The influence of strain on image reconstruction in Bragg coherent X-ray diffraction imaging and ptychography

A quantitative analysis of the effect of strain on phase retrieval in Bragg coherent X-ray diffraction imaging is reported. It is shown in reconstruction simulations that the phase maps of objects with strong step-like phase changes are more precisely retrieved than the corresponding modulus values. The simulations suggest that the reconstruction precision for both phase and modulus can be improved by employing a modulus homogenization (MH) constraint. This approach was tested on experimental data from a highly strained Fe–Al crystal which also features antiphase domain boundaries yielding characteristic π phase shifts of the (001) superlattice reflection. The impact of MH is significant and this study outlines a successful method towards imaging of strong phase objects using the next generation of coherent X-ray sources, including X-ray free-electron lasers.

A monoclinic to tetragonal crossover in (Na0.5Bi0.5TiO3)(1−x)(BaZrO3)xceramic: a lead-free ferroelectric material

A single-phase solid solution of Na0.5Bi0.5TiO3(NBT) and BaZrO3(BZ),i.e.(NBT)(1−x)(BaZrO3)x(x= 0.00, 0.01, 0.03, 0.05 and 0.07), was synthesized using the solid-state reaction method. It is found that the temperature for single-phase formation increases with increasing BZ content. A detailed structural investigation was carried out as a function of composition using Rietveld analysis of powder X-ray diffraction data. The structural refinement reveals a crossover from monoclinic (Cc) to tetragonal (P4mm) symmetry in the composition range 0.03 <x< 0.05. The intensity of superlattice reflection due toa−a−c−tilting of oxygen octahedra in theCcphase was found to decrease with increasing BZ content. The unit-cell volume was also found to increase with BZ content, which was attributed to the higher ionic radii of Ba2+and Zr4+in comparison to Bi3+(Na+) and Ti4+. The structural phase boundary observed in (NBT)(1−x)(BaZrO3)xis concurrent with the observation of morphotropic phase boundary like behaviour.

Photo-modulated dynamic competition between metallic and insulating phases in a layered manganite

ABSTRACTWe show evidence that the competition between the antiferromagnetic metallic phase and the charge- and orbital-ordered insulating phase at the reentrant phase boundary of a layered manganite, LaSr2Mn2O7, can be manipulated using ultrafast optical excitation. The time-dependent evolution of the Jahn-Teller superlattice reflection, the indicator of the formation of charge and orbital order, was measured at different laser fluences. The laser-induced change in the Jahn-Teller reflection intensity shows a reversal of sign between earlier (∼10 ns) and later (∼150 ns) times during the relaxation of the sample. This is consistent with a physics picture whereby the laser excitation modulates the local competition between the metallic and the insulating phases.

Estimation of Order Parameter and Spin Moment of Fe3Pt by White X-Ray Diffraction Method

Diffraction intensities of a single crystal of Fe3Pt alloy have been measured by using white X-rays of synchrotron radiation. The observed intensities have been compared with the calculated ones for fundamental and superlattice reflections. By normalizing the calculated intensities of fundamental reflections to the observed ones and evaluating the ratio of the observed intensity of superlattice reflection to the calculated ones, we have estimated the order parameter as 0.82±0.03. The obtained order parameter has been applied to an analysis of the X-ray magnetic diffraction data to estimate the spin moment of Fe and Pt atoms in this alloy.

Structure And Martensitic Transformation in Ni44Mn43.5Sn12.5-xAlx Heusler Alloys

Alloys with constant Ni/Mn ratio equal to 1.01 of nominal compositions Ni44Mn43.5Sn12.5-xAlx (x = 0, 1, 2 and 3) were induction cast, homogenized in vacuum for 6 hours at 1000ºC, annealed for 1 h at 900 and water quenched for solution treatment (ST). Differential scanning calorimetry (DSC) studies revealed that the quenched alloys undergo martensitic transformation with martensite start temperatures (Ms) ranging from - 140 up to - 80ºC. An increase of Ms temperature with increasing of the aluminum content as well as the linear relationship between Ms and the conductive electron concentration (e/a) was observed. DSC has been used also to estimate the associated entropy change from the transformation heat Q and peak position temperature Tp; corresponding to ΔS ≈ Q/Tp. X-Ray diffraction phase analyses performed at room temperature proved that in all ST alloys the L21 Heusler structure is present. However, a different degree of order of this phase was observed, what was manifested by a decrease of intensity of the 111 superlattice reflection of the L21 structure with an increase of Al content. The ordering behavior was also proven by the transmission electron microscopy (TEM) investigations, particularly electron diffraction patterns. The evolution of microstructure after different treatments was also illustrated by light microscopy observations.

Epitaxial thin films of ordered double perovskite SrLaVMoO6

ABSTRACTEpitaxial thin films of SrLaVMoO6 with an ordered double perovskite structure have been grown on (001) and (111) SrTiO3 substrates by magnetron sputtering. The optimized (111) film exhibited a clear (111) diffraction peak, which is a superlattice reflection of double perovskite unite cell, indicating clear B-site ordering. Temperature dependences of resistivity ρ show metallic behavior and transition point at 140~150 K, of which behavior is reminiscent of the electrical properties of materials showing long-range magnetic or antiferromagnetic order. XPS results of the Mo 3d core level spectra are discussed in terms of the B-site ordering and oxygen nonstoichiometry.

Magnetic and Transport Properties of the Double Perovskite Sr2FeMnO6

The double perovskite Sr2FeMnO6 has been synthesized by the solid-state reaction method. X-ray diffraction and subsequent Rietveld refinement confirm that Sr2FeMnO6 has the double-perovskite structure with a cubic cell (a=7.726 Å) and space group Fm3m (No. 225) . The appearance of the superlattice reflection (111) indicates that Fe and Mn atoms alternatively occupy the center of oxygen octahedron in an ordered way. Based on magnetization hysteresis curve and ESR measurements, a ferrimagnetic to paramagnetic phase transition for the double perovskite Sr2FeMnO6 was observed, and the Neel temperature was determined to be TN ~475K. In paramagnetic phase above TN, the value of gyromagnetic factor was determined to be very close to 2.

Temperature Dependence of Lattice Distortion in Strongly Creep-Deformed Single Crystal Superalloy SC16

Specimens of single crystal superalloy SC16 have been pre-deformed at 1223 K to strain of 15 % under tensile load of 150 MPa. The profiles of the 001 superlattice reflection of the γ’ precipitate phase as well as the profiles of the 002 reflections of the fcc matrix phase and of γ’-precipitate phase have been measured between RT and 1073 K by means of X-ray diffraction (XRD). From these measurements the lattice distortion of the γ’ precipitates and the lattice misfit between γ and γ’ phase have been determined as a function of temperature. The width of the 001 reflection as well as the lattice misfit decreased as a function of temperature. The results are ascribed to the anisotropic arrangement of dislocations at the γ/γ’ interfaces and to the different thermal expansion coefficients of the γ- and γ’-phase. A comparison of lattice misfit with previous measurements on moderately strained specimens suggests that the measured changes in lattice structure are predominantly created in stage I of creep-deformation.

On the crystal structure of Cr2N precipitates in high-nitrogen austenitic stainless steel. III. Neutron diffraction study on the ordered Cr2N superstructure

The ordered structure of Cr2N precipitates in high-nitrogen austenitic steel was investigated utilizing high-resolution neutron powder diffractometry (HRPD). On the basis of the Rietveld refinement of neutron diffraction patterns, the ordered Cr2N superstructure was confirmed to be trigonal (space group P\overline 3 1m), with lattice parameters a = 4.800 (4) and c = 4.472 (5) Å, as suggested in previous transmission electron microscopy studies [Lee, Oh, Han, Lee, Kim & Takaki (2005). Acta Cryst. B61, 137–144; Lee, Kim & Takaki (2006). Acta Cryst. B62, 190–196]. The occupancies of the N atoms in four crystallographic sites [1(a), 1(b), 2(d) and 2(c) Wyckoff sites] were determined to be 1.00 (5), 0.0, 0.74 (9) and 0.12 (3), respectively, reflecting a partial disordering of N atoms along the c axis. The position of the metal atom was specified to be x = 0.346 (8) and z = 0.244 (6), corresponding to a deviation from the ideal position (x = 0.333 and z = 0.250). This deviation caused the (\,{1 \over 3}{1 \over 3}0)-type superlattice reflection to appear. A comparison between the ideal and measured crystal structures of Cr2N was performed using a computer simulation of selected-area diffraction patterns.

Chemical ordering in AlGaN layers grown by MOCVD

ABSTRACTOne of the applications of wurtzite gallium based nitride compounds will be to provide optoelectronic devices from 6.2eV (AlN) to 1.89eV (InN). This will depend on the possibility to grow wurtzite AlGaN and InGaN ternary alloys. As expected, the most difficult region is InGaN due to the large misfit between GaN and InN (= 10%), in this case, ordering, phase separation and growth instabilities have been reported. In the case of AlN and GaN, the misfit is smaller (∼ 2.5%) and one would expect more stable growth. However, it was in this system that ordering along the c axis between AlN and GaN was reported for the first time.In this work, we have found that the growth of AlGaN may be more complicated. Not only the wurtzite lattice can be decreased to simple hexagonal by AlN/GaN ordering along the c axis, but the growth can lead to other types of stackings. Even in the low Al composition range, 10 - 15%, we have found that three processes can operate:1. Ordering into AlN/GaN as two simple hexagonal sublattices.2. The 3:1 ordering which has been recently reported to occur in InGaN.3. A new type of ordering where diffraction experiments (XR and electron diffraction) detect superlattice reflection with a period close to 3 nm. The most adequate model which was found to take this into account shows that, in these growth conditions, the system has preferred to form one AlN cell in between 5 GaN cells, leading to a 5:1 ordering.