Synchrotron Diffraction Techniques Research Articles

The effects of internal stresses on the creep deformation investigated using in-situ synchrotron diffraction and crystal plasticity modelling

In this study we investigated the evolution of meso-scale internal stresses and those effects on local deformation behaviour during incremental plastic and creep deformation in type 316H stainless steel at 550 °C, using in-situ X-ray synchrotron diffraction and crystal plasticity modelling. Owing to the fast data accusation rate of synchrotron diffraction technique, for the first time, the transient behaviour of different grain families was captured during initial fast stress relaxation period of the displacement-controlled creep dwells. Significantly it is found that the evolution of internal stresses during time independent plastic deformation is distinct from that during time dependent creep deformation. During plastic deformation, lattice strains in the {311} grain family exhibit linear behaviour whereas during creep deformation it exhibits non-linear behaviour, instead the {111} grain family exhibit linear behaviour. A novel unified constitutive law was devised within crystal plasticity framework based on the theoretical physics; the model successfully predicts the macroscopic deformation behaviour as well as the distinction between the evolution of meso-scale internal stresses during plastic and creep deformation, therefore, correctly accounting for the effect of internal stresses generated during plastic deformation on the subsequent creep deformation. The validated model has elucidated the grain-neighbouring effects on individual grain deformations.

Breakdown and Reformation of Birnessite in Rechargeable, Bismuth Modified MnO2 Alkaline Batteries

Rechargeable Zn-MnO2 alkaline batteries have been identified as a viable option for the modernization of grid scale energy storage due to their projected cost (<$100/kWh), scalability, and safer components when compared to non-aqueous alternatives.1 For this system to reach its maximum capability, the full Mn4+/2+ redox couple must be reversible over thousands of cycles with high mass loading. This was successfully demonstrated with the incorporation of various electrode constituents which alter the fundamental discharge and charge process of MnO2 to Mn(OH)2.1 Using non-destructive, synchrotron diffraction techniques, we detail the effect of Bi on the breakdown and reformation of birnessite (\U0001d6ff-MnO2) during cycling. Further spectroscopic evidence including Raman and X-ray absorption spectroscopy during cycling also provides crucial insights into amorphous phases and variations in redox activity, with and without additives.Early demonstrations of Bi3+ containing constituents with MnO2 in alkaline batteries increased the cyclability of these systems up to hundreds of cycles with low mass loading.2,3 Later investigations followed up by hypothesizing the presence of Bi3+ could potentially catalyze the reduction of Mn3+ to Mn2+ and that upon subsequent charge promotes the formation of a layered, birnessite, MnO2 structure.4 The quick reduction of Mn3+/2+ is critical because the presence of Mn3+ is associated with the irreversible phase change to the Mn3O4 spinel structure.5 Figure 1 shows the birnessite (001) reflection at ~7.1 Å rapidly shift to lower d-spacings upon discharge until ~6.9 Å then directly convert to Mn(OH)2 where the corresponding ~4.8 Å reflection appears. Notably, no crystalline intermediates such as MnOOH or Mn3O4 form during cycling. Also, upon charge the birnessite (001) reflection does not appear until the third potential plateau even though the Mn(OH)2 (001) reflection fades linearly until the end of charge. Due to these observations, further spectroscopic techniques were used to probe these reactions. Using operando Raman spectroscopy we observed a limited formation of an amorphous Mn3O4 during discharge and a disordered birnessite phase formation occurring on the second plateau of charge in the presence of Bi3+. These investigations provide new insight into the intermediates and formation mechanisms of Mn-based alkaline battery electrodes in the presence of Bi2O3.The presence of both Cu and Bi additives are necessary to stabilize the reversible processes occurring during electrochemical cycling. However, the synergistic relationship between Cu and Bi is not extensively investigated. Previous synchrotron XANES and XRF mapping showed that the Cu2+/0 redox couple was occurring during the Mn3+/2+ reduction. It was hypothesized that the Cu acts as a redox mediator and rapidly forms the Mn2+ species during discharge.6 Here we use a X-ray absorption spectroscopy quick scanning to monitor the Mn, Bi, and Cu redox activity which are all relating to the possible mediation of Mn reduction and oxidation. Using multiple techniques, we provide a complete understanding of the role of both bismuth and copper containing constituents on the redox transitions of MnO2 in alkaline electrolyte. Wholistic investigations combining multiple operando techniques are critical to the development of rechargeable alkaline batteries by advancing the fundamental design of these systems. Figure 1 (a) Operando X-ray diffraction data between 3-7.5 Å of the first cycle of Bi2O3-modified birnessite electrode in KOH electrolyte. (b) Corresponding galvanostatic discharge and charge at a rate of C/3. (c) Lattice parameter ‘c’, the interlayer spacing, of the birnessite phase during the first cycle. (d) The relative peak intensities of the birnessite and Mn(OH)2 (001) reflection during the first cycle.

Experimental observations of large changes in electron density distributions in β−Ge

The electron density distributions in $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ge}$ have been experimentally determined at in situ high-pressure conditions using synchrotron diffraction techniques in a diamond anvil cell. Upon decompression, the electron density along the $c$ axis in tetragonal $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ge}$ displays a sudden drop at 10--11 GPa close to that of the structural \ensuremath{\alpha}-\ensuremath{\beta} transition, while the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ge}$ samples remain as single crystals until transforming to metastable Ge phases around 6.7--8.5 GPa. In contrast to the covalently bonded $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ge}$ that displays only a weak participation of $d$ electrons in the valence band under compression above 7.7 GPa, our experimental results suggest that a large change in $d$-orbital participation can occur in the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ge}$ lattice which has mixed covalent and metallic bonding. The $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ge}$ below 10 GPa may display large fluctuations in electronic properties, which sheds light for exploring novel materials with intriguing electronic and optical properties.

Aluminum dual doping and oxygen transport pathway in novel Sr11Mo4−xAlxO23 oxide-ion solid electrolytes

Three new electrolytes of composition Sr11Mo4-xAlxO23-δ (x = 0.5, 1.0 and 1.5) were synthesized by wet-chemistry procedures (citrate method) as polycrystalline samples. They were crystallographically studied from synchrotron and neutron diffraction techniques. They derive from the Sr11Mo4O23 complex perovskite, which can be rewritten as Sr1.75□0.25SrMoO5.75, highlighting the relationship with double perovskites A2B′B″O6. From these analyses via Rietveld refinement, a dual doping was found from synchrotron data, where the aluminum is equally allocated at both Sr (B′) and Mo (B″) sites. A detailed analysis of the neutron data from Difference Fourier Maps (DFM) unveiled the oxygen delocalization around the Mo site. From the refined crystal structures at room and high temperatures, a Bond Valence Sum Map (BVSM) analysis was useful to find the oxygen pathway in the crystal structure, and the minimum density to allow connections among oxygen atoms. The structural characterization was complemented with thermogravimetric and thermodilatometric analysis. The ionic conductivity was measured from electrochemical impedance spectroscopy (EIS) in wet and dry air and nitrogen atmospheres.

Room-temperature synthesis of a new stable (N2H4)WO3 compound: a route for hydrazine trapping.

A new (N2H4)WO3 compound has been obtained by mixing WO3 and aqueous hydrazine solution at room temperature for 24 h. The reaction is catalyzed by the presence of lithium. X-ray, synchrotron and neutron diffraction techniques have shown that the material crystallizes in trigonal space group P3221 (No. 154). Chains of distorted WO4 tetrahedra extend along the a axis of the unit cell, linked by a corner-sharing oxygen atom: the N2H4 are in the voids between them. The thermal characterization shows that this new compound is stable up to 220°C, greatly beyond the boiling point of N2H4 (114°C); thus making it a promising candidate for catalysis or trapping applications.

Strain evolution in Zr-2.5 wt% Nb observed with synchrotron X-ray diffraction

The mechanical behaviour of hydrides in Zr alloy at low hydrogen concentrations continues to be the subject of research due to wide engineering application and highly complex behaviour of the Zr-H system. Significant structural differences between hydrides and α-Zr cause large dilatational strains associated with formation of the hydride precipitates in zirconium. Two hydride populations were observed to form under different strain states in the α-Zr matrix. The hysteresis of the hydrogen solubility is detected both in intensity and in strain curves for measured δ(111), α(00.2) and α{10.0} peaks. A two-dimensional analysis of α-strain was performed for the axial-hoop, hoop-radial, and axial-radial planes of Zr-2.5 wt% Nb pressure tube material. Variations in the hydrostatic and deviatoric strain components of the α-Zr with temperature were examined. In addition, a change in phase strain during a thermal excursion was considered as a sum of three cooperating components: phase thermal expansion (εT), strains associated with other phase presenting in material (ε[H] or εmatrix), and inter-granular strain within the phase caused by Type II stresses (εother).The present study broadens our understanding of strain evolution in a Zr-H system undergoing hydride dissolution and precipitation. An X-ray synchrotron diffraction technique enables the in-situ following of strain variations in differently oriented grain families during the thermal cycle. In contrast to widely reported the average strain value within the α-phase, the diffraction patterns generated with this technique allow the evaluation of the anisotropic directionality of strains in different phases in Zr-H system generated during phase transformation.

Combination of in-situ diffraction experiments and acoustic emission testing to understand the compression behavior of Mg-Y-Zn alloys containing LPSO phase under different loading conditions

The effect of the orientation of the non-recrystallized grains (non-DRX) and the LPSO phase on plasticity in extruded MgY2Zn1 alloy with a bimodal grain structure have been studied in-situ using the combination of synchrotron diffraction and acoustic emission techniques during compression tests. The adaptive sequential k-means (ASK) procedure was applied to analyze the acoustic emission signal. This method can successful separate the signal for each possible deformation systems. Combining both techniques, the deformation mechanisms that take place during the compression tests under different loading directions have been distinguish. Independently of the loading direction, the beginning of the macroscopic plasticity is always controlled by the activation of basal slip system in the DRX grains. However, highly oriented non-DRX grains and LPSO phase have a strong influence over the compressive deformation of the MgY2Zn1 alloy. The basal planes in the non-DRX grains were oriented parallel to the extrusion direction (ED). Thus, the activation of the {101¯2}〈101¯1¯〉 extension twinning was found to be significant only in the ED mode. In the other two loading cases, TD and 45, the activation of non-basal slip systems has been detected. On the other hand, the reinforcing effect of the elongated LPSO phase is the most effective, if the loading axis is aligned with the fiber direction (extrusion direction) since the load transfer mechanism is more effective. In this loading case, the LPSO can be plastically deformed due to kinking. This deformation mode has been isolated by the ASK procedure and characterized by in-situ scanning electron microscopy.

A Stimuli-Responsive Zirconium Metal-Organic Framework Based on Supermolecular Design.

A flexible, yet very stable metal-organic framework (DUT-98, Zr6 O4 (OH)4 (CPCDC)4 (H2 O)4 , CPCDC=9-(4-carboxyphenyl)-9H-carbazole-3,6-dicarboxylate) was synthesized using a rational supermolecular building block approach based on molecular modelling of metal-organic chains and subsequent virtual interlinking into a 3D MOF. Structural characterization via synchrotron single-crystal X-ray diffraction (SCXRD) revealed the one-dimensional pore architecture of DUT-98, envisioned in silico. After supercritical solvent extraction, distinctive responses towards various gases stimulated reversible structural transformations, as detected using coupled synchrotron diffraction and physisorption techniques. DUT-98 shows a surprisingly low water uptake but a high selectivity for pore opening towards specific gases and vapors (N2 , CO2 , n-butane, alcohols) at characteristic pressure resulting in multiple steps in the adsorption isotherm and hysteretic behavior upon desorption.

A Stimuli‐Responsive Zirconium Metal–Organic Framework Based on Supermolecular Design

AbstractA flexible, yet very stable metal–organic framework (DUT‐98, Zr6O4(OH)4(CPCDC)4(H2O)4, CPCDC=9‐(4‐carboxyphenyl)‐9H‐carbazole‐3,6‐dicarboxylate) was synthesized using a rational supermolecular building block approach based on molecular modelling of metal–organic chains and subsequent virtual interlinking into a 3D MOF. Structural characterization via synchrotron single‐crystal X‐ray diffraction (SCXRD) revealed the one‐dimensional pore architecture of DUT‐98, envisioned in silico. After supercritical solvent extraction, distinctive responses towards various gases stimulated reversible structural transformations, as detected using coupled synchrotron diffraction and physisorption techniques. DUT‐98 shows a surprisingly low water uptake but a high selectivity for pore opening towards specific gases and vapors (N2, CO2, n‐butane, alcohols) at characteristic pressure resulting in multiple steps in the adsorption isotherm and hysteretic behavior upon desorption.

A comprehensive structural analysis of relaxor ferroelectric Cr- and Ni-doped Sr0.61Ba0.39Nb2O6 crystals

A structural features, real compositions, and point defects of nominally pure and Cr- and Ni-doped Sr0.61Ba0.39Nb2O6 (SBN) crystals, which structure contains four chemically-different and ten crystallochemically-different atoms, have been determined using neutron, X-ray single crystal, X-ray powder, and X-ray synchrotron diffraction techniques, and EXAFS/XANES spectroscopy. The location of transition metal ions has been identified based on the analysis of electron (nuclear) density and confirmed by the EXAFS/XANES together with the definition of formal charges of Ni atoms in Ni-doped SBN crystals. The scope and application possibilities of diffraction methods and EXAFS/XANES spectroscopy for the determination of structural features and defect formation in functional bulk nominally-pure and Cr- and Ni-doped Sr0.61Ba0.39Nb2O6 single crystals is established.

Numerical and experimental description of the welding residual stress field in tubular joints for fatigue assessment

In order to achieve fatigue resistant welded structures, it is necessary to manage and control welding process-related factors which affect the fatigue strength. These factors are sometimes present as welding defects and sometimes as inevitable residual stresses which are unwanted by-products of welding processes. This subject has been treated in welding research communities since the 1950s. However, the extent of this effect was unclear and is still a matter of debate. Having advanced predictive tools for accurate determining welding residual stresses will not only lead to the possibility of considering precise welding residual stress effects during life estimation but also can be useful to have effective measures for the subsequent mitigation or modification of the welding residual stress fields. The objective of this study is to describe experimentally and numerically the welding residual stress field in welded tubular joints made of structural steel S355J2H in terms of two different case studies. Residual stresses in cylindrical specimens with dummy welds (case-1) and bead on tube welds (case-2) are determined experimentally by means of x-ray, synchrotron, and neutron diffraction techniques. The SYSWELD software is used to calculate the welding residual stresses numerically by integrating the metallurgical transformation effects in order to compute the link between material microstructure and residual stresses. Thermal and metallurgical calculations couple the temperature field and phase proportions by considering the latent heat of fusion/solidification and the transformations in the transient heat conduction equation. Obtained results which include both thermal and metallurgical history are used as input data for mechanical calculations afterward. The accuracy of the both thermal and structural models is validated through experiments for temperature distribution and residual stresses.

Puzzling calcite-III dimorphism: crystallography, high-pressure behavior, and pathway of single-crystal transitions

High-pressure phase transformations between the polymorphic forms I, II, III, and IIIb of CaCO3 were investigated by analytical in situ high-pressure high-temperature experiments on oriented single-crystal samples. All experiments at non-ambient conditions were carried out by means of Raman scattering, X-ray, and synchrotron diffraction techniques using diamond-anvil cells in the pressure range up to 6.5 GPa. The composite-gasket resistive heating technique was applied for all high-pressure investigations at temperatures up to 550 K. High-pressure Raman spectra reveal distinguishable characteristic spectral differences located in the wave number range of external modes with the occurrence of band splitting and shoulders due to subtle symmetry changes. Constraints from in situ observations suggest a stability field of CaCO3-IIIb at relatively low temperatures adjacent to the calcite-II field. Isothermal compression of calcite provides the sequence from I to II, IIIb, and finally, III, with all transformations showing volume discontinuities. Re-transformation at decreasing pressure from III oversteps the stability field of IIIb and demonstrates the pathway of pressure changes to determine the transition sequence. Clausius–Clapeyron slopes of the phase boundary lines were determined as: ΔP/ΔT = −2.79 ± 0.28 × 10−3 GPa K−1 (I–II); +1.87 ± 0.31 × 10−3 GPa K−1 (II/III); +4.01 ± 0.5 × 10−3 GPa K−1 (II/IIIb); −33.9 ± 0.4 × 10−3 GPa K−1 (IIIb/III). The triple point between phases II, IIIb, and III was determined by intersection and is located at 2.01(7) GPa/338(5) K. The pathway of transition from I over II to IIIb can be interpreted by displacement with small shear involved (by 2.9° on I/II and by 8.2° on II/IIIb). The former triad of calcite-I corresponds to the [20-1] direction in the P21/c unit cell of phase II and to [101] in the pseudomonoclinic C $${\bar{1}}$$ setting of phase IIIb. Crystal structure investigations of triclinic CaCO3-III at non-ambient pressure–temperature conditions confirm the reported structure, and the small changes associated with the variation in P and T explain the broad stability of this structure with respect to variations in P and T. PVT equation of state parameters was determined from experimental data points in the range of 2.20–6.50 GPa at 298–405 K providing $$K_{{{\text{T}}_{0} }}$$ = 87.5(5.1) GPa, (δK T/δT) P = −0.21(0.23) GPa K−1, α 0 = 0.8(21.4) × 10−5 K−1, and α 1 = 1.0(3.7) × 10−7 K−1 using a second-order Birch–Murnaghan equation of state formalism.

Hollandite-type TiO2: a new negative electrode material for sodium-ion batteries

The electrochemical properties of TiO2 with the hollandite structure (TiO2(H)) as a negative electrode material for sodium-ion batteries are reported. TiO2(H) was obtained from hollandite K0.21TiO2 by an oxidation–ion extraction process. Na/TiO2(H) cells exhibit a large first discharge capacity of 280 mA h g−1 down to 0.2 V. After the first discharge the Na/TiO2(H) cells develop a reversible charge–discharge capacity of 85 mA h g−1 at C/8 rate in the 2.5–0.2 V voltage range; this corresponds to the reversible insertion of 0.25 Na per TiO2(H) formula unit. Chronoamperometry and potentiostatic intermittent titration techniques were used to further characterize the electrochemical reaction mechanism. Structural changes in the TiO2(H) electrode upon sodium insertion and extraction have been studied by ex situ XRD and high resolution in situ synchrotron diffraction techniques, for which appropriately modified coin-type cells were used. It is seen that sodium insertion into TiO2(H) is commenced with a single-phase solid solution followed by a structural transition from tetragonal I4/m to monoclinic I2/m symmetry, in which the skeleton framework is retained. The reversible transition includes few structural changes with a small volume change of only 1.1%. Fourier difference maps deduced from SXRD patterns revealed the location of Na ions in 4i sites in the tunnel space. The coordination arrangement around Na ions is distorted capped trigonal prisms formed by seven oxygen atoms. Although still far from the theoretical capacity (335 mA h g−1), the cycling properties at a low insertion potential together with the host framework stability indicate the feasibility of TiO2 with the hollandite structure as a negative electrode material for Na-ion batteries.

Strategies for in situ laser heating in the diamond anvil cell at an X-ray diffraction beamline.

An overview of several innovations regarding in situ laser-heating techniques in the diamond anvil cell at the high-pressure beamline ID27 of the European Synchrotron Radiation Facility is presented. Pyrometry measurements have been adapted to allow simultaneous double-sided temperature measurements with the installation of two additional online laser systems: a CO2 and a pulsed Nd:YAG laser system. This reiteration of laser-heating advancements at ID27 is designed to pave the way for a new generation of state-of-the-art experiments that demand the need for synchrotron diffraction techniques. Experimental examples are provided for each major development. The capabilities of the double pyrometer have been tested with the Nd:YAG continuous-wave lasers but also in a time-resolved configuration using the nanosecond-pulsed Nd:YAG laser on a Fe sample up to 180 GPa and 2900 K. The combination of time-resolved X-ray diffraction with in situ CO2 laser heating is shown with the crystallization of a high-pressure phase of the naturally found pyrite mineral MnS2 (11 GPa, 1100-1650 K).

Adsorption of iodoalkanes on graphite

In this work, we present a study of linear iodoalkanes physisorbed from their bulk liquid and at submonolayer coverages onto a graphite substrate by combining calorimetry, incoherent neutron scattering and synchrotron diffraction techniques. We identify that there is solid monolayer formation for these molecules for alkyl chain lengths of 5–12 carbons at high surface coverages. The monolayer structures of the odd homologues (5, 7, 9 and 11) have been addressed using synchrotron X-ray and neutron scattering at sub-monolayer coverages and indicate that the molecules lie with their carbon backbone parallel to the graphite surface with a Pgb plane group. Evidence for a non-bonded I–I interaction in the monolayers is also discussed.

Welding residual stress behavior under mechanical loading

For an accurate estimation of the fatigue performance of welded joints, not only the initial residual stress field but also its variation under load is decisive. The portion of residual stress which stays stable can shift the load stress range much the same way as the mean stresses do in fatigue loading. The relaxation of residual stresses during fatigue loading however reduces this hazard to the structural health. That is, before considering the influence of residual stresses in fatigue, the effect of fatigue on residual stresses should be understood. In this work, the influence of the quasi-static and cyclic loading on the relaxation of welding residual stresses in small specimens and large components out of different steels namely S235JRG2, S355J2G3, P460NL, S690QL, S960QL, and S1100QL has been investigated experimentally. X-ray diffraction analysis as a reliable method has been used for the determination of residual stress profiles in surface layers. For residual stress analysis in deeper layers, synchrotron and neutron diffraction techniques were applied as complementary methods. A clear recognition of the difference between initial welding residual stresses in small- and large-scale specimens was observed. Tensile residual stresses as high as the yield strength could appear in large-scale welded specimens. That was not the case for small-scale welds due to the low grade of restraint. Nevertheless, in both types of samples, the highest residual stresses were obtained in the weld centerline. At the weld toe which is critical regarding fatigue crack initiation, the magnitude of the residual stresses is lower. Beside of that, it was observed that in the case of relaxation, the first load cycles especially in components out of low-strength steels are decisive. The influence of loading conditions and local mechanical properties on the relaxation of welding residual stresses was investigated based on principles of solid mechanics. The von Mises failure criterion was able to describe the relaxation behavior.

Experimental characterization of electrochemical synthesized Fe nanowires for biomedical applications

Fe based nanomaterials have shown extensive application promises in medical diagnosis and treatment due to their biocompatibility. Using template assisted electrodeposition, iron based nanowires with controllable size, aspect ratio, and magnetic anisotropy have been fabricated. In situ synchrotron diffraction technique has been used to reveal the nanowire growth mechanism and provide real time compositional and crystallographic information. Biocompatibility of the nanowires with Rat-2 fibroblast cells has been evaluated and compared with magnetite nanoparticles. Using an external magnetic field, cell manipulation through the use of these magnetic nanowires has been demonstrated.

Calculation of anisotropic properties of dental enamel from synchrotron data

Obtaining information about the intrinsic structure of polycrystalline materials is of prime importance owing to the anisotropic behaviour of individual crystallites. Grain orientation and its statistical distribution, i.e. the texture, have an important influence on the material properties. Crystallographic orientations play an important role in all kinds of polycrystalline materials such as metallic, geological and biological. Using synchrotron diffraction techniques the texture can be measured with high local and angular resolving power. Here methods are presented which allow the spatial orientation of the crystallites to be determined and information about the anisotropy of mechanical properties, such as elastic modulus or thermal expansion, to obtained. The methods are adapted to all crystal and several sample symmetries as well as to different phases, for example with overlapping diffraction peaks. To demonstrate the abilities of the methods, human dental enamel has been chosen, showing even overlapping diffraction peaks. Likewise it is of special interest to learn more about the orientation and anisotropic properties of dental enamel, since only basic information is available up to now. The texture of enamel has been found to be a tilted fibre texture of high strength (up to 12.5×). The calculated elastic modulus is up to 155 GPa and the thermal expansion up to 22.3 × 10(-6)°C(-1).

In situ synchrotron analysis of lattice rotations in individual grains during stress-induced martensitic transformations in a polycrystalline CuAlBe shape memory alloy

Two synchrotron diffraction techniques, three-dimensional X-ray diffraction and Laue microdiffraction, are applied to studying the deformation behaviour of individual grains embedded in a Cu 74Al 23Be 3 superelastic shape memory alloy. The average lattice rotation and the intragranular heterogeneity of orientations are measured during in situ tensile tests at room temperature for four grains of mean size ∼1 mm. During mechanical loading, all four grains rotate and the mean rotation angle increases with austenite deformation. As the martensitic transformation occurs, the rotation becomes more pronounced, and the grain orientation splits into several sub-domains: the austenite orientation varies on both sides of the martensite variant. The mean disorientation is ∼1°. Upon unloading, the sub-domains collapse and reverse rotation is observed.

Structure of Ferroelectric Silver Niobate AgNbO3

Crystal structure of ferroelectric silver niobate AgNbO3 was determined (Pmc21) by convergent beam electron, electron, neutron, and synchrotron diffraction techniques and first-principles calculations. The atomic displacements along the c axis in Pmc21 AgNbO3 are responsible for the spontaneous polarization, ferroelectricity, and the paraelectric−ferroelectric phase transition.