Periodic Antiphase Boundaries Research Articles

Periodic anti-phase boundaries and crystal superstructures in PtCu3 nanoparticles as fuel cell electrocatalysts

To reduce the cost of the electrocatalyst for the oxygen reduction reaction in proton exchange membrane fuel cells, supported metal alloy nanoparticles are commonly used to decrease the amount of the scarce and expensive Pt. Since the structure of the nanoparticles has a crucial influence on the catalytic properties, advanced characterization methods help elucidate these relationships and ultimately enable more rational synthesis. This work focuses on investigating the presence of one particular planar defect, namely periodic anti-phase boundaries in carbon-supported PtCu3 nanoparticles. The studying of this structural phenomenon was approached with several characterization tools. Ex-situ X-ray diffractograms were used in conjunction with computer simulations and Rietveld analyses to reliably determine the modulated unit cell containing anti-phase boundaries, while their temperature-dependent formation and disappearance were observed with high-temperature X-ray powder diffraction. In addition, their presence was also confirmed by electron diffraction and atomically resolved scanning transmission electron microscopy. Furthermore, the average neighborhood of Cu and Pt atoms was confirmed using extended X-ray absorption fine structure. Finally, the electrocatalytic activity for the oxygen reduction reaction for composites with and without anti-phase boundaries was determined using a thin-film rotating disk electrode, and the performance was found to correlate with the degree of alloy ordering but not with the presence of anti-phase boundaries. Overall, this study represents a significant step towards better control over the atomically precise synthesis of advanced functional metallic materials by providing valuable insight into the formation of planar defects in metallic alloy nanoparticles and their impact on the structure-property relationships of electrocatalysts.

Incommensurately Modulated Structures in Zr-rich PZT: Periodic Nanodomains, Reciprocal Configuration, and Nucleation

Zr-rich PZT is one of the most representative materials with compositions located at the ferroelectric–antiferroelectric (FE–AFE) phase boundary. The study of its incommensurately modulated structure (IMS) is of fundamental importance in understanding the origin of its high-performance stored-energy properties and phase transformation between FE and AFE. In this study, the structural features of the IMS were investigated in detail by transmission electron microscopy. The IMS appears as submicron domains assembled by periodic arrays of two-dimensional nanodomains along a direction with the domain width of about 30 {111} spacings. The nanodomains have dual attributes of both antiphase domains and electric domains. The displacement vector across the periodic antiphase boundaries was determined to be R = [001]. The reciprocal lattice of IMS was constructed and characterized by a set of strong basic reflections of pseudocubic unit cell together with 1/2{ooe} superlattice reflections where 1/2(ooe) superl...

Antiphase Boundaries in the Turbostratically Disordered Misfit Compound (BiSe)(1+δ)NbSe2.

(BiSe)(1+δ)NbSe2 ferecrystals were synthesized in order to determine whether structural modulation in BiSe layers, characterized by periodic antiphase boundaries and Bi-Bi bonding, occurs. Specular X-ray diffraction revealed the formation of the desired compound with a c-axis lattice parameter of 1.21 nm from precursors with a range of initial compositions and initial periodicities. In-plane X-ray diffraction scans could be indexed as hk0 reflections of the constituents, with a rectangular basal BiSe lattice and a trigonal basal NbSe2 lattice. Electron micrographs showed extensive turbostratic disorder in the samples and the presence of periodic antiphase boundaries (approximately 1.5 nm periodicity) in BiSe layers oriented with the [110] direction parallel to the zone axis of the microscope. This indicates that the structural modulation in the BiSe layers is not due to coherency strain resulting from commensurate in-plane lattices. Electrical transport measurements indicate that holes are the dominant charge carrying species, that there is a weak decrease in resistivity as temperature decreases, and that minimal charge transfer occurs from the BiSe to NbSe2 layers. This is consistent with the lack of charge transfer from the BiX to the TX2 layers reported in misfit layer compounds where antiphase boundaries were observed. This suggests that electronic considerations, i.e., localization of electrons in the Bi-Bi pairs at the antiphase boundaries, play a dominant role in stabilizing the structural modulation.

Special features of the long-range order – short-range order phase transition in FCC-based alloys

Results of x-ray analysis of the long-range order in FCC-based alloys with L12, L12(M), L12(MM), and D1a superstructures are generalized. Regularities of the behavior of the long-range order parameter and average size of the thermal and periodic antiphase boundaries (APB) are traced, main mechanisms of temperature order-disorder phase transition are elucidated, and influence of the thermal and periodic APB on the special features of the order-disorder phase transition is revealed. The character of change of the periodic antiphase boundary density determines the special features of the order-disorder phase transition in alloys with longperiod superstructures.

Nanocheckerboard modulations in (NaNd)(MgW)O6

Data is presented for a complex structural and compositional modulation in the perovskite (NaNd)(MgW)O6. This modulation creates a large 14ap×14ap×2ap supercell (ap≈3.9 Å is the lattice parameter of the cubic perovskite aristotype) containing ordered regions with doubled (110) d-spacings in the a-b plane separated by two-dimensional periodic antiphase boundaries and accompanied by a nanocheckerboard pattern. Faint periodic modulations in Z-contrast images suggest an associated periodic variation in composition. The presence of a sodium rich impurity implies the composition of the stable perovskite is nonstoichiometric.

Periodic antiphase boundary spinel-based structures in aluminates, gallates, aluminium oxynitrides and transition aluminas

The crystallographic structures of several transition aluminas possessing a face-centred cubic packing of oxygen anions can be considered as deriving from that of a non-stoichiometric spinel. However, they are not yet known in detail owing to the poor crystallinity of most of the samples studied by X-ray or electron diffraction. Conversely, relatively large crystals have been produced in the case of non-stoichiometric spinels in the alumina-rich (or Ga2O3-rich) parts of the Al2O3–AlN, Al2O3–MgO, Al2O3–NiO, Al2O3–Li2O and Ga2O3–MgO systems. Detailed studies of their diffraction patterns have shown that all these phases possess periodic antiphase boundary (PAPB) structures based on the spinel structure. In the case of the so-called δ-transition aluminas, various structural models have been previously proposed and in this paper we are focusing on the striking similarities between their diffraction patterns and those of several metastable PAPB aluminate structures. This makes it possible to show that at least three distinct PAPB structures must be taken into account in the case of δ-transition aluminas. APB planes are either {100} or {110} whereas APB vectors are either 1/2 ⟨001⟩ or 1/4 ⟨110⟩ when referring to the spinel structure. Neighbouring octahedral and tetrahedral incompatible sites are observed in the vicinity of each APB and cation vacancies are shown to occupy these octahedral sites.

The crystallographic structure of λ transition-alumina compared with the periodic antiphase boundary structure of a Ga2O3-rich Mg gallate

One of the high-temperature metastable phases found in the MgGa2O4–Ga2O3 system is the εMg non-stoichiometric Mg gallate. It has a one-dimensional periodic antiphase boundary (PAPB) structure based on the spinel structure. The APBs are parallel to the {3 1 0} planes and the APB vectors are of the 1/4⟨1 1 0⟩ type when referred to the spinel structure. This generally leads to the existence of 12 monoclinic twin variants in the studied samples. By comparing the X-ray or electronic diffraction patterns of both phases, it will be shown that the recently discovered λ transition-alumina has the same type of structure.

A TEM and neutron diffraction study of the local structure in the rhombohedral phase of lead zirconate titanate

Transmission electron microscopy and neutron diffraction have been used to characterize ceramics and single crystals from the rhombohedral region of the (x = 0.06-0.45) phase diagram. Electron diffraction patterns showed the existence of superlattice reflections of the type , where h = k = l, and , which are not observed by neutron powder diffraction. The analysis of these reflections also revealed satellite spots around the , which are associated with periodic antiphase boundaries. The origin of these superlattice reflections is explained by the existence of local regions presenting antiparallel cation displacements, and models for this are suggested

Novel ordering transitions in off-stoichiometric γ-TiAl intermetallics and the similarities between these and the special point ordering processes in Ni-Mo alloys

Ordering on Ti-rich (002) planes of Al-rich γ-TiAl (Ll0) phase can be described in terms of evolution of superlattice structures consisting of space filling arrangements of two-dimensional (2D) motifs of different shapes. These 2D structural units (tiles) comprising unimolecular clusters of Ti2Al, Ti3Al and Ti4Al stoichiometries and having the shapes of lean and fat rhombi and squares respectively, either singly or in combinations, constitute the building blocks of this variety of superlattice structures. The Al5Ti3 and the Al11Ti7 derivatives of the off-stoichiometric γ-TiAl phase, for instance, can be viewed as identical periodic arrangements of lean rhombi and squares and fat rhombi and squares, respectively. Previous investigations, involving projections from higher dimensional lattices, have shown that these two rather simple structures can be regarded as “unmodulated” periodic approximants of a hypothetical quasiperiodic superlattice (QPSL) structure and that these can transform continuously into the QPSL through an infinitum of long period antiphase boundary (LPAPB) modulated structures. Structures, which identify themselves with some of the intermediate stages of the periodic to quasiperiodic transformation, have been observed in some investigations including the present one. Periodic arrangements of the three types of tiles have also been used to describe the nature of Short Range Order (SRO) and various Long Range Ordered (LRO) phases in Ni-Mo and other f.c.c. alloys which show SRO maxima at 1120 and equivalent points in the reciprocal space. A remarkably similar state of SRO, albeit, with SRO maxima at incommensurate positions, has been observed in this transmission electron microscopy investigation on rapidly solidified Al60Ti40 and Al65Ti35 alloys. Occurrence of curved streaks of diffuse intensity during the early stages of the evolution of some of the Long Range Ordered (LRO) phases is another point which underscores the similarity between the two ordering systems. The results of the present investigation lend additional support to the hypothesis of continuous periodic to quasiperiodic superlattice transformations.

Bimetallic PdCu and PdCu3Particles Prepared by Wet Impregnation-HRTEM Study of the Structure and the Interface with the MgO Substrate

Bimetallic particles of PdCu and PdCu3 are prepared by decomposition of orga- nometallic compounds on MgO micro-cubes. During the annealing between 350 and 400 °C, in reducing atmosphere, PdCu particles grow with the fcc structure, without defined shape. The particles epitaxially (001) oriented on MgO are dilated and accommodated to the substrate. After annealing at 450 ° C, the particles adopt the 03B2 ordered structure. The particles are limited at the edges by (100) and (110) faces. Most of them are oriented (001) on MgO with (110)PdCu ~ (100)MgO, which corresponds to a perfect accommodation of the lattices without deformation compared to the bulk, despite of the reduction in H2. The PdCu3 particles smaller than about 10 nm were found with the 03B1 ordered structure, without periodic anti-phase boundaries.

On the nature of L1 0 ordering in equiatomic AuNi and AuCu thin films grown on Au(001)

The L1 0 ordering of thin epitaxial films having a (001) surface normal subject to elastic constraints imposed by a similarly oriented substrate has been investigated both experimentally and theoretically. Thin AuNi films grown by MBE at room temperature on Au(001) by means of the alternating deposition of Au and Ni are found to possess a L1 0 structure free of periodic antiphase boundaries when growth in controlled in such a way as to ensure that the quantity of Au or Ni deposited is almost exactly equal to one monolayer. If such control is not exercised during growth, a structure having periodic antiphase boundaries is formed. This behaviour stands in contrast to that of AuCu during room temperature MBE growth on Au(001), where a strongly ordered L1 0 structure free of antiphase boundaries is formed even on the codeposition of Au and Cu. The effect of elastic constraints on the state of order in an alloy film which undergoes an L1 0 order-disorder transition is examined as a function of temperature, lattice mismatch, and film thickness within the context of a model which allows for the introduction of dislocations in order to relieve misfit strain. Calculations are performed in detail for the case of AuCu, where particular attention is paid to the coupling between film thickness, the number of misfit dislocations present at equilibrium, and the state of order.

Quantitative analysis of periodic antiphase boundaries by lattice image simulations

Periodic antiphase boundaries in the intermetallic compound Cu3Pd have been studied by high resolution electron microscopy and image simulation. The intensity profiles across the boundary were recorded and the thickness of the boundary compared to the values deduced from X-ray and neutron diffraction data. It is shown that electron microscopy always minimizes the thickness of the boundary. The possible factors for this discrepancy are analysed and discussed.

Annihilation Process of Periodic Antiphase Boundaries in Cu<SUB>3</SUB>Pt Studied with Time Dependent Ginzburg Landau Model

Kinetics of the annihilation process of periodic antiphase boundaries in a Cu 3 Pt alloy has been studied by computer-simulation and transmission electron microscope observation. The kinetic equation was derived from a general equation for the free energy of the system where the ordering and disordering undergo congruently. The excess free energy due to ordering was expressed by a Ginzburg-Landau type expansion, and the boundary energy was assumed to be proportional to the second derivatives of S r and S r 2 , where S r is the local degree of order at the position r in the system

X-ray and electron diffraction study of sartorite. A periodic antiphase boundary structure and polymorphism.

A number of sartorite specimens from Binnathal have been examined by single-crystal X-ray and electron diffractions. The substructure reflections of any crystal picked out from those specimens are consistent with the space group Pbnm and have the subcell with a∼19.6, b∼7.9 and c∼4.2A as reported to date. The reported peculiar mode in diffraction-intensity distribution that it varies from one crystal to another or even from one portion to another in the same crystal has also been observed both in the single-crystal X-ray diffraction and in the selected-area electron diffraction. The mode of intensity distribution of the superstructure reflections, however, has been found to have two distinct types, one has a monoclinic symmetry and the other an orthorhombic symmetry. Crystalline phases showing the former type have been denoted ‘clinosartorite’ and those showing the latter ‘orthosartorite’. In addition to them there are a variety of disordered types as well. The complexity in diffraction patterns can be interpreted essentially as composites of the patterns from the coexisting two distinct types (clinosartorite is frequently twinned). The monoclinic phase has features suggesting a periodic antiphase boundary structure (APBs) with two-dimensional structural modulation. The relationship between the new monoclinic true cell, in which cell edges are chosen in accordance with the structural features, and the conventional cell is described as: aNR=3×aCS–11×cCS, bNR=bCS, cNR=11×cCS, where suffixes NR and CS indicate the newly-derived true cell and the conventional subcell, respectively. On the other hand, ‘orthosartorite’ has the same three cell edges as those reported for the conventional true cell, namely, 3×aCS, bCS, and 11×cCS. The periodic antiphasing and expected modulation of the structure would be rationalized, in terms of a mechanism of reducing lattice strain that may occur owing to the dimensional misfit between Pb–S and As–S links in the structure.

Simulation of the Influence of External Conditions on Long-Period Ordered Phases. Influence of Temperature

The influence of external conditions on the characteristics of long-period structures (LPS) is considered: the temperature influence on the stabilization of the ordered structure with periodic antiphase boundaries is taken as an example. The study is carried out in terms of the phenomenological model which associates long-period phase formation with the cooperative behaviour of atoms in planes with different types of filling. A possibility is found to form a long-period phase in the temperature range in the vicinity of Tc. Calculations of the characteristics of the phase formed and the peculiarities of the X-ray diffraction patterns are made. The possibility to describe the formation and behaviour of a long-period ordered phase in terms of the same model is shown. [Russian Text Ignore]

High-resolution structure imaging of stacking sequences in Y2Fe17Cx(0·6x1·6) magnetic material

Abstract High-resolution electron microscopy has been used to investigate planar crystal defects in magnetic materials of the type Y2Fe17C x (x0·6), which have the rhombohedral Th2Zn17 type of structure. The basal plane stacking sequences are imaged in the [100] orientation. Edge-on and inclined coherent twin boundaries are observed, the latter giving rise to a characteristic coincidence site pattern. The high-resolution images further reveal a high density of conservative and non-conservative antiphase boundaries, which can mutually combine into an antiphase boundary cross-link. Non-conservative antiphase boundaries are also seen to make cross-links between successive twin boundaries. In local areas, a periodic antiphase boundary structure with Y3Fe29C x , stoichiometry is observed. Y2Fe17C x compounds resulting from an annealing treatment at low temperature exhibit an order-disorder phenomenon, based upon the rhombohedral structure. High-resolution structure imaging allows us to confirm the microdomain model that was previously derived on the basis of electron diffraction results only. However, some particular restrictions on the disordered stacking are further derived from the high-resolution images. In Gd2Fe17 compounds without C, an identical order-disorder phenomenon is observed depending on the annealing temperature. The relation of the observed defects with respect to coercivity mechanisms and magnetic anisotropy is discussed.

Formation process of periodic non-conservative antiphase boundary structure in L-Pd5Ce

The low temperature phase of Pd5Ce (L-Pd5Ce) has a one-dimensional long period superstructure (1D-LPS) derived from Ll2. The periodic antiphase boundaries (APBs) are parallel to (110) planes and have a shift vector of 1/2[110]. Hereafter, the indices are referred to the basic lattices of Ll2 As insertion of the APB causes a change in composition, such an APB is called “non-conservative”. Then, a domain size M depends upon the Ce concentration in the alloy. It was found that M increases also with temperature. The temperature dependency of M is attributed to a change of the degree of order within the antiphase domains. In this work, morphology of the non-conservative APBs is observed to clarify the formation process of the 1D-LPS.The alloy of Pd-16.7 at%Ce was prepared by arc melting in argon atmosphere. Disc specimens made from the alloy ingot were first held at 985 K for 260 ks and quenched in iced water to obtain the state of M=∞ or Ll2, followed by annealing for various lengths of time. The annealing temperature was 873 K where the equilibrium value for M is about 3 in unit of (110) lattice spacing of Ll2. Observation was carried out using microscopes JEM-2000FX, JEM-4000EX (HVEM Lab., Kyushu Univ.) and JEM-2000EX (Dept. of Mater. Sci. Tech., Kyushu Univ.).

Modulated structure of Laves phase in a cast Mg-Zn-RE alloy

Comparison of the two SAED patterns (Fig.l) obtained from Laves phase in Mg-9Zn and Mg- 8Zn-1.5RE (RE: rare earth additions made by adding misch metal), shows that the small amount of rare earth additions has a considerble influence on the crystal structure of this phase. The diffraction features suggest that the crystal structure of the Laves phase in the Mg-Zn-RE alloy is modulated by planar translation interfaces which are equivalent to a uniform sequence of antiphase boundaries (APBs) in a long period superstructure. From the splitting distance of the superlattice spots as illustrated in the schematic diagram in Fig. 1(c), the mean half period, which is measured in units of (100) interplanar spacings, is M ≈ 4.86. As shown by Van Dyck et al. the superstructure reflections (or the satellites of the basic spots) of a periodic APB structure are peaked at positions given by: g' = g - (1/d)(m - g.R),

Electron Microscopy of Ordering Related Defects in Alloys.

AbstractRecent results are presented which indicate that the core structure of anti-phase boundaries is sensitive to changes in temperature and alloy composition. Isolated anti-phase boundaries in CoPt3 are found to undergo a structural change when the order-disorder transition temperature is approached from below. Periodic anti-phase boundaries of the long period structures in Cu3Pd, Au3Zn and Au3Cu exhibit similar phenomena. Detailed results on the core structure of periodic boundaries in alloys annealed close to the order-disorder transition are compared to the results of a Monte Carlo analysis for finite temperatures of (100) anti-phase boundaries in L12 alloys.

Structural Studies on the ZrO2–Y2O3 System by Electron Diffraction and Electron Microscopy III

The oxygen-ion displacements in modulated structures for 10–50 mol% Y2O3–ZrO2 can be assigned in terms of the frozen in distortions of the molecular vibrational modes of an oxygen ion cube. This assignment clarifies that the displacement of the 10 mol% Y2O3–ZrO2 structure type (a deformed fluorite structure) decreases and that of the Y2O3 structure type increases as the amount of Y2O3 increases. The displacement of the Y2O3 type already occupies 75% in 20 mol% Y2O3–ZrO2 and 97% in 50 mol% Y2O3–ZrO2 in the microdomains of modulated structures. The decrease of the ionic conductivity with the content of Y2O3 can be attributed to an increase in the volume of microdomains with the Y2O3-type displacement. The arc-shaped diffuse scattering observed in 20 and 40 mol% Y2O3–ZrO2 originates from the presence of various orientations and different intervals of the periodic antiphase boundaries.