Synchrotron-based X-ray Diffraction Research Articles

The formation and evolution of Ni2Cr precipitates in Ni–Cr model alloys as a function of stoichiometry characterized by synchrotron x-ray diffraction

Ni-based alloys containing significant amounts of Cr, may precipitate new phases during long-term service at elevated temperatures. In Ni–Cr model alloys, the formation of Ni2Cr has been found to impact the material properties, including the strength and ductility after isothermal aging below the critical temperature of 590 °C. In this work, we quantify the formation and evolution of long-range ordering in four Ni–Cr binary model alloys (Ni/Cr = 1.8, 2.0, 2.2, 2.4) after isothermal aging up to 10,000 h at temperatures between 373 and 475 °C. The alloys were characterized by hardness testing and synchrotron-based x-ray diffraction to quantify the impact of Ni2Cr phase fraction and precipitate size on mechanical properties. After 500 h of isothermal aging, the formation of Ni2Cr was detected in all four alloys at 475 °C. In the stochiometric alloy samples (Ni/Cr = 2.0), the formation of Ni2Cr was found after 500 and 3000 h aging at 418 and 373 °C, respectively. We found that the matrix lattice contraction and Ni2Cr phase fraction both saturate after early aging times. This is in stark contrast to the hardness and Ni2Cr precipitate size that both continue to increase with increasing aging time. Our results highlight that changes in hardness correlate linearly with Ni2Cr precipitate size rather than phase fraction. This important structure-property relationship can potentially help define Ni–Cr-based component lifetimes directly through an understanding of how Ni2Cr formation impacts strength and ductility. We find that a precipitation hardening model for critical resolved shear stress with weakly coupled dislocations shows good agreement with the material property changes quantified from experimental measurements.

Structural stability of REE-PO4 (REE = Sm,Tb) under swift heavy ion irradiation

Rare earth element (REE) phosphates are an attractive host matrix for long-lived radionuclides from nuclear waste because of the radiation tolerance exhibited by naturally occurring REE phosphate specimens. Here we show that SmPO4 (monazite structure) and TbPO4 (xenotime structure) exhibit a similar amorphization response to swift heavy ion irradiation despite different starting structures. SmPO4 and TbPO4 were irradiated with 1.1 GeV Au ions and analyzed using synchrotron-based X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). The radiation response of the phosphates was evaluated by extracting the amorphous fraction at each irradiation fluence, and a single-impact model was used to determine the amorphous cross section and track diameter. Amorphization within individual ion tracks was confirmed using head-on TEM imaging, and the amorphous track size agreed with the value deduced by XRD analysis. Raman measurements were performed to qualitatively confirm the ion-induced crystalline-to-amorphous transformation which proceeds similarly for both phosphates.

Anomalous strain effect in heteroepitaxial SrRuO3 films on (111) SrTiO3 substrates

We report comprehensive investigations into the structure of high-quality (111)-oriented SrRuO3 films on SrTiO3 substrates to elucidate the effect of (111) heteroepitaxial strain. We found that SrRuO3 film with a thickness of ∼ 40 nm is compressively strained in plane on the substrate with full coherency. Nevertheless, the out-of-plane spacing is almost the same as in the bulk, which is at odds with the conventional paradigm. By probing a series of half-order Bragg reflections using synchrotron-based x-ray diffraction combined with analyses of the scanning transmission electron microscopy images, we discovered that the heteroepitaxial strain is accommodated via significant suppression of the degree of c + octahedral tilting and the formation of three equivalent domain structures on the (111) SrTiO3 substrate. This anomalous effect sheds light on the understanding of an unconventional paradigm of film–substrate coupling for the (111) heteroepitaxial strain.

(Digital Presentation) Modifying LiNiO2 with W Via a Single Step Synthesis Route

LiNiO2 has been long considered as a promising cathode material owing to its high practical energy density. However, structural and surface instabilities, coupled with complexities in the LiNiO2 synthesis, have thus far prevented its commercialisation. To address issues with the material’s stability during synthesis and cycling, the use of an ammonium tungstate flux is demonstrated here to modify both the LiNiO2 crystal structure and primary particle morphology without introducing additional steps into the synthesis. The successful preparation of LiNiO2 modified with an industrially relevant amount of W (< 5 mol %) was confirmed using a combination of electron microscopy and synchrotron-based X-ray diffraction (XRD). Refinement of structural models against the data suggests tungsten dopant ions occupy the Ni site and concurrently induce migration of Ni2+ to the Li sites. Moreover, W enrichment at grain boundaries has been observed under some of the synthesis conditions. Variable temperature XRD was used to highlight the improved stability of the W-doped materials during the calcination at high temperatures. Electrochemical characterisation shows that W-doped LiNiO2 offers improved cycle life at the expense of little specific capacity. The structural consequences of tungsten doping on the behaviour of the material during electrochemical cycling was also investigated using operando XRD, showing reduced mechanical stress upon cycling. In conclusion, we will show that LiNiO2 modified by W with a simple route and no additional processing steps exhibits structural stability at high temperatures, offering a path towards the reliable synthesis of LiNiO2 with controlled morphology, improved chemomechanics and longer cycling life.

Phase Transitions in Amorphous Germanium under Non-Hydrostatic Compression

As the pioneer semiconductor in transistor, germanium (Ge) has been widely applied in information technology for over half a century. Although many phase transitions in Ge have been reported, the complicated phenomena of the phase structures in amorphous Ge under extreme conditions are still not fully investigated. Here, we report the different routes of phase transition in amorphous Ge under different compression conditions utilizing diamond anvil cell (DAC) combined with synchrotron-based X-ray diffraction (XRD) and Raman spectroscopy techniques. Upon non-hydrostatic compression of amorphous Ge, we observed that shear stress facilitates a reversible pressure-induced phase transformation, in contrast to the pressure-quenchable structure under a hydrostatic compression. These findings afford better understanding of the structural behaviors of Ge under extreme conditions, which contributes to more potential applications in the semiconductor field.

Structural Origin of Suppressed Voltage Decay in Single-Crystalline Li-Rich Layered Li[Li0.2 Ni0.2 Mn0.6 ]O2 Cathodes.

Lithium- and manganese-rich layered oxides (LMLOs, ≥ 250 mAh g-1 ) with polycrystalline morphology always suffer from severe voltage decay upon cycling because of the anisotropic lattice strain and oxygen release induced chemo-mechanical breakdown. Herein, a Co-free single-crystalline LMLO, that is, Li[Li0.2 Ni0.2 Mn0.6 ]O2 (LLNMO-SC), is prepared via a Li+ /Na+ ion-exchange reaction. In situ synchrotron-based X-ray diffraction (sXRD) results demonstrate that relatively small changes in lattice parameters and reduced average micro-strain are observed in LLNMO-SC compared to its polycrystalline counterpart (LLNMO-PC) during the charge-discharge process. Specifically, the as-synthesized LLNMO-SC exhibits a unit cell volume change as low as 1.1% during electrochemical cycling. Such low strain characteristics ensure a stable framework for Li-ion insertion/extraction, which considerably enhances the structural stability of LLNMO during long-term cycling. Due to these peculiar benefits, the average discharge voltage of LLNMO-SC decreases by only ≈0.2V after 100 cycles at 28mA g-1 between 2.0 and 4.8V, which is much lower than that of LLNMO-PC (≈0.5V). Such a single-crystalline strategy offers a promising solution to constructing stable high-energy lithium-ion batteries (LIBs).

Carbonation of fresco mural paintings with a dolomitic mortar

An innovative approach involving a collection of experiments to mimic and assess the different conditions (at selected time-pH-[Mg2+]-humidity) existing during the carbonation process in dolomitic mortars is proposed with a combination of micro-analytical imaging techniques, namely: optical and electron microscopy, micro-Fourier-Transform Infrared spectroscopy, Raman spectroscopy and synchrotron-based micro X-ray diffraction. The carbonation process was studied from short time scale to long time scale and from simple aqueous saturated solutions to fresco mock-up samples. These techniques allowed identifying both crystallized and amorphous components and revealing their in-depth distribution in multi-layered systems. The presence of Mg2+ influences the microstructure and composition of dolomitic mortars, resulting in the presence of aragonite concurrent to calcite. In later stages, i.e. once Ca2+ has transformed into carbonate, brucite can lead to the formation of magnesium carbonates and hydroxycarbonates. The results contribute explaining the lability of old dolomitic mortars observed in historical fresco paintings.

Elastic behavior of metal-assisted etched Si/SiGe superlattice nanowires containing dislocations

We systematically investigate structural parameters, such as shape, size, elastic strain, and relaxations, of metal-assisted etched vertically modulated Si/SiGe superlattice nanowires by using electron microscopy, synchrotron-based x-ray diffraction, and numerical linear elasticity theory. A vertical Si/Ge superlattice with atomically flat interfaces is grown by using molecular beam epitaxy on Si-buffered Si(001) substrates. The lattice constants for Si and Ge are 5.43 and 5.66 Å, respectively, which indicate a lattice mismatch of 4.2%. This results in a strained layer in the boundary between Si and Ge leading to dislocations. These substrates serve as the starting material for nanostructuring the surface by using metal-assisted etching. It is shown that the high quality crystalline structure is preserved in the fabrication process, while the lattice mismatch is partially relieved by dislocation formation. Despite this highly effective relaxation path, dislocations present in the parent superlattice do not vanish upon nanostructuring for wires with diameters of down to at least 80 nm. We relate these observations to the applicability of silicon-based nanowires for high-performance thermoelectric generators.

Strain states and unique properties in cold-rolled TiNi shape memory alloys

Martensitic transformation (MT), strain glass transition (SGT) and the corresponding stress-strain responses in cold-rolled Ti49.2Ni50.8 shape memory alloys (SMAs) at various amounts of cold-rolling deformation are investigated. A phase diagram describing all the strain states and transitions among them is established in the temperature – deformation space. It shows a normal sharp, strong first-order B2 to B19′ MT at low deformation levels (cold-rolling thickness reduction εp < 27%), but a continuous B2 to B19′ SGT at high deformation levels (εp ≥ 27%) upon cooling. Superelasticity with small hysteresis, tunable modulus and functional-fatigue resistance properties are achieved by altering the transition behavior from MT to SGT at εp ≥ 27%. Nanodomains of single variant B19′ martensite at the atomic scale are observed by using aberration-corrected scanning transmission electron microscopy in samples with εp = 27%. Further in-situ loading synchrotron-based high energy X-ray diffraction technique shows that the quasi-linear superelasticity associated with the SGT can be attributed to stress-induced continuous growth of B19′ nanodomains. Geometrical phase analysis of high-resolution transmission electron microscopy image reveals a nanoscale network of severe lattice distortion associated with the accumulated high-density of defects at εp ≥ 27%, which is the microstructural origin of the B19′ strain glass state. This work provides a detailed explanation for the longstanding puzzle of unique quasi-linear superelastic response in plastically deformed TiNi SMAs, and may shed light on achieving novel mechanical properties through defect engineering.

Electrical resistivity as a descriptor for classification of amorphous versus crystalline phases of alloys

Discovering new metallic glasses, non-crystalline alloys with unique combinations of mechanical and chemical properties, is a challenging endeavor because it requires exploration of a vast composition space. High-throughput experiments have greatly enhanced the efficiency with which composition-dependent properties of potential glass-forming alloys can be measured, but phase identification remains a bottleneck because slow or expensive techniques such as table-top or synchrotron-based X-ray diffraction measurements are required. In this study, we developed machine learning (ML) models that can classify amorphous and crystalline phases of alloys using electrical resistivity as a primary descriptor. Artificial neural networks were constructed to correlate the electrical resistivities and the X-ray diffractograms of a broad range of combinatorially synthesized alloys. The ML models are found to classify amorphous/crystalline phases in both thin-film libraries and bulk alloys with high accuracy.

Scanning x-ray microscopy: A sub-100 nm probe toward strain and composition in seeded horizontal Ge(110) nanowires

We have spatially investigated lattice spacing, twist, and bending in individual laterally (110)-oriented Ge nanowires (NWs) on pre-patterned Si(001) substrates. A combination of synchrotron-based scanning x-ray diffraction microscopy with an x-ray focus size of 50 nm and numerical finite element calculations on the elastic strain reveals a three-dimensional relaxation scenario, which becomes particularly complex next to NW nucleation points. Despite a lattice mismatch of 4.2%, lattice compliance is preserved, since strain can effectively be released close to the seeding window. Areas in the NWs other than that appear fully relaxed. The resulting NW twist, i.e., lattice rotations around the growth axis, amounts to less than 0.1°.

Crystallisation of phosphates revisited: a multi-step formation process for SrHPO4

Abstract SrHPO4 is used in a multitude of applications, including biomedicine, catalysts, luminescent materials, and batteries. However, the performance of these materials depends on the ability to control the formation and transformation of strontium phosphates. This work focuses on the application of in situ and ex situ measurements, including synchrotron-based X-ray diffraction (XRD) analysis, luminescence of Ce3+ and Eu3+ dopants, light transmission, reflectance, and thermogravimetry to track structural changes in SrHPO4 under different experimental conditions. Ex situ analysis of aliquots revealed favourable crystallisation of β-SrHPO4 through the formation of Sr6H3(PO4)5·2H2O as an intermediate. Furthermore, in situ analysis showed that the reaction mechanism evolves via the initial formation of amorphous strontium phosphate and Sr5(PO4)3OH, which subsequently transforms to γ-SrHPO4. Analysis of the luminescence properties of the lanthanide dopants provided insights into the coordination environments of the substituted Sr2+ sites.

Tracing Low Amounts of Mg in the Doped Cathode Active Material LiNiO2

The resurgence of electromobility drives the need for high energy density cathode materials. LiNiO2 (LNO) meets this demand, based on its high specific capacity in a narrow voltage range and without relying on scare elements. Yet, it has been plagued by various issues, such as poor cycling performance and thermal instability. Adding dopants, such as widely available Mg2+, is a common strategy to balance cycling performance and energy density. Most prior studies focused on large Mg content ranges and were based on laboratory X-ray diffraction. Hence, the influence of Mg2+ addition on the crystal structure remains ambiguous, especially when small amounts are used (≤ 5 mol%; particularly interesting for industrial applications). Here, we present a systematic study of LiNi1−y Mg y O2 (0 ≤ y ≤ 0.05) investigated by high-resolution synchrotron-based X-ray diffraction combined with elemental analysis, electron microscopy and electrochemical testing. The synthetic route relies on the addition of 10 nm Mg(OH)2 nanoparticles prior to the final calcination, as well as on co-precipitation. It is found that Mg2+ mostly occupies the Ni site until saturating at ∼1.7%, then the Li site becomes preferred. This trend in the site occupancies influences the lattice parameters, oxygen coordinate within the unit cell and Ni–O bond distances. Doping also modifies the electrochemical behavior as a cathode material, stabilizing the capacity retention during cycling but sacrificing specific discharge capacity. Laboratory-based operando X-ray diffraction reveals that the increase in capacity retention is due to the suppression of the H2-H3 phase transition and interlayer distance collapse already in 3% Mg-doped LNO. The combination of structural and electrochemical characterization of doped LNO provides useful insights into the structural chemistry of the Mg2+ dopant and can serve as a starting point to understand Mg as a component in multiple dopant strategies for cathode material design and application.

Atomic structure evolution related to the Invar effect in Fe-based bulk metallic glasses

The Invar effect is universally observed in Fe-based bulk metallic glasses. However, there is limited understanding on how this effect manifests at the atomic scale. Here, we use in-situ synchrotron-based high-energy X-ray diffraction to study the structural transformations of (Fe71.2B24Y4.8)96Nb4 and (Fe73.2B22Y4.8)95Mo5 bulk metallic glasses around the Curie temperature to understand the Invar effect they exhibit. The first two diffraction peaks shift in accordance with the macroscopically measured thermal expansion, which reveals the Invar effect. Additionally, the nearest-neighbor Fe–Fe pair distance correlates well with the macroscopic thermal expansion. In-situ X-ray diffraction is thus able to elucidate the Invar effect in Fe-based metallic glasses at the atomic scale. Here, we find that the Invar effect is not just a macroscopic effect but has a clear atomistic equivalent in the average Fe–Fe pair distance and also shows itself in higher-order atomic shells composed of multiple atom species.

Reduction of thermally grown single-phase CuO and Cu2O thin films by in-situ time-resolved XRD

Copper oxide is used as a catalyst or catalyst precursor in chemical reactions that involve hydrogen as a reactant or a product. Controlled reduction and therefore known reduction kinetics of well-defined Cu-oxides in hydrogen is a key issue for the activation of oxide catalysts as well as for other technological fields such as gas sensing. The kinetics and mechanisms of reduction of single-phase CuO and Cu2O films in 5 vol% H2/Ar were investigated by in-situ real time synchrotron-based X-ray diffraction measurements. To this end, the incubation time, the CuO → Cu2O and Cu2O → Cu phase transformation rates, as well as the microstructure and strain state of the evolving Cu, CuO and Cu2O phases were studied during the reduction. Highly porous Cu and Cu-oxides scaffolds were obtained by alternating oxidation and reduction steps, which can be envisaged for a wealth of applications in the fields of catalysis, batteries, water treatment and biomedical applications.

Interpretation of Rubidium-Based Perovskite Recipes toward Electronic Passivation and Ion-Diffusion Mitigation.

Rubidium cation (Rb+ ) addition is witnessed to play a pivotal role in boosting the comprehensive performance of organic-inorganic hybrid perovskite solar cells. However, the origin of such success derived from irreplaceable superiorities brought by Rb+ remains ambiguous. Herein, grain-boundary-including atomic models are adopted for the accurate theoretical analysis of practical Rb+ distribution in perovskite structures. The spatial distribution, covering both the grain interiors and boundaries, is thoroughly identified by virtue of synchrotron-based grazing-incidence X-ray diffraction. On this basis, the prominent elevation of the halogen vacancy formation energy, improved charge-carrier dynamics, and the electronic passivation mechanism in the grain interior are expounded. As evidenced by the increased energy barrier and suppressed microcurrent, the critical role of Rb+ addition in blocking the diffusion pathway along grain boundaries, inhibiting halide phase segregation, and eventually enhancing intrinsic stability is elucidated. Hence, the linkage avalanche effect of occupied location dominated by subtle changes in Rb+ concentration on electronic defects, ion migration, and phase stability is completely investigated in detail, shedding a new light on the advancement of high-efficiency cascade-incorporating strategies and perovskite compositional engineering.

In-situ synchrotron-based high energy X-ray diffraction study of the deformation mechanism of δ-hydrides in a commercially pure titanium

We used by in-situ high energy X-ray diffraction to investigate the deformation behavior of Grade 2 commercially pure titanium that was hydrogen charged to form hydrides. The results showed that the peak broadening in the diffraction patterns are due to the high internal and interphase stresses generated within and around hydrides due to the volume expansion induced by the phase transformation. The hydrides exhibit a typical high strength but brittle secondary phase behavior, which undertakes more elastic strain than matrix and is the location where cracks are first generated. Interestingly, the δ-hydrides sustain larger strains than the matrix, especially after the matrix yields. This study on the deformation mechanism of hydrides in pure titanium provides insight into the hydride deformation behavior and hydrogen embrittlement in both titanium and zirconium.

Enhancement of exciton separation in indoor perovskite photovoltaics by employing conjugated organic chromophores

Indoor photovoltaics (IPVs) are favored for their speciality to provide sustainable and reliable energy for low-power electronic devices. As an ideal IPVs material, perovskite has been well explored in recent years. Herein, we focus on enhancing the photoelectric properties of the perovskite crystal under low light. By introducing N,N′-bis(dimethylaminopropyl-N‴-oxide)-perylene-3,4,9,10-tetracarboxidiimide (PDINO) into the perovskite, the crystallinity degree of the perovskite films are proven to be improved by synchrotron-based grazing incidence X-ray diffraction, thereby effectively enhancing the exciton transmission and separation in the perovskite. By using ultrafast spectroscopy, the influence of the optimized charge separation inside the perovskite under low light has been systematically analyzed. The final optimal perovskite solar cell showed an impressive indoor power conversion efficiency (PCE) of 37.9% under 1000 lux low light, and a PCE of 21.3% under 1 sun illumination. These findings proved doping of the perovskite by strong electron acceptors as a valid strategy for efficient charge transport, paving the way to their applications as IPVs.

Inhomogeneous electric field-induced structural changes in soft lead zirconate titanate ferroelectric ceramics

Under the application of an external voltage, high electric field concentrations can develop around the interdigitated electrode edges inside multilayer ceramic actuators (MLCAs). The spatial distribution of the local electrical field can create local inhomogeneity in the electromechanical response. To investigate the complex field inhomogeneity in MLCAs, partially electroded Nb-doped PbZrxTi1-xO3 samples were investigated via synchrotron-based high-energy X-ray diffraction (XRD) as a function of applied electric field. These in situ experiments allowed us to probe the structural changes as a function of position relative to the electrode edge and calculate the local degree of domain alignment, from which the local electric field directions were inferred. The domain switching behavior, both in amplitude and orientation, was found to be spatially dependent across the inactive regions in partially electroded samples. Specifically, the degree of domain alignment and field-induced phase transitions are amplified near the electrode edge. The orientation-dependent phase transitions are also amplified for the tetragonal composition near the morphotropic phase boundary (MPB), i.e., the Nb-doped PbZr0.53Ti0.47O3 composition. Finite element analysis (FEA) shows spatially-dependent, inhomogeneous electric field distributions in the partial-electrode samples, which closely match the experimentally inferred local electric field directions from XRD. The correlation of FEA and experimental data from XRD corroborates that the ferroelectric domain orientation distributions are being directed, primarily, in the direction of the electric field.

Vacancy induced anomalies in the electrical transport properties of Ag-doped Zn1−xCdxSb (x = 0.375) solid solutions

Zinc antimonide (ZnSb) and its solid solution with CdSb are well-known p-type thermoelectric materials. Electrical transport properties of doped ZnSb exhibit certain anomalies: (a) non-monotonic changes in the electrical properties with temperature and (b) occurrence of a reversible hysteresis loop in electrical transport data when thermally cycled. The objective of this study was to investigate the underlying cause of these behaviors. Ag-doped compositions of (Zn0.625Cd0.375)1−δAgδSb (δ = 0, 0.02, and 0.04) solid solutions have been prepared by melt-synthesis—rapid compaction—annealing process. Measurement of the electrical conductivity (σ), Seebeck coefficient (S), and Hall coefficient (RH) (room temperature to 673 K) displayed the characteristic hysteresis behavior on thermal cycling along with the unusual rise in the charge carrier concentration (n) around 500 K. Aside from that, it was found that cooling rates dramatically influence room temperature properties. Analysis of synchrotron-based x-ray diffraction data by Rietveld refinement indicates that Ag-doping results in the formation of Zn vacancies [Vzn]. Also, a sharp drop in the concentration of Zn vacancies, [Vzn] around 550 K was observed and could be correlated with the changes in n values. This correlation between changes in [Vzn] and n has been used to explain the observed electrical anomalies, which are a consequence of the repeated annihilation and creation of Zn vacancies with temperature changes.