High-pressure study of low-compressibility Ta2N

Room temperature synchrotron x-ray diffraction experiments were performed on nanocrystal Ta₂N in a diamond anvil cell to a pressure of 55.48 GPa. This nitride is a well-known kind of high-hardness material, and it is found to be highly incompressible. The structure is stable with no phase transitions observed in this pressure range. The zero-pressure bulk modulus and its pressure derivative at ambient pressure are B₀ = 360 ± 3 GPa, B₀' = 4, and in room conditions, the a and c parameters are 3.054 Å, 4.996 Å, respectively. The bulk modulus of Ta₂N is greater than those of TaC, ε-TaN, Cr₂N and MoN. The differences in bulk moduli might be due to the differences in structure and the cohesive energy among these phases.

Halogen-free chelated orthoborate ionic liquids and organic ionic plastic crystals

Five halogen-free orthoborate salts comprised of three different cations (cholinium, pyrrolidinium and imidazolium) and two orthoborate anions, bis(mandelato)borate and bis(salicylato)borate, were synthesised and characterised by DSC, X-ray diffraction and NMR. DSC measurements revealed that glass transition points of these orthoborate salts are in the temperature range from −18 to −2 °C. In addition, it was found that [EMPy][BScB] and [EMIm][BScB] salts have solid–solid phase transitions below their melting points, i.e. they exhibit typical features of plastic crystals. Salts of the bis(salicylato)borate anion [BScB]− have higher melting points compared with corresponding salts of the bis(mandelato)borate anion [BMB]−. Single crystal X-ray diffraction crystallography (for [Chol][BScB] crystals) and solid-state multinuclear (13C, 11B and 15N) NMR spectroscopy were employed for the structural characterisation of [Chol][BScB], [EMPy][BScB] and [EMIm][BScB], which are solids at room temperature: a strong interaction between [BScB]− anions and [Chol]+ cations was identified as (i) hydrogen bonding between OH of [Chol]+ and carbonyl groups of [BScB]− and (ii) as the inductive C–Hπ interaction. In the other salt, [EMIm][BScB], anions exhibit ππ stacking in combination with C–Hπ interactions with [EMIm]+ cations. These interactions were not identified in [EMPy][BScB] probably because of the lack of aromaticity in cations of the latter system. Our data on the formation of a lanthanum complex with bis(salicylato)borate in the liquid mixture of La3+(aq) with [Chol][BScB] suggest that this class of novel ILs can be potentially used in the extraction processes of metal ions of rare earth elements.

Structure and transport properties of a plastic crystal ion conductor : Diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate

Understanding the ion transport behavior of organic ionic plastic crystals (OIPCs) is crucial for their potential application as solid electrolytes in various electrochemical devices such as lithium batteries. In the present work, the ion transport mechanism is elucidated by analyzing experimental data (single-crystal XRD, multinuclear solid-state NMR, DSC, ionic conductivity, and SEM) as well as the theoretical simulations (second moment-based solid static NMR line width simulations) for the OIPC diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P1,2,2,4][PF6]). This material displays rich phase behavior and advantageous ionic conductivities, with three solid–solid phase transitions and a highly “plastic” and conductive final solid phase in which the conductivity reaches 10–3 S cm–1. The crystal structure shows unique channel-like packing of the cations, which may allow the anions to diffuse more easily than the cations at lower temperatures. The strongly phase-dependent static NMR line widths of the 1H, 19F, and 31P nuclei in this material have been well simulated by different levels of molecular motions in different phases. Thus, drawing together of the analytical and computational techniques has allowed the construction of a transport mechanism for [P1,2,2,4][PF6]. It is also anticipated that utilization of these techniques will allow a more detailed understanding of the transport mechanisms of other plastic crystal electrolyte materials.

Combinational rate effects on the performance of nano-grained pseudoelastic Nitinols

Combinational loading-unloading rate effects were studied on the behavior of NiTi shape memory alloys (SMAs) under nanoindentation loads. While combinational loading rates showed negligible effects on the performance of NiTi SMAs, the combinational unloading rates did show significant effects on hysteresis energy. The heating-cooling phenomenon during the loading stage and the sole cooling during the unloading stage explain the effects. This study elucidates the nature of thermomechanical SMAs' behaviors during complex compressive loadings with the presence of solid-state phase transition.

Modelling ion-pair geometries and dynamics in a 1-ethyl-1-methylpyrrolidinium-based ion-conductive crystal

Full conformational and energy explorations are conducted on an organic ionic plastic crystal, 1-ethyl-1-methylpyrrolidium tetrafluoroborate [C2 mpyr][BF4 ]. The onsets of various stages of dynamic behaviour, which appear to account for low-temperature solid-solid phase transitions, are investigated by using quantum-chemical simulations. It is suggested that pseudorotation of the pyrrolidine ring occurs in the first instance; the partial rotation of the entire cation subsequently occurs and may be accompanied by reorientation of the ethyl chain as the temperature increases further. A cation-anion configuration, whereby BF4 (-) interacts with the C2 mpy cation from the side of the ring, is the most likely structure in the low-temperature phase IV region. These interpretations are supported by (13) C nuclear magnetic resonance chemical-shift analysis.

Spin-crossover, mesomorphic and thermoelectrical properties of cobalt(II) complexes with alkylated N3-Schiff bases

Three new cobalt(ii) complexes, [Co(L12)2](BF4)2 (1), [Co(L14)2](BF4)2·H2O (2) and [Co(L16)2](BF4)2·H2O (3), where L12-16 are N3-Schiff bases appended with linear C12-16 carbon chains at the nitrogen atoms, were obtained in good yields by facile one-pot reactions. The single crystal X-ray structure of complex 1 shows a tetragonally compressed CoN6 coordination geometry. The melting temperatures of 1-3 were lower than 373 K, while their decomposition temperatures were above 473 K. All complexes have high-spin Co(ii) centres at 300 K and exhibit a columnar mesophase above 383 K. Complexes 1 and 3 showed normal thermal spin-crossover behaviour with weak hysteresis loops at about 320 K. Hence, these complexes showed uncoupled phase transitions (class iiia). The values for the Seebeck coefficient (Se) of the cobalt redox couples formed from 1 and 2 were 1.89 ± 0.02 mV K-1 and 1.92 ± 0.08 mV K-1, respectively, identifying them as potential thermoelectrochemical materials.

Crystallographic variant selection in α–β brass

The transformation texture of α/β brass with a diffusional Widmanstätten α growth morphology has been investigated. Electron micrographs and electron backscattered diffraction was used to determine that the orientation relationship between the β phase and the α associated with nucleation at β grain boundaries was (44.3°) left angle bracket1 1 6right-pointing angle bracket. Crystallographic variant selection was observed across those prior β/β grain boundaries, but this has little effect on the transformation texture due to the crystal symmetry. The effect of the crystallographic variant selection on texture is further weakened by nucleation of diffusional transformed α in the grain interior.

Crystallographic variant selection in Ti-6Al-4V

Transformation textures in the two-phase alloy Ti–6Al–4V have been studied. Samples were heated into the fully β phase condition and then slow cooled to allow diffusional transformation to α. This produced a microstructure of grain boundary α encircling colonies of Widmanstätten α. Electron backscattered diffraction (EBSD) texture measurements showed that the α texture was markedly sharper than that calculated on a basis of equal variant probability, indicating that significant variant selection was occurring during diffusional transformation. Investigation of the α variants produced across prior β grain boundaries has shown that the selection of variants during transformation is highly dependant on the crystallography of those boundaries. The effect of this crystallographic variant selection on the transformation texture has been modelled.

Having a baby and being in school : researching pregnant and parenting young people and their educational transitions

This paper reports on the 2002 pilot phase (in a provincial city) of a continuing study of pregnant and parenting young people and their movements in and out of school (and other educational) settings. It presents an overview of methodological approaches employed and dilemmas encountered, data collected and readings of that data, and an indication of how issues identified from the pilot study have informed the directions and emphases of an expanded investigation for 2003 and beyond. The paper draws on specific cases to identify how young people negotiate their way in and out of school during this phase of their lives. It offers an insight into how young people see themselves ''becoming somebody'' in and around other identity work they engage in while pregnant and parenting at school. The research provides knowledge about the intersection between the institutional and individual complexities of leaving and staying-on at school, including an account of the academic and social reasons for leaving or returning to school and school responses to student pregnancy and parenting.

Order–disorder transitions in poly(3,4-ethylenedioxythiophene)

The commonly held perception that high conductivity in conducting polymers is linked to a high level of π-stacking order in the material is shown here to be of lesser importance in highly conducting poly(3,4-ethylenedioxythiophene) (PEDT), which has been prepared by chemical vapour phase polymerisation. Despite the fact that there is a highly energetic phase transition about 130 °C (110 J/g), and that this transition corresponds to a loss of the long-range π-stacking as observed in grazing angle XRD, the conductivity remains unchanged beyond the transition and only decreases by a factor of two when heating to above 200 °C. The XRD data suggest that order in two dimension remains above the phase transition measured by DSC and this order is sufficient to maintain a high level of electronic conductivity. Furthermore, as the ligand on the iron salt used in the synthesis is varied, the conductivity of the PEDT varies over two orders of magnitude. These phenomena cannot be explained by different degree of doping or crystallinity and it is proposed that the iron salt has an ordering effect during the vapour phase polymerisation.

Unusual phase behaviour of the organic ionic plastic crystal N,N-dimethylpyrrolidinium tetrafluoroborate

Analysis of N,N-dimethylpyrrolidinium tetrafluoroborate by ¹H and ¹¹B NMR, Raman spectroscopy and powder XRD shows that this organic ionic plastic crystal material exhibits unusual phase behaviour. ¹H NMR analysis indicates that the mobility of the pyrrolidinium cation decreases when the material is heated into phase I, while the X-ray diffraction pattern changes from a simple, one peak structure in phase II to a more complex pattern in phase I. The possible origins of these unusual transitions are discussed.

High-pressure structural transitions of Sc2O3 by X-ray diffraction, raman spectra, and ab initio calculations

The high-pressure behavior of scandium oxide (Sc₂O₃) has been investigated by angle-dispersive synchrotron powder X-ray diffraction and Raman spectroscopy techniques in a diamond anvil cell up to 46.2 and 42 GPa, respectively. An irreversible structural transformation of Sc₂O₃ from the cubic phase to a monoclinic high-pressure phase was observed at 36 GPa. Subsequent ab initio calculations for Sc₂O₃ predicted the phase transition from the cubic to monoclinic phase but at a much lower pressure. The same calculations predicted a second phase transition at 77 GPa from the monoclinic to hexagonal phase.

Structure and phase stability of NiMnGa ferromagnetic shape memory alloys by experimental and ab-initio techniques

This paper summarizes some of our recent results on crystal structure, microstructure, orientation relationship between martensitic variants and crystallographic features of martensitic transformation in Ni-Mn-Ga FSMAs. It was shown that Ni53Mn25Ga22 has a tetragonal I4/mmm martensitic structure at room temperature. The neighboring martensitic variants in Ni53Mn25Ga22 have a compound twinning relationship with the twinning elements K1={112}, K2={11-2}, η1=<11-1>, η2=<111>, P={1-10} and s=0.379. The ratio of the relative amounts of twins within the same initial austenite grain is ~1.70. The main orientation relationship between austenite and martensite is Kurdjumov-Sachs (K-S) relationship. Based on the crystallographic phenomenological theory, the calculated habit plane is {0.690 -0.102 0.716}A (5.95° from {101}A), and the magnitude, direction and shear angle of the macroscopic transformation shear are 0.121, <-0.709 0.105 0.698>A (6.04° from <-101>A) and 6.88°, respectively.

Application of white-beam X-ray microdiffraction for the study of mineralogical phase identification in ancient Egyptian pigments

High-brightness synchrotron X-rays together with precision achromatic focusing optics on beamline 7.3.3 at the Advanced Light Source have been applied for Laue microdiffraction analysis of mineralogical phases in Egyptian pigments. Although this task is usually performed using monochromatic X-ray diffraction, the Laue technique was both faster and more reliable for the present sample. In this approach, white-beam diffraction patterns are collected as the sample is raster scanned across the incident beam (0.8 µm × 0.8 µm). The complex Laue diffraction patterns arising from illumination of multiple grains are indexed using the white-beam crystallographic software package XMAS, enabling a mineralogical map as a function of sample position. This methodology has been applied to determine the mineralogy of colour pigments taken from the ancient Egyptian coffin of Tjeseb, a priestess of the Apis bull dating from the Third Intermediate to Late period, 25th Dynasty to early 26th Dynasty (747 to 600 BC). For all pigments, a ground layer of calcite and quartz was identified. For the blue pigment, cuprorivaite (CuCaSi4O10) was found to be the primary colouring agent with a grain size ranging from ∼10 to 50 µm. In the green and yellow samples, malachite [Cu2(OH)2CO3] and goethite [FeO(OH)] were identified, respectively. Grain sizes from these pigments were significantly smaller. It was possible to index some malachite grains up to ∼20 µm in size, while the majority of goethite grains displayed a nanocrystalline particle size. The inability to obtain a complete mineralogical map for goethite highlights the fact that the incident probe size is considerably larger than the grain size. This limit will continue to improve as the present trend is toward focusing optics approaching the diffraction limit (∼1000× smaller beam area).