High-resolution Synchrotron X-ray Diffraction Research Articles

Exchange-bias field induced by surface inhomogeneities in ferromagnetic/charge-ordered bilayer structure

Epitaxial bilayer structure consisting of ferromagnetic metallic Pr0.7Sr0.3MnO3 (PSMO) and charge-ordered insulator La0.5Ca0.5MnO3 (LCMO) was fabricated on (001) SrTiO3 substrate by pulsed laser deposition. High-resolution synchrotron X-ray diffraction showed high quality of epitaxial layer. However, besides diffraction peaks from PSMO layer, LCMO layer and SrTiO3 substrate, we observed an additional shoulder peak, which might stem from the inhomogeneities of composition in PSMO/LCMO. Further the atomic force microscopy measurement showed the presence of non-stoichiometric large particulates at surface, imparting an overall inhomogeneous composition to the film. This implied that the variation of crystalline structure of PSMO/LCMO occurred due to inhomogeneous composition. Moreover, studies on magnetic properties showed that surface inhomogeneities and ferromagnetic clusters at the PSMO/LCMO interface probably influenced the ferromagnetism of the bilayer film together, tuning exchange bias effect.

Crystal Structure and Catalytic Behavior in Olefin Epoxidation of a One-Dimensional Tungsten Oxide/Bipyridine Hybrid.

The tungsten oxide/2,2'-bipyridine hybrid material [WO3(2,2'-bpy)]·nH2O (n = 1-2) (1) has been prepared in near quantitative yield by the reaction of H2WO4, 2,2'-bpy, and H2O in the mole ratio of ca. 1:2:700 at 160 °C for 98 h in a rotating Teflon-lined digestion bomb. The solid-state structure of 1 was solved and refined through Rietveld analysis of high-resolution synchrotron X-ray diffraction data collected for the microcrystalline powder. The material, crystallizing in the orthorhombic space group Iba2, is composed of a one-dimensional organic-inorganic hybrid polymer, ∞(1)[WO3(2,2'-bpy)], topologically identical to that found in the previously reported anhydrous phases [MO3(2,2'-bpy)] (M = Mo, W). While in the latter the N,N'-chelated 2,2'-bpy ligands of adjacent corner-shared {MO4N2} octahedra are positioned on the same side of the 1D chain, in 1 the 2,2'-bpy ligands alternate above and below the chain. The catalytic behavior of compound 1 for the epoxidation of cis-cyclooctene was compared with that for several other tungsten- or molybdenum-based (pre)catalysts, including the hybrid polymer [MoO3(2,2'-bpy)]. While the latter exhibits superior performance when tert-butyl hydroperoxide (TBHP) is used as the oxidant, compound 1 is superior when aqueous hydrogen peroxide is used, allowing near-quantitative conversion of the olefin to the epoxide. With H2O2, compounds 1 and [MoO3(2,2'-bpy)] act as sources of soluble active species, namely, the oxodiperoxo complex [MO(O2)2(2,2'-bpy)], which is formed in situ. Compounds 1 and [WO(O2)2(2,2'-bpy)] (2) were further tested in the epoxidation of cyclododecene, trans-2-octene, 1-octene, (R)-limonene, and styrene. The structure of 2 was determined by single-crystal X-ray diffraction and found to be isotypical with the molybdenum analogue.

Processing and crystallographic structure of non-equilibrium Si-doped HfO2

Si-doped HfO2 was confirmed to exist as a non-equilibrium state. The crystallographic structures of Si-doped HfO2 were studied using high-resolution synchrotron X-ray diffraction and the Rietveld refinement method. Incorporation of Si into HfO2 and diffusion of Si out of (Hf,Si)O2 were determined as a function of calcination temperature. Higher thermal energy input at elevated calcination temperatures resulted in the formation of HfSiO4, which is the expected major secondary phase in Si-doped HfO2. The effect of SiO2 particle size (nano- and micron-sized) on the formation of Si-doped HfO2 was also determined. Nano-crystalline SiO2 was found to incorporate into HfO2 more readily.

Low-moment ferrimagnetic phase of the Heusler compound Cr2CoAl

Synthesizing half-metallic fully compensated ferrimagnets that form in the inverse Heusler phase could lead to superior spintronic devices. These materials would have high spin polarization at room temperature with very little fringing magnetic fields. Previous theoretical studies indicated that Cr2CoAl should form in a stable inverse Heusler lattice due to its low activation energy. Here, stoichiometric Cr2CoAl samples were arc-melted and annealed at varying temperatures, followed by studies of their structural and magnetic properties. High-resolution synchrotron X-ray diffraction revealed a chemically ordered Heusler phase in addition to CoAl and Cr phases. Soft X-ray magnetic circular dichroism revealed that the Cr and Co magnetic moments are antiferromagnetically oriented leading to the observed low magnetic moment in Cr2CoAl.

Spin-driven symmetry breaking in the frustrated fcc pyrite MnS2

We report the characterisation of natural samples of the cubic pyrite mineral MnS2 using very high resolution synchrotron x-ray diffraction techniques. At low temperatures we find a new low temperature polymorph, which results from coupling between magnetic and lattice degrees of freedom. Below the magnetic ordering temperature TN = 48 K, we detect a pseudo-tetragonal distortion with a tiny c/a ratio of 1.0006. The structure can be refined in the space group Pbca The symmetry lowering reduces magnetic frustration in the fcc Mn2+ lattice and is likely responsible for the previously reported lock-in of the magnetic propagation vector. This behaviour is similar to the spin-Peierls phase transitions reported in other three-dimensional Heisenberg magnets like the chromate spinels.

Crystal structure determination of incommensurate modulated martensite in Ni–Mn–In Heusler alloys

The crystal structure of modulated martensite in Mn-rich off-stoichiometric Ni2Mn1.44In0.56 alloy was determined with high-resolution powder neutron diffraction and synchrotron X-ray diffraction in the frame of (3+1)-dimensional superspace theory. The average crystal structure and the modulation wave vector were firstly derived by analyzing the reflection separations induced by the martensitic transformation on the basis of the transformation orientation inheritance. This treatment could be applied to predetermine the modulated structures of materials with displacive structural transformation. The crystal structure of modulated martensite was finally refined by the Rietveld method. Results show that the martensite possesses an incommensurate 6M modulated structure of superspace group I2/m(α0γ)00, with lattice parameters a=4.3919(4)Å, b=5.6202(1)Å, c=4.3315(7)Å, and β=93.044(1)°, and the modulation wave vector q=0.343(7) c*. The detailed site occupations for extra-Mn atoms with respect to the stoichiometric case were investigated by ab initio calculations. The extra-Mn atoms have a preference to be uniformly dispersed. A threefold layered superstructure in the 3-dimensional space was proposed to approximately describe the incommensurate modulated structure. This 6M superstructure model is considered to be representative for off-stoichiometric Ni–(Co)–Mn–In modulated martensite with martensitic transformation around room temperature. The present study is expected to offer an important basis for reliable crystallographic and microstructural characterizations on Ni–Mn–In alloys, so as to understand the underlying mechanisms of their multifunctional magneto-responsive properties.

Anomalies in magnetoelastic properties of DyFe11.2Nb0.8 compound

The structural and magnetic properties of DyFe11.2Nb0.8 compound have been investigated by high intensity, high resolution synchrotron x-ray diffraction, ac magnetic susceptibility, and dc magnetization measurements as well as Mössbauer spectroscopy (5–300 K). The easy magnetization direction at room temperature is along the c-axis. With decreasing temperature, the magnetocrystalline anisotropy changes from easy-axis to easy-cone at Tsr2 (∼202 K), then to easy plane at Tsr1 (∼94 K). The thermal expansion coefficient obtained from the synchrotron x-ray study exhibits clear anomalies around both of the spin reorientation transition temperatures. The 57Fe hyperfine interaction parameters and magnetic moments values have been determined for the 8i, 8j, and 8f sites from the Mössbauer spectra.

Effect of surface inhomogeneities on crystalline structure and magnetic properties of epitaxial Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3 film on (001) SrTiO3

Epitaxil Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3 (PSMO/LCMO) bilayer structure was grown on (001)-oriented SrTiO3 (STO) substrate by pulsed laser deposition. High-resolution synchrotron X-ray diffraction showed high quality of epitaxial layer. However, besides diffraction peaks from PSMO layer, LCMO layer and STO substrate, we observed an additional shoulder peak, which might stem from the inhomogeneities of composition in the film. Grazing incidence X-ray reflectivity (GIXRR) was employed to investigate the surface and interfacial crystalline structure of PSMO/LCMO. The fitting results of GIXRR indicated negligible interdiffusion and roughness at interface. Further atomic force microscopy measurement showed the presence of non-stoichiometric large particulates at surface, imparting an overall inhomogeneous composition to the film. This implied that the variation of crystalline structure of PSMO/LCMO film occurred due to inhomogeneous composition. Moreover, studies on magnetic properties showed that surface inhomogeneities and ferromagnetic clusters at the PSMO/LCMO interface probably influenced the ferromagnetism of the bilayer film together, tuning exchange bias in the film.

High Capacity Positive Electrode Material for Room Temperature Na Ion Battery: NaxMn2/3Co1/6Ni1/6O2

A layered compound, NaxMn2/3Co1/6Ni1/6O2 (NMCN), prepared by a solid-state reaction, served as a high capacity positive electrode material with the reversible capacity of ca. 200 mA · h · g− 1using a Na foil as the counter electrode. A high-resolution synchrotron X-ray diffraction (XRD) analysis revealed that, whereas the NMCN sample had the P3 structure (R3m) when the precursor was heated below 700°C, the P2 structure (P63/mmc) was obtained when heated above 800°C. The latter excels over the former in both capacity and cycle stability; i.e., the initial discharge capacities and potentials of the P2 and P3-NMCN were 216 mA · h · g− 1, 2.87 V vs. Na+/Na and 206 mA · h · g− 1, 2.78 V vs. Na+/Na, respectively. The P2-MNCN retained 80% of its initial discharge capacity while the P3-NMCN dropped to 69% after 30 cycles in the potential range of 1.0–4.5 V vs. Na+/Na. An in-situ XRD analysis revealed that the lattice volume of the P2-NMCN contracts and expands by 20% during the charge and discharge (i.e., Na extraction and insertion) without drastically changing its crystallographic structure.

Lanthanide Orthoantimonate Light Emitters: Structural, Vibrational, and Optical Properties

Lanthanide orthoantimonates represented by the chemical formula LnSbO4 (Ln are all the lanthanides elements with exception of cerium and promethium) were synthesized via solid-state reactions. The crystalline structures of LnSbO4 were determined by high-resolution Synchrotron X-ray diffraction and the Rietveld method. The samples exhibited monoclinic structures with two different arrangements as a function of the ionic radius of the lanthanide metal. For compounds with the largest ionic radii (Ln = La and Pr), the P21/n space group was determined, while compounds with intermediate and smallest ionic radii (Ln = Nd–Lu) were described under the P21/c setting. Raman spectroscopy was employed to study the vibrational features of all compounds, allowing us to determine the characteristic phonons for each structure and, consequently, to establish the relationship between chemical environment and vibrational properties. Optical features of typical SmSbO4 and TbSbO4 were thoroughly investigated, and the results indicate that the orthoantimonates are promising luminescent inorganic materials. The photoluminescence spectra (emission and excitation) of both compounds were obtained as a function of temperature under ultraviolet radiation, showing strong orange (Sm) and green (Tb) emissions, respectively. Chromaticity diagrams (CIE) were also determined for all LnSbO4 series, aiming to bring forward the color coordinates for these emitters tuned by the chemical environment and temperature.

Pyrite formation via kinetic intermediates through low-temperature solid-state metathesis.

The preparation of materials with limited phase stabilities yet high kinetic activation barriers is challenging. Knowledge of their possible formation pathways aids in addressing these challenges. Metathesis reactions present an approach to circumvent these barriers; however, solid-state metathesis reactions are often too rapid from extensive self-heating to understand the reaction. The stoichiometric reaction of MCl2 salts (M = Mn, Fe, Co, Ni, Cu, Zn) with Na2S2 enables the formation of pyrite (FeS2), CoS2, and NiS2 at low temperatures (250-350 °C). Na2S2 has the same polyanionic dimer as found in the pyrite structure, which would suggest the possibility of a facile ion-exchange reaction. However, from high-resolution synchrotron X-ray diffraction and differential scanning calorimetry, the energetic driving force does not appear to result solely from NaCl formation but also from formation of intermediate and pyrite phases. It is apparent that the reaction proceeds through polyanionic disproportionation and formation of a low-density alkali-rich intermediate, followed by anionic comproportionation and atomic rearrangement into the pyrite phase. These results have profound implications for the use of low-temperature metathesis in achieving materials by design.

The Global and Local Symmetries of Nanostructured Ferroelectric Relaxor 0.94(Bi0.5Na0.5)TiO3 – 0.06BaTiO3

Novel submicrometer-grained 0.94(Bi0.5Na0.5)TiO3 – 0.06 BaTiO3 ceramic, prepared from nanopowder by a combination of hot-pressing and recrystallization, is investigated by high-resolution synchrotron x-ray diffraction and x-ray absorption near-edge spectroscopy (XANES). The simplest and best-fitting Rietveld refinement of the performed wide-Q diffraction experiment corresponds to a three-phase system formed by a ferroelectric rhombohedral R3c phase, a globally cubic Pmm phase and a small amount of nanosized (∼12 nm) quasi-cubic phase. The globally cubic phase and the nanosized one built up a complex, nanostructured, configuration that accounts for the reported dielectric relaxor behavior. XANES reveals that the departures from centro-symmetry at the unit cell scale for the precursor powder and for the sintered ceramic are practically the same.

Hirshfeld atom refinement for modelling strong hydrogen bonds.

High-resolution low-temperature synchrotron X-ray diffraction data of the salt L-phenylalaninium hydrogen maleate are used to test the new automated iterative Hirshfeld atom refinement (HAR) procedure for the modelling of strong hydrogen bonds. The HAR models used present the first examples of Z' > 1 treatments in the framework of wavefunction-based refinement methods. L-Phenylalaninium hydrogen maleate exhibits several hydrogen bonds in its crystal structure, of which the shortest and the most challenging to model is the O-H...O intramolecular hydrogen bond present in the hydrogen maleate anion (O...O distance is about 2.41 Å). In particular, the reconstruction of the electron density in the hydrogen maleate moiety and the determination of hydrogen-atom properties [positions, bond distances and anisotropic displacement parameters (ADPs)] are the focus of the study. For comparison to the HAR results, different spherical (independent atom model, IAM) and aspherical (free multipole model, MM; transferable aspherical atom model, TAAM) X-ray refinement techniques as well as results from a low-temperature neutron-diffraction experiment are employed. Hydrogen-atom ADPs are furthermore compared to those derived from a TLS/rigid-body (SHADE) treatment of the X-ray structures. The reference neutron-diffraction experiment reveals a truly symmetric hydrogen bond in the hydrogen maleate anion. Only with HAR is it possible to freely refine hydrogen-atom positions and ADPs from the X-ray data, which leads to the best electron-density model and the closest agreement with the structural parameters derived from the neutron-diffraction experiment, e.g. the symmetric hydrogen position can be reproduced. The multipole-based refinement techniques (MM and TAAM) yield slightly asymmetric positions, whereas the IAM yields a significantly asymmetric position.

Thermoelectric Properties of Undoped High Purity Higher Manganese Silicides Grown by Chemical Vapor Transport

Semiconducting higher manganese silicides (HMS), with a nominal composition of MnSi1.73, are particularly promising thermoelectric materials because of their elemental abundance, nontoxicity, and reported ZT of around 0.4 at 800 K for undoped samples. However, embedded MnSi impurities naturally form during the melt growth of HMS materials. The influences of such naturally occurring MnSi impurities within bulk HMS have yet to be carefully studied. Herein, we report the synthesis of high-purity MnSi-free single crystals of HMS by chemical vapor transport and the thermoelectric properties of consolidated HMS samples prepared by spark plasma sintering (SPS). The high purity of the HMS crystals is verified by scanning and transmission electron microscopy, electron diffraction, and synchrotron high-resolution X-ray diffraction. Despite successfully growing high purity HMS single crystals, we find that MnSi will nevertheless precipitate from HMS after SPS processing. In-situ sychrotron high-resolution X-ray diff...

Structural phase stability and electric field induced relaxor–ferroelectric phase transition in (1 − x)(Bi0.5Na0.5)TiO3–xBaTiO3 ceramics

Structure, phase transition, and dielectric relaxor behavior have been investigated in morphotropic phase boundary compositions of (1−x)(Bi0.5Na0.5)TiO3-xBaTiO3 ceramics for x=0.06, 0.07, and 0.075. In-situ structural thermal stability has been analyzed by using high-resolution synchrotron X-ray diffraction as a function of temperature. A transition sequence of rhombohedral+tetragonal−tetragonal−cubic phases is observed upon heating. The “rhombohedral+tetragonal” indicates a coexistence of rhombohedral R3c and tetragonal P4bm phases. The two-phase coexistence extends from room temperature to 200°C. Zero-field dielectric permittivity reveals a frequency dispersion at low temperature region (T<Tm) and polar nanoregions below Burns temperature (TB). The two-phase coexistence plays a vital role for frequency dispersion below Tm. Dielectric spectra of pre-poled samples reveal a field-induced ferroelectric−relaxor phase transition at depolarization temperature Td.

Structural change and phase coexistence upon magnetic ordering in the magnetodielectric spinelMn3O4

Cooperative Jahn-Teller ordering is well-known to drive the cubic $Fd\overline{3}m$ to tetragonal $I$4$_1$/$amd$ structural distortion in Mn$_3$O$_4$ at 1170 $^{\circ}$C. Further structural distortion occurs in Mn$_3$O$_4$ upon magnetic ordering at 42 K. Employing high-resolution variable-temperature synchrotron x-ray diffraction we show that tetragonal $I$4$_1$/$amd$ and orthorhombic $Fddd$ phases coexist, with nearly equal fractions, below the N\'eel temperature of Mn$_3$O$_4$. Significant variation of the orthorhombic $a$ and $b$ lattice constants from the tetragonal $a$ lattice constant is observed. Structural phase coexistence in Mn$_3$O$_4$ is attributed to large strains due to the lattice mismatch between the tetragonal $I$4$_1$/$amd$ and the orthorhombic $Fddd$ phases. Strain tensors determined from Rietveld refinement show a highly strained matrix of the $I4_1/amd$ phase that accommodates the nucleated orthorhombic $Fddd$ phase in the phase coexistence regime. A comparison of the deformation observed in Mn$_3$O$_4$ to structural deformations of other magnetic spinels shows that phase coexistence may be a common theme when structural distortions occur below 50 K.

Applying high resolution SyXRD analysis on sulfate attacked concrete field samples

High resolution synchrotron X-ray diffraction (SyXRD) was applied for a microstructural profile analysis of concrete deterioration after sulfate attack. The cement matrices consist of ordinary Portland cement and different amounts of supplementary cementitious materials, such as fly ash, natural pozzolana and granulated blast furnace slag. The changes of the phase composition were determined along the direction of sulfate ingress. This approach allows the identification of reaction fronts and zones of different phase compositions and conclusions about the mechanisms of sulfate attack. Two reaction fronts were localized in the initial 4mm from the sample surface. The mechanism of deterioration caused by the exposition in the sulfate-bearing soil is discussed. SyXRD is shown to be a reliable method for investigation of cementitious materials with aggregates embedded in natural environments.

Structural transformations in R3Cu4Sn4 (R = Ho, Er, Tm) intermetallic compounds

The family of ternary intermetallic compounds R3T4X4 (R = rare-earth; T = d-element; X = Si, Ge, Sn) comprises over 100 members that have a generic orthorhombic structure (space group Immm, #71) at room temperature. We have observed a monoclinic ↔ orthorhombic crystallographic transformation in Ho3Cu4Sn4, Er3Cu4Sn4 and Tm3Cu4Sn4 by high-resolution synchrotron X-ray and neutron powder diffraction. We show that the temperature at which this transformation occurs scales linearly with the ionic radius of the rare-earth. No such transformation was observed in Dy3Cu4Sn4, down to 1.7 K, consistent with this scaling.

Magnetoelastic driven Negative Thermal Expansion in a Framework Antiferromagnet

Metal-organic frameworks (MOFs) have recently attracted great attention for their multiferroic, magnetocaloric and low dimensional magnetic order. These properties depend on the precise magnetic interactions in frameworks and a deeper understanding of the coupling between the lattice of these materials and their magnetic order is required to underpin the future developments of these promising compounds. Neutron diffraction is the ideal technique for such studies but its application to studies of the magnetic structure of MOFs has been limited to date due to challenges in solving the structure of these dilute yet complex magnetic compounds. We have recently examined the magnetic properties and structure of a new dicarboxylate framework, cobalt adipate, Co(C6H8O4), which adopts P21/c monoclinic symmetry.[1] This compound has been found to order antiferromagnetically at low temperature and fits to neutron diffraction data have shown it adopts Pb21/c magnetic symmetry. Its magnetic structure features sheets of Co cations coupled antiferromagnetically in two dimensions through carboxylate groups. The emergence of this order is accompanied by magnetoelastic coupling, which drives anisotropic negative thermal expansion along the a-axis at low temperature. This is the first evidence for such behaviour in a MOF and we find that both this behaviour and the spin orientation in this material are controlled by the presence of weak ferromagnetic dipole-dipole coupling between the layers of tetrahedral cobalt. Variable temperature high resolution synchrotron X-ray and neutron powder diffraction have also revealed that the monoclinic angle of Co adipate decreases on cooling, passing through a metrically orthorhombic phase without any indication of a phase transition. This unusual behaviour has been rationalised by examining the thermal expansion of the framework along its principal axis, highlighting the importance of such analysis in low symmetry materials.

Two types of adjacent dimer layers in the low-temperature phase of BaCuSi2O6

The low-temperature crystal structure of BaCuSi2O6has been investigated with high-resolution synchrotron x-ray and neutron powder diffraction techniques and has been found to be on average (ignoring the incommensurate modulation) orthorhombic, with the most probable space group Ibam. The Cu-Cu dimers in this material are forming two types of layers with distinctly different interatomic distances. Subtle changes also modify the partially frustrated interlayer Cu-Cu exchange paths. The present results corroborate the interpretation of low-temperature nuclear magnetic resonance and inelastic neutron scattering data in terms of distinct dimer layers. The experimentally determined low-temperature crystal structure of BaCuSi2O6is discussed in terms of its relation to the newer findings of theory and of the complementary experiments.