective conjugation length and increasing the hybridization of alkali metal and carbon orbitals. These changes probably result in the observed conductivity enhancement, the dramatic increase in ESR linewidth (6 – 12 G to 40 – 60 G) and the decrease in unit cell parameter.

Die spontane Spaltung von 3,3‐Diäthyl‐5‐methylpiperidin‐2,4‐dion(Methyprylon) in die optischen Isomeren. Eine Rötgenstrukturanalyse eines isodimorphen Systems von Mischkristallen

The Spontaneous Resolution of the 3,3‐Diethyl‐5‐methylpiperidine‐2,4‐dione (Methyprylon) into its Optical Isomers. An X‐Ray Structure Analysis of an Isodimoophous System of Mixed CrystalsRepeated recrystallizations from acetone of a racemic mixture of the title compound successively result in a spontaneous resolution into optical isomers [1]. In this process batches of mixed crystals of increased optical purity are formed [2]. This phenomena was investigated on a molecular basis using X‐ray diffraction methods. The mixed crystals belong to the space group P212121 and their diffraction patterns give no indication of major disorders. There is a linear relationship between the changes in unit cell parameters a and b and the optical rotation of the sample. The structures of the mixed crystals from four fractions belonging to two different crystalline modifications have been determined. The molecular shape is such that most of the atomic positions of the two enantiomers superimpose almost perfectly. Occupation factors determined for the asymmetric C‐ sites agree well with the optical activity of the crystals fraction.

Defect structures in the brannerite-type vanadates. III. Preparation and study of Cu 2+1− x− yCu 1+yф xV 2−2 x− yMo 2 x+ yO 6 ( xmax = 0.23; ymax = 0.27)

Phases of the formula Cu 2+ 1− x−y Cu 1+ y ф x V 2−2 x−y Mo 2 x+y O 6, where ф x represents a vacancy at Cu 2+ site, have been synthesized and characterized by X-ray diffraction, DTA, and magnetic susceptibility measurements. The extent of their homogeneity range has been established. All crystallize in the structure closely related to the brannerite type with the symmetry reduced from monoclinic to triclinic because of a Jahn-Teller distortion of CuO 6 octahedra. On heating they undergo a phase transformation to a monoclinic phase. Unit cell dimensions and transition temperatures were compiled for eight samples of the solid solution, and correlations were established between the chemical composition ( x, y) and the structural parameters. Changes in unit cell parameters involve the expansion of the lattice and the systematic evolution of the triclinic unit cell to the monoclinic one.

Phase Transitions in Single Crystals of L-DPPCand DL-DPPC

Abstract By use of thin layered single micro-crystals obtained from xylene suspensions, the polymorphism of L- and DL-dipalmitoylphosphatidylcholines (DPPC) was studied by electron and X-ray diffractions, microscopic observations, and thermal analyses, with particular attention to the structural transformation in the bilayer plane associated with a thermotropic phase transition, which occurred at about 60°C for dihydrated crystals. At this phase transition, the disappearance of the optical anisotropy in the bilayer plane of the low temperature phase and large changes in crystal dimensions were observed microscopically, and by diffraction studies corresponding changes in unit cell parameters and intensity distributions were observed. The effect of water on thermotropic phase transition behaviours of DPPC's were also examined. The combination of the present experimental methods and the use of monodomain single crystals facilitate the analysis of molecular structures and packings in bilayer plane, which will...

Magnetic and electrical properties and Mössbauer effect in the solid solution Fe(Nb 1− xW x)O 4 (0 ⩽ x ⩽ 1)

The solid solution Fe(Nb 1− x W x )O 4 (0 ≦ x ≦ 1) with wolframite-type structure has been prepared under argon atmosphere at 1000°C. The change of unit cell parameters is not linear, owing to the zigzag chain of octahedra along the c-axis. The results of magnetic susceptibility and Mössbauer effect measurements indicate the existence of a mixed valence state of iron ions.

Changes in mordenite upon various pretreatments. Part 1.—Structural rearrangements

The structural rearrangements of two mordenites, one dealuminated, upon various heat treatments have been studied by X-ray diffraction and i.r. spectrometry. The simultaneous changes in unit cell parameters and in i.r. band frequency or intensity allow some conclusions to be drawn. The removal of chemical aluminium creates a large number of SiOH groups while simultaneously a partial recrystallization occurs with formation of new Si—O bonds. Dry air treatment of the two zeolites does not modify them much. Heating the mordenite, dealuminated or not, under a wet atmosphere at 500°C removes a different type of aluminium atoms with simultaneous structural rearrangement giving new Si—O bonds.

Neodymium incorporation into YAl‐borate crystals in preparation from solutions in molten potassium trimolybdate

AbstractThe limits of neodymium substitution for yttrium in (Y, Nd)Al3(BO3)4 crystals in preparation from solutions in molten potassium trimolybdate have been studied. The maximum neodymium concentration in the crystals was 72 at.%, this corresponding to the ratio Nd:Y = 1 in the solution. When this ratio exceeds unity, only potassium‐neodymium molybdate is formed. The distribution coefficient for neodymium is greater than unity and decreases both with the increasing Nd/Y ratio in the melt and with decreasing growth rate of the crystals. The unit cell parameters change in the series YAl3(BO3)4NdAl3(BO3)4 in a non‐steady manner, this indicating that the structure is inclined to disordering.

Correlation between adsorption heats and change in unit cell parameters of zeolite CaA during the adsorption of water vapor

1. A correlation was made between the energy and deformation characteristics for the sorption system: zeolite CaA-H2O. 2. Two molecular mechanisms were discussed for the reaction of H2O with Ca cations and the penetration of H2O molecules into sodalite cells.

Magnetic property in the system BaCo 1− xNi xO 3 (x = 0 ∼ 1)

The system ofBaCo 1−xNi xO 3 (x = 0 ∼ 1) with a two-layer hexagonal perovskite-type structure are prepared at an oxygen pressure of 2000 bars. The change in unit cell parameters is not linear. Magnetic measurements indicate that BaCoO 3, BaCo 0.9Ni 0.1O 3 and BaCo 0.8Ni 0.2O 3 are antiferromagnetic with T N = 8, 10,and10°K, respectively.

A phase transformation with no change in space group symmetry; octafluoronaphthalene

A solid-state phase transformation in octafluoronaphthalene has been discovered at 266.5K on cooling, and at 15K higher on heating. The symmetry of both phases is found to be the same, namely monoclinic with space group P21/c. The unit cell parameters change by up to 10%, but the integrity of a single crystal, which shatters on cooling, is good enough for a single-crystal structure determination. This has been done in both phases to a sufficient accuracy that a mechanism for the transformation can be proposed. Molecules which lie parallel to one another shear to a new parallel position, the shear movement being equal to one carbon-carbon bond of the naphthalene skeleton. In this process the molecules reorient, but are still related by the same symmetry operations. This transformation, although not unique, is probably the first of its kind to be discovered in molecular systems.

Lattice-Frequency Studies of Crystalline and Fold Structure in Polyethylene

The assignment of the 71 cm−1 band in the infrared spectrum of polyethylene to the B1u translational lattice vibration has been confirmed by dichroism studies on an a-axis oriented sample. This permits confident use of the Tasumi-Krimm calculations for the dependence of this frequency of unit-cell parameters. The results of these calculations have been applied to the analysis of the observed lattice frequency differences between odd and even n-paraffins, and it is shown that the frequency shifts are interpretable in terms of changes in unit cell parameters resulting from different methyl end-group packing. The variation of the lattice frequency with degree of branching observed in a series of low-density polyethylenes can be understood on the basis of incorporation of branches in the lattice. The dependence of the lattice frequency in high-density polyethylene on the physical state of the specimen can be correlated with different constraints imposed by the fold regions on the chain packing. These results indicate that the folds must be considered to be ``tight'' rather than ``loose.''