Per-Erik Werner

TREOR, a semi-exhaustive trial-and-error powder indexing program for all symmetries

An indexing program, TREOR, mainly based on trial-and-error methods is described. The program contains separate routines for cubic, tetragonal, hexagonal, orthorhombic, monoclinic and triclinic symmetries. Ten years usage has been analysed to improve the original program. For monoclinic indexing a specific short-axis test has been developed. The over-all success rate of the program has been found to be better than 90%, and considerably more for orthorhombic and higher symmetries.

New techniques for indexing: N-TREOR in EXPO

Indexing of a powder diffraction pattern is still a critical point in procedures aiming at solving crystal structures from powder data. New code has been associated to the program TREOR90 in order to define an efficient peak search procedure, to modify the crystallographic decisions coded into TREOR90 to make it more exhaustive, to refine the selected unit cell automatically, and to make the entire procedure user friendly, via a graphical interface. The new program, called N-TREOR, has been integrated into the package EXPO to create a suite of programs able to provide a structural model from the analysis of the experimental pattern. N-TREOR is also available as a stand-alone program.

Advances in powder diffraction pattern indexing: N-TREOR09

Powder pattern indexing can still be a challenge, despite the great recent advances in theoretical approaches, computer speed and experimental devices. More plausible unit cells, belonging to diffe ...

Crystallization kinetics and morphology of poly(propylene-stat-ethylene) fractions

The crystallization and melting behaviour and the morphology of fractions of poly(propylene-stat-ethylene) with 2.7–11.0 mol% ethylene were studied by differential scanning calorimetry, wide-angle X-ray scattering, polarized light microscopy and transmission electron microscopy, after etching with permanganic acid. The inclusion of ethylene co-repeating units in isotactic polypropylene (0–11.0 mol% ethylene) caused approximately linear decreases in kinetic and equilibrium melting temperatures and in the glass transition temperature with increasing ethylene content. X-ray scattering showed that the content of the γ form increased with increasing ethylene content, increasing crystallization temperature and decreasing cooling rate from the molten state. It was shown for one of the copolymers (8.7 mol% ethylene) that during heating approximately 50% of the γ form was converted to the α form before the final melting of the sample. The rest of the γ crystals melted without transformation to the α form. The multimodality of the crystal melting above the crystallization temperature in the polymers with a more uniform crystal structure was caused by recrystallization during heating, whereas polymers with appreciable contents of both α and γ forms exhibited multimodal melting at all the heating rates adopted. The size of the low temperature melting peak as assessed at a heating rate of 40 K min−1 was approximately proportional to the initial content of the γ form. By comparison with the spherulitic structure of homopolymers, that of the copolymers was coarser with internal and peripheral pockets of molten material during spherulite growth. The crystal lamellae exhibited more curvature in the copolymer samples than in the homopolymer.

Structure of a high-pressure phase of vanadium pentoxide, β-V2O5

A high-pressure phase of vanadium pentoxide, denoted β-V2O5, has been prepared at P = 6.0 GPa and T = 1073 K. The crystal structure of β-V2O5 has been studied by X-ray and neutron powder diffraction, and high-resolution transmission electron microscopy. The V atoms are six-coordinated within distorted VO6 octahedra. The structure is built up of quadruple units of edge-sharing VO6 octahedra linked by sharing edges along [010] and mutually connected by sharing corners along [001]. This arrangement forms layers of V4O10 composition in planes parallel to (100). The layers are mutually held together by weak forces. β-V2O5 is metastable and transforms to α-V2O5 at 643–653 K under ambient pressure. Structural relationships between β- and α-V2O5, and between β-V2O5 and B-Ta2O5-type structures are discussed. The high-pressure β-V2O5 layer structure can be considered as the parent of a new series of vanadium oxide bronzes with cations intercalated between the layers.

The structure of the low temperature phase Mg2NiH4(LT)

The crystal structure of a low temperature modification of Mg2NiH4(LT), stable below 235°C, has been determined from Guinier-Hagg powder diffraction data. The unit cell dimensions are a = 6.497(2)A, b = 6.414(1)A, c = 6.601(2)A and β = 93.23(2)°. The structure has been refined by profile analysis from a sample also containing MgH2 and small amounts of two other phases, viz. the high temperature modification Mg2NiH3.9(HT) and Mg2NiHx(LT). It is indicated that the phase transformation of Mg2NiH3.9 (HT) at 235°C is eutectoid, giving mainly Mg2NiH4(LT) but also small amounts of a less hydrogen containing phase Mg2NiHx(LT) (x≈2).

Crystal structure determination and rietveld refinement of Zn(OH)(NO3) · H2O

Abstract The crystal structure of Zn(OH)(NO3) · H2O has been studied by X-ray diffraction methods using fibrous microcrystals and powder. The cell dimensions are a = 17.951 A, b = 3.2600 A, c = 14.272 A, and β = 114.91°. The space group is P2 1 c . The structure has been refined by the Rietveld full-profile technique. The results obtained from different data sets and the corresponding discrepancy indicators are compared. The structural chemistry of the compound is discussed and an interpretation of its topotactic transformation into Zn3(OH)4(NO3)2 is presented.

Crystal structure and morphology of melt-crystallized poly(propylene-stat-ethylene) fractions

Abstract Crystallinity, crystal structure and lamellar thickness in melt-crystallized samples of poly(propylene-stat-ethylene) fractions with 2.7–11.0 mol% ethylene comonomer and of approximately constant tacticity were assessed by wide- and small-angle X-ray scattering, differential scanning calorimetry and infra-red spectroscopy. Most of the samples were crystallized under isothermal conditions at 373 K. In comparison with an isotactic homopolymer of polypropylene, the copolymers showed lower crystallinity, melting enthalpy and average length of 3 1 helices, a slightly larger unit cell, a longer long period and an invariant lamellar thickness. The X-ray crystallinity of the copolymers remained approximately constant with increasing ethylene content, whereas the γ-crystallinity increased and the heat of fusion decreased moderately. It is suggested that the ethylene units are partially included in the crystals, and that this causes the invariance in crystallinity and crystal thickness. The observed gradual decrease in average 3 1 helix length with increasing ethylene content as assessed by infra-red spectroscopy is in accordance with this suggestion.

The structure refinement of boehmite (-A100H) and the study of its structural variability based on Guinier-Hägg powder data

Abstract The extent of structural variability of boehmite has been studied by the comparison of the structure parameters of a synthetic and a natural mineral sample. Profile-fitting refinement based on Guinier-Hagg type powder data was used to determine the actual structures. It is also concluded that the small structural differences found do not interfere with the established practice of taking boehmite as reference in X-ray diffractometric quantitative phase analysis of minerals.

Determination of the composition of BaNbO3 using profile refinement and phase analysis

Abstract A small but distinct shift of 0.2% in the unit cell axis of BaNbO 3 has been observed between samples having gross compositions on different sides of the BaNbO 3 -Ba 2 Nb 5 O 9 binary line in the Ba-Nb-O system. To investigate if this shift was due to a variation of the Ba-occupancy, profile refinements using the Rietveld technique were made. The outcome of these refinements are discussed.