Long-range Crystallographic Order Research Articles

Solid-State 183W NMR Spectroscopy as a High-Resolution Probe of Polyoxotungstate Structures and Dynamics.

Polyoxometalates such as ammonium paratungstate (APT) constitute an important class of metal oxides with applications for catalysis, (opto)electronics, and functional materials. Structural analyses of solid polyoxometalates mostly rely on X-ray or neutron diffraction techniques, which are largely limited to compounds that can be isolated with long-range crystallographic order. While 183W NMR has been shown to probe polyoxotungstate structures and dynamics in solution, its application to solids has been extremely limited. Here, state-of-the-art methods for the detection of solid-state 183W NMR spectra are tested and compared for APT in different hydration states. The highly resolved solid-state spectra distinguish each crystallographically distinct site in the tungstate structure. Furthermore, the 183W chemical shifts are shown to be highly sensitive to the local structure, dynamics, and symmetry of APT, establishing solid-state 183W NMR spectroscopy as a potent probe for analysis of polyoxotungstates and other tungsten-derived materials to complement solution NMR and diffraction-based techniques.

The Spatial Correlation and Anisotropy of β-(AlxGa1-x)2O3 Single Crystal.

The long-range crystallographic order and anisotropy in β-(AlxGa1-x)2O3 (x = 0.0, 0.06, 0.11, 0.17, 0.26) crystals, prepared by optical floating zone method with different Al composition, is systematically studied by spatial correlation model and using an angle-resolved polarized Raman spectroscopy. Alloying with aluminum is seen as causing Raman peaks to blue shift while their full widths at half maxima broadened. As x increased, the correlation length (CL) of the Raman modes decreased. By changing x, the CL is more strongly affected for low-frequency phonons than the modes in the high-frequency region. For each Raman mode, the CL is decreased by increasing temperature. The results of angle-resolved polarized Raman spectroscopy have revealed that the intensities of β-(AlxGa1-x)2O3 peaks are highly polarization dependent, with significant effects on the anisotropy with alloying. As the Al composition increased, the anisotropy of Raman tensor elements was enhanced for the two strongest phonon modes in the low-frequency range, while the anisotropy of the sharpest Raman phonon modes in the high-frequency region decreased. Our comprehensive study has provided meaningful results for comprehending the long-range orderliness and anisotropy in technologically important β-(AlxGa1-x)2O3 crystals.

Anion-polarisation-directed short-range-order in antiperovskite Li2FeSO.

Short-range ordering in cation-disordered cathodes can have a significant effect on their electrochemical properties. Here, we characterise the cation short-range order in the antiperovskite cathode material Li2FeSO, using density functional theory, Monte Carlo simulations, and synchrotron X-ray pair-distribution-function data. We predict partial short-range cation-ordering, characterised by favourable OLi4Fe2 oxygen coordination with a preference for polar cis-OLi4Fe2 over non-polar trans-OLi4Fe2 configurations. This preference for polar cation configurations produces long-range disorder, in agreement with experimental data. The predicted short-range-order preference contrasts with that for a simple point-charge model, which instead predicts preferential trans-OLi4Fe2 oxygen coordination and corresponding long-range crystallographic order. The absence of long-range order in Li2FeSO can therefore be attributed to the relative stability of cis-OLi4Fe2 and other non-OLi4Fe2 oxygen-coordination motifs. We show that this effect is associated with the polarisation of oxide and sulfide anions in polar coordination environments, which stabilises these polar short-range cation orderings. We propose that similar anion-polarisation-directed short-range-ordering may be present in other heterocationic materials that contain cations with different formal charges. Our analysis illustrates the limitations of using simple point-charge models to predict the structure of cation-disordered materials, where other factors, such as anion polarisation, may play a critical role in directing both short- and long-range structural correlations.

Short- and Long-Range Cation Disorder in (AgxCu1–x)2ZnSnSe4 Kesterites

Understanding the nature of and controlling the cation disorder in kesterite-based absorber materials remain a crucial challenge for improving their photovoltaic (PV) performances. Herein, the combination of neutron diffraction and synchrotron-based X-ray absorption techniques was implemented to investigate the relationships among cation disorder, defect concentration, overall long-range crystallographic order, and local atomic-scale structure for (AgxCu1–x)2ZnSnSe4 (ACZTSe) material. The joint Rietveld refinement technique was used to directly reveal the effect of cation substitution and quantify the concentration of defects in Ag-alloyed stoichiometric and nonstoichiometric Cu2ZnSnSe4 (CZTSe). The results showed that 10%-Ag-alloyed nonstoichiometric ACZTSe had the lowest concentration of detrimental antisite CuZn defects (∼8 × 1019 defects per cm–3), which was two times lower than pristine and five times lower than the stoichiometric compositions. Moreover, Ag incorporation maintained the concentrations of beneficial Cu vacancies (VCu) and antisite ZnCu defects to >2 × 1020 defects per cm–3. X-ray absorption measurements were performed to verify the degree of disorder through the changes in bond length and coordination number. Therefore, the incorporation of Ag into the CZTSe lattice could control the distribution of antisite defects, in the form of short- and long-range site disorder. This study paves the way to systematically understand and further improve the properties of kesterite-based materials for different energy applications.

Sr2Pt8-x As: a layered incommensurately modulated metal with saturated resistivity.

The high-pressure synthesis and incommensurately modulated structure are reported for the new compound Sr2Pt8-x As, with x = 0.715 (5). The structure consists of Sr2Pt3As layers alternating with Pt-only corrugated grids. Ab initio calculations predict a metallic character with a dominant role of the Pt d electrons. The electrical resistivity (ρ) and Seebeck coefficient confirm the metallic character, but surprisingly, ρ showed a near-flat temperature dependence. This observation fits the description of the Mooij correlation for electrical resistivity in disordered metals, originally developed for statistically distributed point defects. The discussed material has a long-range crystallographic order, but the high concentration of Pt vacancies, incommensurately ordered, strongly influences the electronic conduction properties. This result extends the range of validity of the Mooij correlation to long-range ordered incommensurately modulated vacancies. Motivated by the layered structure, the resistivity anisotropy was measured in a focused-ion-beam micro-fabricated well oriented single crystal. A low resistivity anisotropy indicates that the layers are electrically coupled and conduction channels along different directions are intermixed.

The interaction between stearic acid monolayers and butane under elevated pressures

A study on the interaction between butane and lipid membranes is presented. Monomolecular films of stearic acid were prepared on a water surface and were exposed to dense gas phases of n-butane and isobutane. From X-ray reflectivity measurements and grazing incidence X-ray diffraction, the accumulation of gas molecules at the liquid–gas interface was analyzed. We show that the gas molecules penetrate into the lipid membrane and accumulate between the head group and tail group of the lipids. This process goes in hand with an increase of the long-range lateral crystallographic order within the lipid film. This and the formation of multilayer islands points to an expulsion of lipids by the gas molecules. At higher pressures close to the condensation pressure of the studied gases, the vertical and lateral order is lost, indicating an adsorbed liquid film in which the lipids are dissolved.

X-ray spectroscopic studies of the electronic structure of chromium-basedp-type transparent conducting oxides

The valence band structure of $p$-type transparent oxides---crystalline ${\text{Mg}}_{x}{\text{Cr}}_{2\ensuremath{-}x}{\text{O}}_{3}$ and nanocrystalline ${\text{Cu}}_{x}{\text{CrO}}_{y}$---is analyzed as a function of incoming photon energy. The valence band of both $p$-type transparent conducting oxides shows striking similarities to measurements on crystalline ${\text{CuCrO}}_{2}$:Mg with all films showing that chromium states compose the top of the valence band, suggesting that the valence-band structure is dominated by the presence of the Cr-${\mathrm{O}}_{6}$ octahedra. A comparison of the valence band between the best performing $p$-type, crystalline ${\mathrm{CuCrO}}_{2}$:Mg, with crystalline ${\text{Mg}}_{x}{\text{Cr}}_{2\ensuremath{-}x}{\text{O}}_{3}$ and nanocrystalline ${\text{Cu}}_{x}{\text{CrO}}_{y}$ shows that the chromium $3d$ states are fixed irrespective of changes in long-range crystallographic order. This indicates little spatial overlap between adjacent Cr $3d$ states. This further confirms the conduction mechanism via hopping for chromium based $p$-type TCOs as the Cr $3d$ states are localized within the Cr-${\mathrm{O}}_{6}$ octahedra.

Thermal evolution of nanocrystalline co-sputtered Ni–Zr alloy films: Structural, magnetic and MD simulation studies

Monophasic and homogeneous Ni10Zr7 nanocrystalline alloy films were successfully grown at room temperature by co-sputtering in an indigenously developed three-gun DC/RF magnetron sputtering unit. The films could be produced with long-range crystallographic and chemical order in the alloy, thus overcoming the widely acknowledged inherent proclivity of the glass forming Ni–Zr couple towards amorphization. Crystallinity of these alloys is a desirable feature with regard to improved efficacy in applications such as hydrogen storage, catalytic activity and nuclear reactor engineering, to name a few. Thermal stability of this crystalline phase, being vital for transition to viable applications, was investigated through systematic annealing of the alloy films at 473 K, 673 K and 923 K for various durations. While the films were stable at 473 K, the effect of annealing at 673 K was to create segregation into nanocrystalline Ni (superparamagnetic) and amorphous Ni + Zr (non-magnetic) phases. Detailed analyses of the physical and magnetic structures before and after annealing were performed through several techniques effectual in analyzing stratified configurations and the findings were all consistent with each other. Polarized neutron and X-ray reflectometry, grazing incidence x-ray diffraction, time-of-flight secondary ion mass spectroscopy and X-ray photoelectron spectroscopy were used to gauge phase separation at nanometer length scales. SQUID based magnetometry was used to investigate macroscopic magnetic properties. Simulated annealing performed on this system using molecular dynamic calculations corroborated well with the experimental results. This study provides a thorough understanding of the creation and thermal evolution of a crystalline Ni–Zr alloy.

Variation of Piezoelectric properties and mechanisms across the relaxor-like/Ferroelectric continuum in BiFeO3- (K0.5Bi0.5)TiO3-PbTiO3 ceramics.

1- x - y)BiFeO3-x(K0.5Bi0.5)TiO3-yPbTiO3 (BFKBT- PT) piezoelectric ceramics were investigated across the compositional space and contrasted against the xBiFeO3- (1-x)(K0.5Bi0.5)TiO3 (BF-KBT) system, whereby a range of relaxor-like/ferroelectric behavior was observed. Structural and piezoelectric properties were closely related to the PbTiO3 concentration; below a critical concentration, relaxor-like behavior was identified. The mechanisms governing the piezoelectric behavior were investigated with structural, electrical, and imaging techniques. X-ray diffraction established that longrange non-centrosymmetric crystallographic order was evident above a critical PbTiO3 concentration, y > 0.1125. Commensurate with the structural analysis, electric-field-induced strain responses showed electrostrictive behavior in the PbTiO3-reduced compositions, with increased piezoelectric switching in PbTiO3-rich compositions. Positive-up-negative-down (PUND) analysis was used to confirm electric-field-induced polarization measurements, elucidating that the addition of PbTiO3 increased the switchable polarization and ferroelectric ordering. Piezoresponse force microscopy (PFM) of the BF-KBT-PT system exhibited typical domain patterns above a critical PbTiO3 threshold, with no ferroelectric domains observed in the BF-KBT system in the pseudocubic region. Doping of BiFeO3-PbTiO3 has been unsuccessful in the search for hightemperature materials that offer satisfactory piezoelectric properties; however, this system demonstrates that the partial substitution of alternative end-members can be an effective method. The partial substitution of PbTiO3 into BF-KBT enables long-range non-centrosymmetric crystallographic order, resulting in increased polar order and TC, compared with the pseudocubic region. The search for novel high-temperature piezoelectric ceramics can therefore exploit the accommodating nature of the perovskite family, which allows significant variance in chemical and physical characters in the exploration of new solid-solutions.

Solid-state processing of oxidation-resistant molybdenum borosilicide composites for ultra-high-temperature applications

The high-temperature capabilities of multi-phase composites based on Mo5Si3Bx are examined after solid-state processing and pulsed laser deposition (PLD) coating fabrication approaches. These composites are prepared by mechanical alloying of elemental powders and densified by vacuum hot pressing, which is a scalable processing approach. Chemical analyses of the hot-pressed compacts reveal a consistent 15–22 percent loss of boron, which is primarily due to the high-temperature hot-pressing step. Composites possessing sufficient amounts of boron are evaluated by thermogravimetric studies in temperatures up to 1650 °C in air. One composition demonstrates oxidative stability after long-term (100 h) isothermal conditions, as well as thermal cycling to simulate solar-thermal operation. Hot-pressed samples of composites consisting of Mo5Si3Bx + MoSi2 + MoB are also employed as deposition targets for PLD trials. X-ray diffraction analysis of the resulting films indicates the absence of long-range crystallographic order.

Structural and magnetic properties of epitaxial Co2FeAl films grown on MgO substrates for different growth temperatures

We report the correlation between the crystalline structure, electronic structure and magnetic properties of Co2FeAl films as a function of growing temperature both experimentally and theoretically. The Co2FeAl film grown at room temperature is initially in the partially disordered B2 state, but then it gains a much higher ordered structure with increasing growing temperature due to its transition from short-range to long-range crystallographic order by surface diffusion. Electron energy loss spectroscopy measurements reveals that the increase in the I(L3)/I(L2) ratio of Co can be attributed to the enhanced ferromagnetic exchange interaction between neighboring Co atoms and the fact that the Co contribution is more dominant than the Fe contribution. As the growing temperature increases, many more unoccupied 3d states in Co are observed, hence the Gilbert damping constant increases due to a strong spin–orbit interaction. We also present the results of highly accurate quasiparticle self-consistent GW calculations and confirm that Co2FeAl in an ideal L21 structure is indeed a half-metal with a well-defined band gap in the minority spin channel.

Multilevel modular mesocrystalline organization in red coral

Biominerals can achieve complex shapes as aggregates of crystalline building blocks. In the red coral skeleton, we observe that these building blocks are arranged into eight hierarchical levels of similarly (but not identically) oriented modules. The modules in each hierarchical level assemble into larger units that comprise the next higher level of the hierarchy, and consist themselves of smaller, oriented modules. EBSD and TEM studies show that the degree of crystallographic misorientation between the building blocks decreases with decreasing module size. We observe this organization down to a few nanometers. Thus, the transition from imperfect crystallographic order at millimeter scale to nearly perfect single crystalline domains at nanometer scale is progressive. The concept of mesocrystal involves the three-dimensional crystallographic organization of nanoparticles into a highly ordered mesostructure. We add to this concept the notion of modularity. This modularity has potential implications for the origin of complex biomineral shapes in nature. A multilevel modular organization with small intermodular misorientations combines a simple construction scheme, ruled by crystallographic laws, with the possibility of complex shapes. If the observations we have made on red coral extend to other biominerals, long-range crystallographic order and interfaces at all scales may be key to how some biominerals achieve complex shapes adapted to the environment in which they grow.

High-Pressure Studies of 1,3,5,7-Cyclooctatetraene: Experiment and Theory

High-pressure studies of 1,3,5,7-cyclooctatetraene have been performed by using Raman spectroscopy up to 16 GPa and compared with complementary density functional calculations. Angular-dispersive X-ray diffraction studies were also performed in the solid state at elevated pressure. The lattice constants of solid 1,3,5,7-cyclooctatetraene obtained from the X-ray diffraction pattern taken at 3.8 GPa and room temperature are in good agreement with theoretical results. At least two phase transitions were observed during pressure increase followed by the loss of long-range crystallographic order, which was also associated with a strong pressure-induced luminescence that allowed estimation of band gap alterations with pressure.

Mosaic nanostructure of TiO2 with rutile short-range atomic order

A nanolaminate film of six 36nm TiO2–7nm Al2O3 bilayers is sputter deposited at room temperature and examined by high resolution transmission electron microscopy (HRTEM). Neither the TiO2 nor the Al2O3 layers have long-range crystallographic order. Previous Raman spectroscopy of the nanolaminate showed that short-range atomic order in the TiO2 component is characteristic of bulk rutile. The HRTEM images of the Al2O3 layers consist entirely of random contrast speckle characteristic of a material with no atomic ordering beyond the nearest-neighbor level. However, the predominant feature in the images of the TiO2 layers is a mosaic structure, with fewer regions of random contrast speckle. The mosaic consists of four repetitive elements: (1) domains of {110} planes terminating along ⟨100⟩ directions, (2) planar faults along ⟨100⟩ directions, (3) {110} facets in steps along the [001] direction, and (4) a herringbone structure of short strands of (110) and (−110) planes on either side of a ⟨100⟩ midrib. We show how two combined growth operations can generate this nanostructure: These operations are the preferential three-dimensional growth of a rutile nucleus with a {110} habit and the formation of growth faults with 12⟨10−1⟩{011} and 12⟨10−1⟩{121} displacement vectors. The results explicitly show that TiO2 with rutile short-range atomic order self-assembles into units beyond the nearest-neighbor level. This behavior is different from oxides that are continuous random network formers, such as SiO2 and Al3O3, in which the metal-oxygen bonds are predominantly covalent.

Near-ultraviolet optical absorption behavior of TiO2–Al2O3 multilayer films

The fundamental optical absorption edge of sputter-deposited titania-alumina (TiO2–Al2O3) multilayers on fused SiO2 substrates is studied by near ultraviolet-visible spectrophotometry. We examine a family of films with bilayer architecture Λ=9–72nm TiO2∕7nm Al2O3 (TiO2 volume fraction from 0.56 to 0.91). Neither the TiO2 or Al2O3 layers have long-range crystallographic order. The absorption coefficient α in the region of optical density from 2 to 7 is determined as a function of energy E. The first significant finding is that the onset of absorption for these multilayers is determined by their TiO2 component, independent of TiO2 layer thickness. The nondirect optical gap Eo is determined by extrapolating linear α1∕2 vs E curves to α=0. Eo=2.95 (±0.02)eV for all multilayers, identical to Eo for a single layer TiO2 film with rutile short-range atomic order. The second significant finding is that the strength of α in the multilayers is diminished compared to a Vegard’s rule analysis based on the weighted sum of α(TiO2) and α(Al2O3). A modified Vegard’s rule model is proposed to account for the possible electronic effect of Al2O3 on TiO2 at an interface.

Structural and magnetic characterizationof single-crystalline Fe/MgO/Fe magneto-tunnel junctions grown on GaAs(001) and InP(001)

We report on the structural and magneticcharacterization of Ni/Fe/MgO/Fe structures grown on GaAs(001) andInP(001). High-resolution transmission electron microscopy imagesconfirm a good crystallographic order of the structures grown onGaAs(001). On the contrary, the structures grown on InP(001) do not showany long-range crystallographic order. For structures grown onGaAs(001), magneto optic Kerr effect analysis shows steps in thehysteresis loops corresponding to magnetization reversal processes thatoccur independently in both ferromagnetic (FM) films. No FMcoupling between the bottom Fe and the top Ni/Fe films is detected.The coercivity of the bottom Fe film in the junction is increased incomparison to the Fe/GaAs(001) film of the same thickness most likelydue to crystallographic defects. The defects are created in the topNi/Fe bilayer and are spread over the whole single-crystallinestructure. For structures grown on InP(001), a strong FMcoupling is deduced from the hysteresis loops and is interpreted interms of the orange-peel coupling.

A detailed structural characterization of quartz on heating through the α–β phase transition

AbstractTotal neutron scattering measurements, analysed using a modification of the reverse Monte Carlo modelling method to account for long-range crystallographic order, have been used to describe the temperature-dependent behaviour of the structure of quartz. Two key observations are reported. First, the symmetry change associated with the displacive α–β phase transition is observed in both the long-range and short-range structural correlations. Secondly, some aspects of the structure, such as the Si–O bond length and the thermally-induced dynamic disorder, the latter of which sets in significantly below the transition, are relatively insensitive to the phase transition. These results are used to show that the α-domain model of the β-phase disorder is inappropriate and that the classical soft-mode picture of the phase transition is too simplistic. Instead, it is argued that the structural behaviour is best described in terms of its ability to respond to low-frequency, high-amplitude vibrational modes. This view is supported by additional single-crystal diffuse neutron scattering measurements.

Influence of thermal treatment on the phase properties of HoNi 2B 2C

The influence of thermal treatment on the superconducting properties of the quaternary compound HoNi 2B 2C and its correlation to the volume of the unit cell is investigated by X-ray diffractometry and ac-susceptibility measurements. Our results reveal that the annealing process carried out on are-melted intermetallic samples of HoNi 2B 2C strongly influences the interplay between superconductivity and magnetism in this compound. The re-entrant behaviour, observed for polycrystalline HoNi 2B 2C samples annealed at 1100°C for 4 days, can be transformed into pure-superconducting behaviour by subsequent annealing at 800°C for 10 days and vice versa. The results can be explained by mutual substitution of atoms on lattice sites which varies the degree of the long-range crystallographic order. Partially irreversible behaviour can be attributed to the formation of vacancies on lattice sites.

Strain-induced distortion of the bulk bands of gadolinium.

Thin films of gadolinium, approximately 8 ML thick, have been grown on the corrugated (112) surface of molybdenum and the electronic structure has been investigated with angle-resolved photoelectron spectroscopy. The unfavorable lattice match between the ``steplike'' Mo(112) substrate and the preferred hexagonally ordered Gd film results in an incommensurate Gd structure that appears to be ordered, but strained along the direction of the corrugations. The hexagonal Gd lattice is expanded by more than 20% along the ``step'' lines of the substrate, as determined from the reduced Brillouin-zone size along the \ensuremath{\Gamma}\ensuremath{\Sigma}M high-symmetry line and low-energy electron diffraction. The induced strain substantially alters the conventional Gd 5d/6s bulk bands to exhibit a dispersion opposite to that of the relaxed Gd(0001) structure. Dislocations destroy the long-range crystallographic order in the direction orthogonal to the corrugations, which results in the localization of the bands along the \ensuremath{\Gamma}TK symmetry line. \textcopyright{} 1996 The American Physical Society.

MBE growth and characterization of epitaxial TiO 2 and Nb-doped TiO 2 films

Epitaxial TiO 2 and Nb-doped TiO 2 thin films have been grown on (110) TiO 2 rutile substrates by molecular beam epitaxy using elemental Ti and Nb sources along with an electron cyclotron resonance oxygen plasma source. Film composition was measured by X-ray photoelectron spectroscopy. Reflection high-energy electron diffraction and low-energy electron diffraction patterns reveal excellent long-range crystallographic order and a rutile structure for both TiO 2 and Nb-doped TiO 2 films. The high degree of similarity in Ti and Nb core-level X-ray photoelectron diffraction angular distributions establishes that Nb is substitutionally incorporated in the rutile lattice. Analysis of the Nb 3d 5 2 core-level binding energy suggests am Nb 4+ oxidation state.