Types Of Antiphase Boundaries Research Articles

Regulation of antiphase boundary density in Fe3O4 thin films and its effect on the electrical and magnetic properties

Antiphase boundaries (APBs) widely exist in Fe3O4 thin films and affect significantly their electrical and magnetic properties. Here, effects of APBs on the electrical and magnetic properties of Fe3O4 thin films have been quantitatively investigated by a combined study of pulsed laser deposition, transmission electron microscopy (TEM) and first-principles calculations. It was revealed that the substrates affected greatly the density of APBs in Fe3O4 thin films but did not change the atomic structures of APBs. The APB density in the Fe3O4 (111) thin film grown on the MgO (111) substrate was one order of magnitude larger than that on the Al2O3 (0001) substrate. Atomic-scale TEM characterizations revealed that the APBs in the Fe3O4 thin films on both substrates had three types of atomic structures. Due to the higher APB density in the Fe3O4/MgO thin film, the electrical resistivity at 300 K was two orders of magnitude larger than that of the Fe3O4/Al2O3 thin film. Meanwhile, the saturation magnetization and the coercivity of the Fe3O4/MgO thin film were reduced by 86 % and 35 %, respectively. First-principles calculations suggested that the antiferromagnetic coupling was easily formed at all the three types of APBs, which led to the decrease of spin polarization and the increase of electrical resistance of the Fe3O4 thin films.

Structure and Charge Carrier Separation Promotion Effects of Antiphase Boundaries in Cesium Lead Bromide.

Defects in lead halide perovskites (LHPs) may have a significant impact on charge carrier separation, but the roles of the defects are not fully understood. Here, using aberration-corrected scanning transmission electron microscopy (STEM), different types of antiphase boundaries (APBs) are discovered in CsPbBr3 platelets. APBs with a displacement vector of 1/4[111] are characterized by double layers of CsBr layers at the (110) or (001) planes, while APBs at the (112) planes are formed through edge sharing of PbBr6 ̵octahedra. Significant lattice distortions are determined at (001) and (110) APBs on the basis of quantitative analyses of STEM images. Density functional theory calculations demonstrate that all three types of APBs can induce band offsets at their valence bands and conduction bands. The APBs are intended to promote the separation of photogenerated charge carriers in LHPs. These findings provide a crystal engineering technique for enhancing the optoelectronic properties of LHPs by controlling defects.

Atomic origin of magnetic coupling of antiphase boundaries in magnetite thin films

Revealing the magnetic coupling nature of boundary defects is crucial for in-depth understanding of the behavior and properties of magnetic materials and devices. Here, magnetite (i.e., Fe3O4) thin films were grown epitaxially on (100) SrTiO3 single-crystal substrates by pulsed laser deposition. Atomic-scale scanning transmission electron microscopy characterizations reveal that three types of antiphase boundaries (APBs) are formed in the Fe3O4 thin film. They are the (100) APB that is formed on the (100) plane with a crystal translation of (1/4)a[011¯], the type I and type II (110) APBs that are both formed on the (110) plane with the same crystal translation of (1/4)a[101] but different terminated atomic planes. The type I (110) APB is terminated at the atomic plane with mixed tetrahedral- and octahedral-sites Fe atoms, the type II (110) APB is terminated at the octahedral-site Fe plane. First-principles calculations reveal that the (100) APB and the type I (110) APB tend to form the ferromagnetic coupling that will not decrease the spin polarization of Fe3O4 films, while the type II (110) APB prefers to form the antiferromagnetic coupling that will degrade the magnetic properties. The magnetic coupling modes of the APBs are closely related to the Fe-O-Fe bond angles across the boundaries.

Impact of Antiphase Boundaries on Structural, Magnetic and Vibrational Properties of Fe3Al.

We performed a quantum-mechanical study of the effect of antiphase boundaries (APBs) on structural, magnetic and vibrational properties of FeAl compound. The studied APBs have the {001} crystallographic orientation of their sharp interfaces and they are characterized by a 1/2〈111〉 shift of atomic planes. There are two types of APB interfaces formed by either two adjacent planes of Fe atoms or by two adjacent planes containing both Fe and Al atoms. The averaged APB interface energy is found to be 80 mJ/m and we estimate the APB interface energy of each of the two types of interfaces to be within the range of 40–120 mJ/m. The studied APBs affect local magnetic moments of Fe atoms near the defects, increasing magnetic moments of Fe atoms by as much as 11.8% and reducing those of Fe atoms by up to 4%. When comparing phonons in the FeAl with and without APBs within the harmonic approximation, we find a very strong influence of APBs. In particular, we have found a significant reduction of gap in frequencies that separates phonon modes below 7.9 THz and above 9.2 THz in the defect-free FeAl. All the APBs-induced changes result in a higher free energy, lower entropy and partly also a lower harmonic phonon energy in FeAl with APBs when compared with those in the defect-free bulk FeAl.

Structural-Phase Low-Stability States of BCC-Alloys with APB-Complexes in the Course of an Order-Disorder Phase Transition

Using the Monte Carlo method, the influence of the APB-complexes (a pair of shear-induced APBs along the direction and a pair of thermal APBs along the direction) on the low-stability states of β-brass is investigated during an order – disorder phase transition in the CuZn alloy as an example. It is shown that the formation energy of a complex of thermal APBs is higher than that of a complex of shear APBs. The contribution of APBs into disordering is essential up to the temperature of the order – disorder phase transition. The most significant fact for the long-range order in this system is the appearance of a defect in the form of an APB; the difference in the types of APBs and the plane of their occurrence do not exert a significant influence on the long-range order behavior with the temperature variations. The types of antiphase boundaries do affect the structural-energy characteristics of the system at the temperatures below that of the phase transition. It is obvious that a system with structure defects is less ordered than a defect-free system. The presence of a defect in the form of an APB favors the onset of disordering at lower temperatures: reduction of ordering in the alloy begins in the case of thermal APBs at lower temperatures compared to the case of shear APBs. In the CuZn alloy with a complex of thermal APBs along the direction the first distortions of the structural order invariably appear near the Zn–Zn boundary. In the alloy with a complex of shear-induced APBs along the direction the distortions of the structural order are only observed in the regions where the boundaries intersect. The presence of antiphase boundaries influences the alloy stability during heating. The CuZn alloy without distortions of structural defects is more stable than that with APBs. It is shown that the process of disordering is accompanied by the smearing and faceting of the boundaries.

Electron-Beam-Induced Antiphase Boundary Reconstructions in a ZrO2-LSMO Pillar-Matrix System.

The availability of aberration correctors for the probe-forming lenses makes simultaneous modification and characterization of materials down to atomic scale inside a transmission electron microscopy (TEM) realizable. In this work, we report on the electron-beam-induced reconstructions of three types of antiphase boundaries (APBs) in a probe-aberration-corrected TEM. With the utilization of high-angle annular dark-field scanning transmission electron microscopy (STEM), annular bright-field STEM, and electron energy-loss spectroscopy, the motion of both heavy element Mn and light element O atomic columns under moderate electron beam irradiation are revealed at atomic resolution. Besides, Mn segregated in the APBs was observed to have reduced valence states which can be directly correlated with oxygen loss. Charge states of the APBs are finally discussed on the basis of these experimental results. This study provides support for the design of radiation-engineering solid-oxide fuel cell materials.

Linking Atomic Structure and Local Chemistry at Manganese‐Segregated Antiphase Boundaries in ZrO2–La2/3Sr1/3MnO3 Thin Films

This paper presents direct experimental evidence of Mn segregation at three types of antiphase boundaries in La2/3Sr1/3MnO3 thin films doped with ZrO2. The local atomic structure of these antiphase boundaries is investigated by high‐angle annular dark‐field and annular bright‐field imaging in a scanning transmission electron microscope. Chemical composition and cation valence are determined by electron energy‐loss spectroscopy. The evidence that strain relaxation and ZrMn substitution are driving forces for the formation of the antiphase boundaries is found in this study. Analysis of atomic structure, image contrast, Mn valence state, and Mn occupancy shows that the antiphase boundaries are charge‐neutral with minimized internal electric fields.

Types and configurations of domain walls in ferroelectric Bi4Ti3O12 single crystals

In the ferroelectric Bi4Ti3O12, the major component P s(a) of spontaneous polarization P s lies along the a axis; the component P s(c) along the c axis is small. The two switchable components are expected to make up various types of domain walls (DWs). According to group-theory analysis, 11 permissible types of DWs are predicted to exist theoretically, and they include five types of ferroelectric DWs, one type of anti-phase boundary (APB) and five types of APB-combined ferroelectric DWs. The five types of ferroelectric DWs are P s(a)-90° DWs, P s(a)-180° DWs, P s(c)-180° DWs, P s(a)-90°/P s(c)-180° DWs and P s-180° DWs. In Bi4Ti3O12 single crystals, just the five types of ferroelectric DWs were observed using transmission electron microscopy, with no trace of APBs or APB-combined ferroelectric DWs seen. The P s(a)-90° domains are lamellar and do not have even thickness. Both the P s(a)-90° DWs and P s(a)-90°/P s(c)-180° DWs lie mainly on the (110) plane, but often fold to the (001) plane. The P s(a)-180° domains are predominantly flaky. Both the P s(a)-180° DWs and P s-180° DWs lie mainly on the (001) plane. The P s(c)-180° DWs observed are irregularly curved.

Stability of Edges and Extended Defects on Boron Nitride and Graphene Monolayers: The Role of Chemical Environment

We perform ab initio calculations that indicate that the relative stability of antiphase boundaries (APBs) with armchair and zigzag chiralities in monolayer boron nitride (BN) is strongly dependent on the chemical potentials of boron (B) and nitrogen (N) in the synthesis process. For a realistic interval of chemical potentials, ranging from gas-phase to solid-state sources, a stoichiometric armchair APB with tetragons and octagons in its core and no homopolar bonds (N–N or B–B) is found to be the most stable in the intrinsic region of the chemical-potential interval, while for N-rich (B-rich) environments a zigzag-oriented boundary with a core consisting of hexagons only, containing N–N (B–B) bonds is found to be the most stable. Zigzag boundaries with core structures consisting of pentagons and octagons, containing homopolar bonds, are also considered in both undoped and carbon-doped forms. We find that at the typical temperatures of BN–nanostructure synthesis several of these zigzag boundaries should coexist with the armchair one, under N-rich or B-rich conditions. This stability transition arises from a competition between homopolar-bond and elastic-energy costs in the core of the APBs, in contrast with the case of graphene, where the zigzag geometry with pentagons and octagons is the most stable. We also address the stability of N- and B-terminated edges of BN triangular islands and find a robust stability for N-terminated edges, across a wide range of chemical potential values, in agreement with the experimentally observed tendency of BN islands to display N-terminated zigzag edges. We predict that fine-tuning the chemical potentials with a B-rich source where B atoms are in a bonding environment of low stability may lead to the formation of B-terminated zigzag edges. The electronic structure of the two types of APBs in BN is shown to be particularly distinct, with the zigzag APB depicting defect-like deep electronic bands in the band gap, while the armchair APB shows bulk-like shallow electronic bands.

Structures and Development Mechanism of the Anti-Phase Boundaries in Sr2FeMoO6

High-quality Sr2FeMoO6 (SFMO) was achieved and examined by X-ray analysis via the Rietveld structure refinement. Detail structure of anti-phase boundaries (APBs) in SFMO was investigated by means of high-resolution transmission electron microscopy. We discovered that two types of APBs were simultaneously present inside core-shell grains, wherein high-density maze-type APBs were in the core and swirl-type APBs were in the shell part. It is highlighted that the atom arrangements of structures of both types of APBs are confirmed to be zigzag. Underlying present discovery and the formation mechanism of SFMO, a mechanism concerning development of APBs in SFMO was proposed, which clarifies not only the simultaneous existing of maze- and swirl-type APBs but also their related morphology formation.

Monte Carlo simulation of antiphase boundaries and growth of antiphase domains in Al5Ti3 phase in Al-rich γ-TiAl intermetallics

Predominantly two kinds of antiphase boundaries (APBs) form in Al5Ti3, which is an Al-rich ordered derivative of the γ-TiAl (L10) phase. This phase can be viewed as a periodic arrangement of lean rhombs and squares on the Ti-rich (002) planes of the tetragonal L10. Energies of the two types of APBs were varied in a Monte Carlo simulation by suitably changing the pair interaction parameters. APBs of both types form boundaries of Al5Ti3 antiphase domains (APDs), which coarsen with time. An important observation in this regard is that mostly facetted APBs form at lower ageing temperatures, whereas curved APBs appear to form at relatively higher ageing temperatures. The findings of this work suggest that there exists a critical temperature, akin to the roughening transition temperature for crystals, that marks the transition from facetted to curved APBs.

Occurrence of short-range-ordered incommensurate states in γ-TiAl

This paper examines a lesser-known form of quasiperiodicity in alloys, namely, quasiperiodicity in superlattice structures. Unlike quasicrystals, a quasiperiodic superlattice (QPSL) structure has an underlying periodic lattice and it is the arrangement of the two atomic species on this lattice that makes the superlattice quasiperiodic. Central to the discussion will be a QPSL structure, comprising three different building blocks (tiles), whose approximants are the Al5Ti3 and Al11Ti7 structures in Al-rich γ-TiAl. The three types of tiles represent unimolecular clusters of three different superlattice structures derived from γ-TiAl. Two different types of antiphase boundaries (APBs) occur in Al5Ti3. Their energies have been changed continuously in a Monte Carlo simulation to study transformations, either to the QPSL or to known periodic superlattice structures in Al–Ti. Some of the intermediate stages of transformation are short-range ordered states, which exhibit intensity maxima at incommensurate positions in the reciprocal space and different geometrical patterns of diffuse intensity. Very good agreement has been found in the simulation results, the results of a 4D to 2D projection scheme and TEM observations.

CBED and LACBED: characterization of antiphase boundaries

Convergent-beam electron diffraction (CBED) and large-angle convergent-beam electron diffraction (LACBED) techniques are well adapted to the characterization of several types of crystal defects. In fact, dislocations, grain boundaries and stacking faults have already been successfully characterized with these methods. In the present paper, we describe the CBED and LACBED characterization of another type of crystal defect showing a special interest in materials science: antiphase boundaries (APBs). The first part of the paper is devoted to the determination of the effects of antiphase boundaries on CBED and LACBED patterns that could be expected from a theoretical point of view. It indicates that the superlattice excess lines present on these patterns are split into two lines with equal intensity when the incident beam is located on an APB. In the second part, we experimentally test these theoretical predictions on a specimen showing two different known types of antiphase boundaries. In a third part we indicate how these methods could be used to identify unknown APBs in a specimen. Finally, the advantages and disadvantages of both methods for the characterization of antiphase boundaries are discussed.

Origin of roughening in epitaxial growth of silicon on Si(0 0 1) and Ge(0 0 1) surfaces

It is shown that the formation of (2×1) reconstructed islands during the growth of Si on Si(001) and Ge(001) surfaces at moderate temperatures and the consequent anisotropic diffusion and sticking lead to roughening. As Si islands nucleate and expand, the dimer rows in neighboring rows need not, in general, align with each other. An anti-phase boundary (APB) will be found if two growing islands meet, but their internal dimer rows are not in the same registry. One type of APB runs perpendicular to the substrate rows, whereas the other runs along the substrate rows. As has been pointed out by Hamers et al. [J. Vac. Sci. Technol. A 8 (1990) 195] this first type of APB is a preferential center for nucleation of next layer islands and thus naturally leads to roughening. Here we show that the other type of APB leads, in combination with the anisotropy in sticking and diffusion of dimers, to the formation of long B-type double layer steps and narrow trenches. We argue that it is the kinetic suppression of filling of these trenches and not, as previously suggested the existence of step edge barrier for the double layer step edge that leads to roughening.

Antiphase boundaries on low-energy-ion bombarded Ge(001)

Surface vacancy and adatom clusters have been created on Ge(001) by bombarding the surface with 800 eV argon ions at various substrate temperatures ranging from room temperature to 600 K. The vacancies preferentially annihilate at the ends rather than at the sides of the dimer rows, resulting in monolayer deep vacancy islands which are elongated in a direction of the dimer rows of the upper terrace. As vacancy islands nucleate and expand, the dimer rows in neighbouring vacancy islands need not, in general, align with each other. An antiphase boundary will develop if two growing vacancy islands meet, but their internal dimer rows are not in the same registry. In contrast to Si(001), where only one type of antiphase boundary is found, we have found three different types of antiphase boundaries on Ge(001). Higher dose (> several monolayers) room temperature ion bombardment followed by annealing at temperatures in the range 400–500 K results in a surface which contains a high density of valleys. In addition to the preference for the annihilation of dimer vacancies at descending versus ascending steps we also suggest that the development of antiphase boundaries drives the roughening of this surface. Finally, several atomic rearrangement events, which might be induced by the tunneling process, are observed after low-dose ion bombardment at room temperature.

Epitaxial growth of silicon on Si(001) by scanning tunneling microscopy

The epitaxial growth of silicon on Si(001)-(2×1) substrates at temperatures between 580 and 850 K is studied using scanning tunneling microscopy (STM). The growth is strongly anisotropic, forming long narrow structures only a few dimers wide but more than 100 Å long. Models are proposed for the two types of antiphase boundaries that are observed on the epitaxially grown surfaces.

Long-period structures in the Nb–Ga system

The alloys NbGa3 - x form composition-driven long-period superstructures derived from the D022 by the introduction of non-conservative antiphase boundaries along (001) planes. The displacement vectors of successive antiphase boundaries are alternately ¼ [201] and ¼ [021]. In alloys with a composition which deviates less from NbGa3 than Nb5Ga13 successive antiphase boundaries all have the same displacement vector, either ¼ [201] or ¼ [021]. The alloy Nb5Ga13 is the first material which has a long-period superstructure based on the alternation of two types of antiphase boundaries.

Diffraction from arrays of antiphase boundaries in ordered III-V alloys

Recent electron diffraction studies have found ordered phases in AlxGa1-xAs, GaAsxSb1-x, and InxGa1-xAs alloy systems, and these ordered phases are likely to be found in many other III-V ternary alloys as well. The presence of ordered phases in these alloys was detected in the diffraction patterns through the appearance of superstructure reflections between the Bragg peaks (Fig. 1). The ordered phase observed in the AlxGa1-xAs and InxGa1-xAs systems is of the CuAu-I type, whereas in GaAsxSb1-x this phase and a chalcopyrite type ordered phase can be present simultaneously. The degree of order in these alloys is strongly dependent on the growth conditions, and during the growth of these alloys, high surface mobility of the depositing species is essential for the onset of ordering. Thus, the growth on atomically flat (110) surfaces usually produces much stronger ordering than the growth on (100) surfaces. The degree of order is also affected by the presence of antiphase boundaries (APBs) in the ordered phase. As shown in Fig. 2(a), a perfectly ordered In0.5Ga0.5As structure grown along the <110> direction consists of alternating InAs and GaAs monolayers, but due to local growth fluctuations, two types of APBs can occur: one involves two consecutive InAs monolayers and the other involves two consecutive GaAs monolayers.

Non-periodic antiphase boundaries in Au 3Zn observed by high resolution electron microscopy

The non-periodic antiphase boundaries of the Au 3Zn(H)-structure have been studied by means of high resolution electron microscopic structure imaging. At atomic resolution level two different types of antiphase boundaries were resolved. The antiphase vectors for these boundaries are R = 1 2 [101] and R = 1 2 [112] . The APB's 1 2 [101] are generally inclined slightly from the (100) plane and APB's 1 2 [112] consist of successive stripes lying step by step parallel to the [100] direction.

Distribution of domain sizes during overlayer growth

The distribution of antiphase boundaries created in an overlayer after quenching from a two-dimensional lattice gas phase is described by Markovian disorder. We show that many types of antiphase boundaries with different widths are possible. This one-dimensional distribution is used to describe the growth of oxygen p(2 × 1) antiphase domains along the doubly-spaced direction on a W(112) surface after quenching from a disordered, lattice gas state.