Anisotropic Microstrain Research Articles

Spin chiral interactions modulated Seebeck coefficient and improved optical properties by Zn doping in CuCrO2 crystallites

Future electronics hold a special promise for flexible, optically translucent thermoelectric devices. Here, we demonstrate the better thermoelectric performance of p-type transparent CuCrO2 crystallites by the insertion of non-magnetic Zn2+ in magnetic Cr3+ site at room and high temperatures. Doping induced increase in charge carrier density generally leads to elevated carrier scattering, which in turn diminishes the voltage generated per unit tem-perature difference and causes a decrease in the Seebeck coefficient. Balancing the Seebeck coefficient and conductivity is a crucial challenge in the design of thermoelectric materials. By introducing non-magnetic doping, the anti-ferromagnetic order of Cr3+ ions is destabilized, resulting in a modification of the spin configuration. This alteration gives rise to spin chiral interactions, which, in turn, contribute to the generation of an exceptionally large Seebeck coefficient of 582 μV/K at 100 °C. The insertion also generates anisotropic microstrains in the structure and the structural changes were investigated by XRD analysis, FTIR, and Raman spectroscopy. Carrier concentration and carrier mobility were measured by Hall measurements techniques and has shown increasing and decreasing trend respectively with the increase of the doping percentage of Zn2+. Optical studies were done by UV–Vis analysis and found the maximum band gap of 3.04 eV for 0.5 wt% doped CuCrO2 and the absorption spectra revealed the defect induced in the structure comprehending the Raman analysis. The enhancement of carrier concentration and mobility led to increased conductivity of 2096 S/m and power factor of 411 μW/mK2 for the sample 0.3 wt% at 700 °C. The Seebeck coefficient of the sample dropped as the temperature rose, but it was maintained in the range of 440–580 μV/K, which is found as the primary characteristic of Zn doping. Thermal conductivity is found as decreased with doping and the highest ZT obtained is 0.199 for 0.3 wt% doped sample at 700 °C. The higher thermoelectric performance and optical transmission at visible wavelength can be put together in optoelectronic devices.

Comment on "Structural transition and superconductivity in hydrothermally synthesized FeX (X = S, Se)" by U. Pachmayr, N. Fehn and D. Johrendt, Chem. Commun., 2016, 52, 194.

The occurrence of triclinic structural modification of β-FeSe was reported in the literature; in particular this polymorph was claimed to be observed at low temperature in samples prepared by hydrothermal synthesis. The establishment of triclinic symmetry was argued based on peculiar features characterizing some selected X-ray powder diffraction lines. Using high-resolution synchrotron X-ray powder diffraction, the same features were observed for an aged β-FeSe sample prepared by a solid state synthesis technique. Moreover, by refining the anisotropic microstrain parameters it was demonstrated that the diffraction pattern can be fairly fitted by applying the well-recognized orthorhombic structural model adopted for the low temperature phase of β-FeSe. This result indicates that the occurrence of the triclinic polymorph for β-FeSe can be ruled out.

Synchrotron XRD: Enlightening the processing-properties correlation in (Bi0.5Na0.5)0.96Ba0.04TiO3 (BNBT4) ceramics

High-resolution X-ray diffraction is applied to investigate macro and micro crystal structure changes across different synthesis routes in BNBT4 piezoceramics. Synthetic powders were obtained by conventional sol-gel auto combustion and mixed oxides methods; then samples were sintered by combining hot uniaxial pressing and subsequent recrystallization. The crystal structure of the ceramic obtained by sol-gel is rhombohedral (R3c) and the one obtained from mixed oxides is monoclinic (Cc). In the sol-gel case, a poling process produces an intense 001 texture parallel to the polarizing electric field. Poling the mixed oxides ceramics quickly transforms the structure from monoclinic to rhombohedral while texture and anisotropic micro-strains develop further as the poling increases. A correlation between detected structural transformations and variations in selected physical properties is suggested. The anisotropy of the critical temperatures for dielectric and electromechanical depolarization is lower for mixed oxides ceramics than for sol-gel ceramics.

A well-controlled cracks and gliding-free single-crystal Ni-rich cathode for long-cycle-life lithium-ion batteries

Single-crystalline (SC) high energy nickel (Ni)-rich cathodes play a key role as a potential cathode material in lithium-ion batteries (LIBs) to address the challenges in a hierarchical structure of their secondary particles by decreasing phase boundaries and materials surfaces. The SC LiNi0.78Mn0.12Co0.1O2 (SC-NMC78) cathode with primary particles of several micron-sized particles are developed and thoroughly investigated in this study, demonstrating superior cycling performance, along with significantly enhanced structural reliability after long-term cycling. The improved SC-NMC78 has an octahedral SC morphology with a modest grain size, which reduces the lithium-ion diffusion route and enhances structural stability. The SC-NMC78 offers a high discharge capacity of 175 and 155 mAh g−1 at 0.2 and 1 C, respectively, and better capacity retention of 132 mAh g−1 after 200 cycles at 1 C as a cathode in LIBs. The cycled SC-NMC78 particles exhibited no lattice gliding and micro-cracks, demonstrating that the SC shape may substantially reduce anisotropic micro-strain. This efficient, repeatable, and customizable method for producing SC Ni-rich cathodes without any additives should accelerate their commercialization. The density functional theory also proved that the low global hardness of Ni2+ in SC-NMC78 and optimized content of Ni/Li exchange were well-consistent with the experimental findings.

Mode Crystallography Analysis through the Structural Phase Transition and Magnetic Critical Behavior of the Lacunar Spinel GaMo4Se8.

In the lacunar spinels, with the formula AB4X8, transition-metal ions form tightly bound B4 clusters resulting in exotic physical properties such as the stabilization of Néel-type skyrmion lattices, which hold great promise for energy-efficient switching devices. These properties are governed by the symmetry of these compounds with distortion of the parent noncentrosymmetric F4̅3m space group to the polar R3m, with recent observation of a coexisting Imm2 low-temperature phase. In this study, through powder neutron diffraction, we further confirm that a metastable Imm2 coexists with the R3m phase in GaMo4Se8 and we present its structure. By applying the mode crystallography approach to the distortions together with anisotropic microstrain broadening analysis, we postulate that the formation origin of the minority Imm2 phase stems from the high compressive stress observed in the R3m phase. Bond valence sum analysis also suggests a change in electronic configuration in the transition to Imm2 which could have implications on the electrical properties of the compound. We further establish the nature of the magnetic phase transition using critical exponent analysis obtained from single-crystal magnetization measurements which shows a mixture of tricritical mean-field and 3D Heisenberg behavior [β = 0.22(4), γ = 1.19(1), and δ = 6.42(1)]. Magnetoentropic mapping performed on a single crystal reveals the signature of a positive entropy region near the magnetic phase transition which corresponds to the skyrmion phase field observed in a polycrystalline sample.

Structural Details of BaTiO3 Nano-Powders Deduced from the Anisotropic XRD Peak Broadening.

In this study, nano-BaTiO3 (BTO) powders were obtained via the solvothermal method at different reaction times and were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The results were compared with those obtained for a larger crystallite size BTO powder (BTO-m). The sizes of the cuboid crystallites (as determined by XRD and TEM) ranged from about 18 to 24 nm, depending on the reaction time. The evolution with temperature of the structure parameters of nano-BTO was monitored by means of X-ray diffraction and Raman spectroscopy and no signs of phase transition were found up to 170 °C. Careful monitoring of the dependence of the XRD peak widths on the hkl indices showed that the effect of the cubic crystallite shape upon the XRD peak widths was buried by the effect of hidden tetragonal line splits and by anisotropic microstrain. The good correlation of the line widths with the tetragonal split amplitudes, observed especially for BTO-m above the transition temperature, indicates tetragonal deformations, as also revealed by Raman spectroscopy. The large anisotropic microstrain shown by the nano-powders, which had a maximum value in the <100> directions, was considered evidence of the phenomenon of surface relaxation of cubic crystallites edged by {100} faces. The observed behavior of the nano-BTO structures with increasing temperature may suggest a correlation between the surface relaxation and tetragonal deformation in the nano-cubes. The experimental results for both nano-BTO and mezoscale-BTO are in agreement with the core-shell model.

Highly-Dispersed Submicrometer Single-Crystal Nickel-Rich Layered Cathode: Spray Synthesis and Accelerated Lithium-Ion Transport.

For conventional polycrystalline Ni-rich cathode material consisting of numerous primary particles in disordered orientation, the crystal anisotropy in charge/discharge process results in the poor rate capability and rapid capacity degradation. In this work, highly-dispersed submicron single-crystal LiNi0.8 Co0.15 Al0.05 O2 (SC-NCA) cathode is efficiently prepared by spray pyrolysis (SP) technique followed by a simple solid-state lithiation reaction. Porous Ni0.8 Co0.15 Al0.05 Ox precursor prepared via SP exhibits high chemical activity for lithiation reaction, enabling the fabrication of single-crystal cathode at a relatively low temperature. In this way, the contradiction between high crystallinity and cation disordering is well balanced. The resulted optimized SC-NCA shows polyhedral single-crystal morphology with moderate grain size (≈1 μm), which are beneficial to shortening the Li+ diffusion path and improving the structural stability. As cathode for lithium ion batteries, SC-NCA delivers a high discharge capacity of 202 and 140 mAh g-1 at 0.1 and 10 C, respectively, and maintains superior capacity retention of 161 mAh g-1 after 200 cycles at 1C. No micro-crack is observed in the cycled SC-NCA particles, indicating such single-crystal morphology can greatly relieve the anisotropic micro-strain. This effective, continuous and adaptable strategy for preparing single-crystal Ni-rich cathode without any additive may accelerate their practical application.

In Situ Observation for Deformation-Induced Martensite Transformation (DIMT) during Tensile Deformation of 304 Stainless Steel Using Neutron Diffraction. PART I: Mechanical Response

304 stainless steel is one of the most common stainless steels due to its excellent corrosion resistance and mechanical properties. Typically, a good balance between ductility and strength derives from deformation-induced martensite transformation (DIMT), but this mechanism has not been fully explained. In this study, we conducted in situ neutron diffraction measurements during the tensile deformation of commercial 304 stainless steel (at room temperature) by means of a Time-Of-Flight type neutron diffractometer, iMATERIA (BL20), at J-PARC MLF (Japan Proton Accelerator Research Complex, Materials and Life Science Experimental Facility), Japan. The fractions of α′-(BCC) and ε-(HCP) martensite were quantitatively determined by Rietveld-texture analysis, as well as the anisotropic microstrains. The strain hardening behavior corresponded well to the microstrain development in the austenite phase. Hence, the authors concluded that the existence of martensite was not a direct cause of hardening, because the dominant austenite phase strengthened to equivalent values as in the martensite phase. Moreover, the transformation-induced plasticity (TRIP) mechanism in austenitic steels is different from that of low-alloy bainitic TRIP steels.

Entropy stabilized single-phase (Hf,Nb,Ta,Ti,Zr)B2 solid solution powders obtained via carbo/boro-thermal reduction

This report deals with the synthesis of entropy stabilized (ES) single-phase (Hf,Nb,Ta,Ti,Zr)B2 powders, with specific surface area of about 1.6 m2/g, in a AlB2-type structure using individual transition metal (TM) oxides and, for the first time, elemental boron and carbon as precursors to feed the carbo/boro-thermal (CBT) reduction. Elemental B and C were intimately mixed into a mixture of five TM oxide powders, TM = Hf, Nb, Ta, Ti and Zr through an high energy planetary milling. ES single-phase TM diboride solid solution powders were obtained by a synthesis process consisting of a CBT reduction followed by solid solution formation. A B:C molar ratio = 1.27 (per 1 M mass of metals) was adjusted leading the CBT reduction to completion (i.e., full conversion of TM oxides), applying the synthesis temperature of 2123 K under vacuum. The micro-strain intended as deviation of some inter-planar distances of the entropy stabilized AlB2-type solid solution lattice, was analyzed by x-ray diffraction: a strong anisotropic micro-strain was found, and was attributed to the compositional disorder due to the coexistence of differing TM with different atomic radius.

X-ray diffraction Rietveld analysis and Bond Valence analysis of nano titania containing oxygen vacancies synthesized via sol-gel route

Nano titanium dioxide (TiO2) synthesized by sol-gel route was calcined at different temperatures, the powder calcined at 673 K indicates biphasic mixture of brookite and anatase. On the other hand the powder calcined at 773 K exhibited three phase mixture of anatase, brookite and rutile phases. The microstrain present in the multiphasic powder was studied by the X-ray diffraction Williamson - Hall method. Detailed Rietveld analysis with line profile analysis combined with spherical harmonics indicated the anisotropic microstrain in the calcined powders. The strain was found to reduce with calcination temperature. The anisotropic microstrain is correlated to oxygen vacancies, which influence the crystal structure parameters of the phases in terms of their TiO6 octahederal bond length/bond angle values. The bond valence sum analysis of the powders confirms for the oxygen vacancy in the titania powders.

Reflection splitting-induced microstrain broadening

Crystal structure determination on the basis of powder diffraction data frequently involves the question how the given diffraction data with some appreciably hkl-dependent line broadening should be interpreted. In many cases, such line broadening may either: (i) reasonably well be reconciled with a certain high-symmetry structure model or (ii) with a variant of the former with lower symmetry crystal family, which frequently will give a somewhat better fit in Rietveld refinement. In this work, it is shown mathematically that symmetry reduction induced reflection spitting masked by other line broadening contributions, thus leading to some reflection splitting-induced line broadening, shows a similar hkl dependence as typically adopted for anisotropic microstrain broadening with respect to the high-symmetry structure. This implies that Rietveld refinement on the basis of the low-symmetry model (including typically isotropic line broadening) and on the basis of the high-symmetry model with anisotropic microstrain broadening can both lead to similar qualities of the fit. Hence, the refinement results for both possibilities should be carefully considered in combination with possibly available additional information (e.g. results of first-principles calculations) to arrive at adequate conclusions concerning the true symmetry of the material under investigation.

Structural analysis of the foldecture derived from racemic peptide foldamers

The molecular packing structure of an elongated parallelogram plate shaped foldecture composed of a 1:1 racemic mixture of 11-helical peptide foldamers was resolved by powder X-ray diffraction (PXRD) analysis. A comprehensive Rietveld refinement procedure compensated for powder texture and identified the principal face of the foldecture. Each foldamer makes head-to-tail intermolecular hydrogen bonds, creating extended chains of single enantiomers that form a network of hydrophobic close contacts with foldamers of both the opposite and the same chiralities. An isosurface for anisotropic microstrain was calculated and found to be smallest along the x-axis, which is parallel to the network of intermolecular hydrogen bonds. Comparison with the single crystal structure found molecular packing motifs to be almost identical—a result infrequently observed in enantiopure foldectures. This is the first powder X-ray diffraction structural analysis of a foldecture composed of multiple components.

Microstructure and charge trapping assessment in highly reactive mixed phase TiO2 photocatalysts

The structural-microstructural characteristics and interfacial charge transfer are key issues to the development of efficient mixed phase TiO2 photocatalysts. In this work, the interplay of lattice deformation and microstrains as well as the identification of charge trapping sites and electron transfer were investigated for a series of nanostructured titania photocatalysts by X-ray powder diffraction analysis, Raman and electron paramagnetic resonance (EPR) spectroscopy. These mixed phase nanomaterials were selected as model sol-gel TiO2 systems based on their exceptional photocatalytic performance over a wide range of hazardous water pollutants (including degradation/mineralization of phenol, 2,4-dichlorophenoxyacetic acid and imazalil) under UV light. Lattice contraction with respect to the bulk anatase together with anisotropic microstrains was identified for the smallest (11nm) anatase nanoparticles. Both effects gradually relaxed with the increase of calcination temperature and the concomitant particle growth, with microstrains scaling linearly with the relative change of the c-axis lattice constant and the broadening of the main anatase Raman mode. The growth of anatase nanoparticles at 1023K with minimal lattice deformation and microstrains resulted in the optimal photocatalytic efficiency, outperforming the benchmark Aeroxide® P25 catalyst. This mixed phase catalyst comprised also larger, though more strained, rutile nanocrystals than P25, and presented an additional deeper electron trapping lattice site according to light-induced EPR measurements. More importantly, electron transfer from rutile to anatase lattice traps was identified by EPR under visible light in the mixed phase photocatalyst. The improved crystal quality of the anatase nanocrystals combined with the enhanced charge separation in anatase/rutile interfaces is concluded crucial to the design of competent solar photocatalytic nanomaterials.

Structural disorder, anisotropic micro-strain and cation vacancies in thermo-electric lead chalcogenides

Thermoelectric materials can interconvert heat and electricity, and the extraordinary thermoelectric properties of lead chalcogenides (PbX, X = S, Se, Te) attract immense scientific interest. A key topic is the role of the cation in reaching a very low thermal conductivity necessary for efficient energy conversion. Here we present new structural insights about the deceptively simple rock-salt lead chalcogenides through a comparative multi-temperature synchrotron powder X-ray diffraction study. For the first time, the presence of anisotropic microstrain broadening as well as lead vacancies are quantified for all three compounds. The microstrain implies extended breakage of cubic symmetry as a sign of the incipient ferroelectric nature of PbX. The degree of microstrain is correlated to the transition pressure of a symmetry reducing phase transition, and this trend can be explained by anion mediated s-p hybridization on lead. The observed number of vacancies is greatest for PbS (4-8%), but two samples of PbS show different cation occupancy, and thus sample-dependent vacancies might be the property that unifies conflicting results reported for PbX. Gram-Charlier analysis identifies a local non-spherical distribution of Pb; however, model unbiased maximum entropy analysis indicates that any static displacement of Pb, if present, is less than 0.2 Å at 100 K.

Williamson–Hall analysis and the dielectric behavior of complex perovskites (BaM0.9M′0.1)0.984O3 (M=Ti, Zr; M′=Nb, Ta)

The long range average structure and local distortions of perovskite type solid solutions (BaTi0.9M′0.1)0.984O3 and (BaZr0.9M′0.1)0.984O3 (M′=Nb, Ta) were examined by high-resolution synchrotron X-ray diffraction. A Rietveld refinement indicated that all four solid solutions had simple cubic symmetry, where the lattice volume varied according to the size of the cationic constituents. As revealed by Williamson–Hall analysis and scanning electron microscopy, both (BaTi0.9M′0.1)0.984O3 had denser and enlarged grains than the parental BaTiO3, and had anisotropic microstrain that are indicative of the non-periodic atomic displacements along the [100] directions of the cubic cell. (BaTi0.9Nb0.1)0.984O3 and (BaTi0.9Ta0.1)0.984O3 exhibited relaxor-like dielectric behavior with Curie temperatures of 230K and 195K, respectively. On the other hand, (BaZr0.9Nb0.1)0.984O3 and (BaZr0.9Ta0.1)0.984O3 showed almost unchanged dielectric behavior compared to that of BaZrO3.

Evidence of Development of New Spin Orders Benefiting to Enhance Magnetic Properties in Co2+-Doped Delafossite-Type Oxide CuCrO2

This paper reports the effect of high-spin Co2+-doped CuCrO2 delafossite-type oxide on the structure and physical properties. X-ray diffraction and Raman spectroscopy show that the structure is maintained for all Co-doped samples for chromium. The incorporation of this element generates anisotropic microstrains in the structure. The temperature dependence of zero field-cooling magnetization was measured. All samples exhibit an AFM transition around 24 K. The high-spin state and the shift due to the exchange splitting of the conduction band suggest strong hybridization between carriers in the Cr 3d t2g band and the t2g states of the high-spin Co2+ to develop other spin orders benefiting to enhance magnetic susceptibility and support the evidence of new FM transition. The coupling between the magnetic order and ferroelectric order is also characterized.

ZrSe3-Type Variant of TiS3: Structure and Thermoelectric Properties

A dense TiS3 sample has been processed by Spark Plasma Sintering. The structural analysis, obtained by coupling powder X-ray diffraction and transmission electron microscopy (TEM), shows that the A-variant of the ZrSe3-type structure is stabilized for the first time. Defects along the main atomic layer stacking directions are evidenced by high-resolution TEM, which explain the peculiar X-ray powder diffraction patterns, with strongly anisotropic microstrains. The presence of these structural defects might explain the existence of a metal-to-insulator transition with a charge localization below TMI ≈ 325 K. Large absolute values of the Seebeck coefficient, in the range −700 ≤ S ≤ −600 μV·K–1, are observed for 100 ≤ T ≤ 600 K, together with a low thermal conductivity, κ = 2 W·K–1·m–1 at 600 K. The T–1 dependence of the lattice part of κ indicates its phononic character. As the charge carrier concentration measured by Hall effect is too low, n = 1.24 × 1018 cm–3, extra doping would be necessary to decrease its too high electrical resistivity (ρ300K ∼ 1.4 Ω·cm) for thermoelectric applications.

Effect of Zn Substitution on the Structural and Physical Properties of Delafossite-Type Oxide CuCrO2

In this paper, we report the effect of non-magnetic Zn-doped CuCrO2. The structure, Raman spectroscopy, magnetic properties, dielectric permittivity, and electric polarization have been investigated. The incorporation of Zn2+ generates very anisotropic microstrains in the structure. The temperature dependence of magnetic susceptibly for all samples exhibits a magnetic dilution and paramagnetic behavior at high temperature. It is argued that non-magnetic substitution destabilizes the antiferromagnetic order of Cr3+ ions and modulates the spin configuration. The coupling between the magnetic order, dielectric permittivity and polarization is also characterized.

Anisotropic microstrain broadening in cementite, Fe3C, caused by thermal microstress: comparison between prediction and results from diffraction-line profile analysis

X-ray powder diffraction data of an Fe3C powder consisting of polycrystalline particles shows pronouncedly anisotropic microstrain broadening of the Bragg reflections. The extent and anisotropy of the broadening can quantitatively be attributed to thermal microstresses induced by anisotropic thermal shrinkage from the preparation temperature of 873 K to the ambient analysis temperature, in conjunction with the elastic anisotropy of Fe3C.

Crystal structure determination of Hägg carbide, χ-Fe5C2 by first-principles calculations and Rietveld refinement

Abstract X-ray powder-diffraction data recorded using different wave lengths as well as neutron powder diffraction data on Hägg carbide, χ-Fe5C2, were evaluated by Rietveld or Pawley refinements, respectively. Likewise, employing different starting models, first-principles calculations using density functional theory (DFT) involving structure optimisation with respect to energy were performed for χ-Fe5C2. The results from diffraction and DFT imply a crystal structure having a monoclinic C2/c symmetry with a quite regular (monocapped) trigonal-prismatic coordination of C by Fe atoms. The anisotropy of the microstrain broadening observed in the powder-diffraction patterns agrees with the anisotropy of the reciprocal Young’s module obtained from elastic constants calculated by DFT. The anisotropic microstrain broadening can to some degree, be modelled allowing for a triclinic distortion of the metric of χ-Fe5C2 (deviation of the lattice angle γ from 90°) involving reflection spitting, which mimics the hkl-dependently broadened reflections. This distortion corresponds to the most compliant shear direction of the monoclinic χ-Fe5C2. The anisotropic microstrain broadening results from microstress induced e.g. by anisotropic thermal expansion inducing misfit between the grains, in association with the intrinsic anisotropic elastic compliance of χ-Fe5C2. This anisotropic microstrain broadening was likely the origin of previous proposals of triclinic P-1 space-group symmetry for the crystal structure of χ-Fe5C2, which is rejected in the present work.