Antisite Point Defects Research Articles

Atomic-Scale Direct Observation on Cation Antisite Intermixing and Identification of Charge Conduction Mechanism in p-Type CuBi2O4 for Photocathodes

Cu-based p-type semiconducting oxides have been investigated for their potential as water-reduction photocathodes in order to enhance energy conversion efficiency in photoelectrochemical cells. Among these oxides, CuBi2O4 has recently gained significant attention as a promising candidate for converting light energy into electrochemical reactions because of its good light absorption property with an adequate bandgap. Despite the importance of identifying the major defect structure to comprehend the electronic conduction behavior, however, no direct experimental analysis has been suggested yet. While the formation of Cu cation vacancies may be discussed as a possible explanation of the p-type conduction behavior, direct evidence regarding the atomic-scale structural and chemical characteristics is still required.In this research, a combination of advanced characterization techniques is employed to figure out a representative defect structure affecting the optoelectronic properties in CuBi2O4. Cs-corrected scanning transmission electron microscopy (STEM) analysis is conducted to identify a noteworthy occurrence of antisite point defects. Atomic-level energy-dispersive X-ray spectroscopy (EDS) analysis is also employed simultaneously, to clearly show the intermixing of BiCu-CuBi antisite cations as a crucial type of point defect in CuBi2O4. Additionally, a combined result of optical and electrical measurement and density functional theory (DFT) calculations demonstrates the presence of local Cu 3d polaron states in the bandgap. A new type of charge conduction mechanism is created by the hopping of hole-polarons between Cu sites, but it is seriously hindered by the BiCu cations occupying Cu sites. A higher degree of intermixing leads to a greater activation energy and decreased electronic conductivity due to the hindrance of hole-polaron hopping, as evidenced in the Arrhenius plot.Overall, this research provides valuable insights into the role of antisite defects in CuBi2O4 and their impact on its potential as a photocathode material. The results highlight the significance of the combination of atomic-scale structure analysis and theoretical calculations to identify the possible existence of defects and gain a comprehensive understanding of the electric characteristics in complex oxides for (photo)electrocatalysis. By understanding how antisite defects affect electronic conductivity, various strategies can be developed by controlling the possible defects to optimize not only CuBi2O4 but also other complex oxides for energy-harvesting applications. Figure 1

Si‐Doping of Low‐Temperature‐Grown GaAs Heterostructures on (100) and (111)A GaAs Substrates

Utilizing the amphoteric property of silicon impurity in GaAs (111)A for acceptor doping of low‐temperature‐grown (LTG‐) GaAs films and LTG‐GaAs‐based heterostructures for THz photoconductive applications is proposed and realized. The electronic properties of superlattice structures {LTG‐GaAs/GaAs:Si} grown by molecular beam epitaxy on (100) and (111)A GaAs substrates are experimentally investigated, where GaAs:Si are silicon‐doped GaAs layers grown at standard conditions. The use of GaAs substrates with surface orientation (111)A results in spatially indirect p‐type doping of LTG‐GaAs layers. In addition, the charge redistribution at the LTG‐GaAs/GaAs:Si interfaces, due to the presence of silicon atoms in GaAs:Si layers and antisite point defects in adjacent LTG‐GaAs layers, is also analyzed by means of band structure modeling.

Low-energy irradiation induced giant quasilinear superelasticity over wide temperature range in NiTi shape memory alloys

Continuous strain glass transition (STGT) in shape memory alloys (SMAs) has attracted much attention and shows potential applications in biomedical, robotics, and micro-electromechanical systems due to its quasilinear superelasticity (SE). However, the reported strain glass system in NiTi alloys through doping antisite point defects can only produce local $R$-phase martensitic domains and show small recoverable strain ($\ensuremath{\sim}1%$), which limits its wide applications. Here, we propose a method to design B19\ensuremath{'} strain glass with large recoverable strain in NiTi binary alloys by introducing interstitial atoms and vacancies through low-energy irradiation by integrating molecular dynamics and phase field modeling. The interstitial atoms play the most important role to transform the normal martensitic transformation (MT) to STGT. A complete phase diagram is established to describe the relationship between MT/STGT and irradiation energy. The system after large irradiation energy $(\ensuremath{\sim}5.3\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.16em}{0ex}}\mathrm{keV}/\mathrm{c}{\mathrm{m}}^{2})$ has shown obvious frequency dependence of storage modulus, continuous volume fraction change, and B19\ensuremath{'} martensitic nanodomains, which confirm the existence of B19\ensuremath{'} STGT. This B19\ensuremath{'} strain glass has shown large recoverable strain $(\ensuremath{\sim}5.8%)$ over a wide temperature range (from 100 to 300 K), which can be attributed to the continuous nucleation and growth of martensitic nanodomains in this temperature range. Our calculations theoretically proposed a method to design strain glass systems with giant quasilinear SE by interstitial defects and may stimulate the application of SMAs.

Characterization of Cu-rich and Zn-poor Cu2ZnSnS4 single crystal grown by vertical Bridgman technique

To date, although a number of studies have focused on understanding the fundamental properties of Cu-poor/Zn-rich Cu2ZnSnS4 (CZTS) single crystals, little attention has been paid to investigate the physical properties of those with a Cu-rich / Zn-poor chemical composition. Therefore, in this study, the structural and electrical properties of Cu-rich/Zn-poor CZTS single crystal grown by the Bridgman technique have been studied in order to fill this gap in literature. XRD and Raman findings confirm the growth of CZTS single crystal having a kesterite phase without secondary phases. Hall and temperature dependent conductivity measurements reveal the presence of VCu and CuZn antisite point defects in CZTS during the growth stage with thermal activation energies of 15 meV and ∼100 meV, respectively. Hall mobility, resistivity and hole carrier concentration values at room temperature were found to be 1.2 cm2/V.s, 16.2 Ω.cm and 3.1 × 1017 cm−3, respectively.

Direct measurements of space-charge-regions in individual nanometer-scale crystals of Al2O3-rich magnesium-aluminate-spinel using off-axis electron holography and electron energy-loss spectroscopy

Space charge regions (SCR) in ionic solids, a result of an uneven distribution of point defects, alter their thermodynamic, transport, mechanical, and optical properties.We examine the formation of SCR in nanometer-scale sized crystals of magnesium-aluminate-spinel with excess Al2O3, MgO·1.27Al2O3. For this composition, the expected dominant point defects are anti-sites and cation vacancies.We determine the types, distribution and formation energies of charged point defects that form the SCR. Off-axis electron holography and electron energy-loss spectroscopy enabled to measure the electrostatic potential and distribution of Al·Mg anti-site charged point defects, respectively in individual crystals.We extract the width of the SCR, LSCR, between 1.7 and 2.1 nm. We measured the formation energy of charged vacancies following two extreme states, only aluminum or magnesium vacancies resulting in EfV″′Al=1.09±0.16eV and EfV″Mg=1.04±0.19eV, respectively.For crystals considerably larger than LSCR, V'''Al and VʺMg cation vacancies are the majority species at the particle core resulting in a negative charge density. When approaching the surface of the crystal, the space charge region is richer in Al·Mg defects.Finally, we examine the SCR as the crystal size is reduced, approaching LSCR. Measurements at particle cores of these crystals results in a positive charge due to the overlap of the SCR.

First Principles Study on the Thermodynamic and Elastic Mechanical Stability of Mg2X (X = Si,Ge) Intermetallics with (anti) Vacancy Point Defects

In this paper, based on the density functional theory, through thermodynamic and mechanical stability criteria, the crystal cell model of intermetallic compounds with vacancy and anti-site point defects is constructed and the lattice constant, formation heat, binding energy, elastic constant, and elastic modulus of Mg2X (X = Si, Ge) intermetallics with or without point defects are calculated. The results show that the difference in the atomic radius leads to the instability and distortion of crystal cells with point defects; Mg2X are easier to form vacancy defects than anti-site defects on the X (X = Si, Ge) lattice site, and form anti-site defects on the Mg lattice site. Generally, the point defect is more likely to appear at the Mg position than at the Si or Ge position. Among the four kinds of point defects, the anti-site defect x M g is the easiest to form. The structure of intermetallics without defects is more stable than that with defects, and the structure of the intermetallics with point defects at the Mg position is more stable than that at the Si/Ge position. The anti-site and vacancy defects will reduce the material’s resistance to volume deformation shear strain, and positive elastic deformation, and increase the mechanical instability of the elastic deformation of the material. Compared with the anti-site point defect, the void point defect can lead to the mechanical instability of the transverse deformation of the material and improve the plasticity of the material. The research in this paper is helpful for the analysis of the mechanical stability of the elastic deformation of Mg2X (X = Si, Ge) intermetallics under the service condition that it is easy to produce vacancy and anti-site defects.

Pervasive Cation Vacancies and Antisite Defects in Copper Indium Diselenide (CuInSe2) Nanocrystals

Copper indium diselenide (CuInSe2) is a prototype ternary compound and group I–III–VI semiconductor with useful optoelectronic properties. CuInSe2 nanocrystals have been of significant interest because of their size-tunable optical properties and lack of toxic heavy metals. Because of the particular vacancy and antisite substitutional point defects in CuInSe2, large stoichiometric deviations can be tolerated, sometimes leading to the so-called ordered vacancy compounds (OVCs). Here, we use Raman spectroscopy of oleylamine-capped CuInSe2 nanocrystals and ab initio lattice dynamics modeling to study the concentration and arrangements of (2vCu– + InCu2+) defect pairs in the nanocrystals. The nanocrystals have randomly distributed defect pairs that become mobile under light excitation and accumulate, as in OVCs, along the [100] direction. Because the high concentration of vacancies in CuInSe2 nanocrystals is compensated by InCu2+ antisite defects, these nanocrystals do not exhibit an optical plasmon resonance...

Domain structures and Prco antisite point defects in double-perovskite PrBaCo2O5+δ and PrBa0.8Ca0.2Co2O5+δ

Owing to the excellent mixed-ionic and electronic conductivity and fast oxygen kinetics at reduced temperature (<800 °C), double-perovskite oxides such as PrBaCo2O5+δ exhibit excellent properties as an oxygen electrode for solid oxide fuel cells (SOFCs). Using transmission electron microscopy (TEM), we revealed high-density antiphase domain boundaries (APBs) and 90° domain walls in PrBaCo2O5+δ grains. Besides the regular lamellar 90° domain walls in {021} planes, irregular fine 90° domains are attached to the curved APBs. Electron energy-loss spectroscopy (EELS) reveals the composition variation across some of the 90° domain walls. There are fewer Co and more Ba ions approaching the 90° domain walls, while the changes in Pr and O ions are not detectable. We assume that the extra Ba2+ cations replace the Pr3+ cations, while the Pr3+ cations go to the Co site to form PrCo antisite point defects and become Pr4+. In this case, the Pr4+ cations will help to balance the local charges and have compatible ionic radius with that of Co3+. The local strain field around the 90° domain walls play a crucial role in the stabilization of such PrCo antisite point defects. The antisite point defects have been observed in our high-resolution TEM images and aberration-corrected high-angle annular dark-field (HAADF) scanning TEM images. After Ca2+ doped into PrBaCo2O5+δ to improve the structure stability, we observed tweed structures in the PrBa0.8Ca0.2Co2O5+δ grain. The tweed structure is composed of high-density intersected needle-shaped 90° domain walls, which is linked to a strong local strain field and composition variation. Even when the temperature is increased to 750 °C, the domain structures are still stable as revealed by our in situ TEM investigation. Therefore, the influence of the domain structures and the PrCo antisite defects on the ionic and electric conductivities must be considered.

Prediction on elastic and thermal properties of defective L12–Al3Li intermetallics

The effects of Al vacancy, Li vacancy, Al anti-site and Li anti-site point defects on the elastic and thermal properties of L12–Al3Li intermetallic compound were investigated by the first-principle pseudopotential plane-wave calculations. The results show that Li anti-site and Al anti-site can easily form in L12–Al3Li. Li vacancy slightly increases hardness and the Debye temperature, whereas Li anti-site, Al anti-site, and Al vacancy impose the opposite influence. All these point defects have no obvious impact on the brittleness of L12–Al3Li. Under the Debye temperature, the specific heat of L12–Al3Li weakly decreases with the content of Li vacancy, but increases with those of Li anti-site, Li anti-site, Al anti-site, and Al vacancy.

Effect of anti-site point defects on the mechanical and thermodynamic properties of MgZn2, MgCu2 Laves phases: A first-principle study

A theoretical investigation on the effect of anti-site point defects on mechanical and thermodynamic properties of MgZn2 and MgCu2 based on the first-principles calculations has been implemented. The results show that Mg anti-site defect on Zn or Cu site plays a reinforcing role, while exhibiting a more brittle behavior. However, the defect phase of Cu anti-sites on Mg sublattice shows a ductile tendency. The temperature-dependent thermodynamic properties are also predicted along with Debye-Grüneisen model including Debye temperature, thermal expansion coefficient and vibrational heat capacity as well as vibrational entropy. Taking into account the contribution of the lattice vibration and thermal electronic excitations, the Helmholtz free energy F can be calculated, suggests that anti-site defects caused by the occupancy of Mg at the Zn or Cu site are thermodynamically unstable compared to the defect-free phases. In addition, the total amount of charge transfer, the overall and the local difference charge densities are further discussed to analyze the mechanism of mechanical properties.

Deciphering chemical order/disorder and material properties at the single-atom level.

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling 'real' materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily on average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. This work combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure-property relationships at the fundamental level.

Intrinsic "Vacancy Point Defect" Induced Electrochemiluminescence from Coreless Supertetrahedral Chalcogenide Nanocluster.

A deep understanding of distinct functional differences of various defects in semiconductor materials is conducive to effectively control and rationally tune defect-induced functionalities. However, such research goals remain a substantial challenge due to great difficulties in identifying the defect types and distinguishing their own roles, especially when various defects coexist in bulk or nanoscale material. Hereby, we subtly selected a molecular-type semiconductor material as structural mode composed of supertetrahedral chalcogenide Cd-In-S nanoclusters (NCs) with intrinsic vacancy point defect at the core site and antisite point defects at the surface of supertetrahedron and successfully established the correlation of those point defects with their own electrochemiluminescence (ECL) behaviors. The multichannel ECL properties were recorded, and the corresponding reaction mechanisms were also proposed. The predominant radiation recombination path of ECL emission peak at 585 nm was significantly distinguished from asymmetrically broad PL emission with a peak at 490 nm. In addition, the ECL performance of the coreless supertetrahedral chalcogenide nanocluster can be modulated by atomically precise doping of monomanganese ion at the core vacant site. A relatively high ECL efficiency of 2.1% was also gained. Actually, this is the first investigation of ECL behavior of semiconductor materials based on supertetrahedral chalcogenide nanocluster in aqueous solution. Current research may open up a new avenue to probe the roles of various different defects with defined composition and position in the NC. The versatile and bright ECL properties of Cd-In-S NC combined with tunable ECL potential and ECL peak suggest that the new kind of NC-based ECL material may hold great promising for its potential applications in electrochemical analysis, sensing, and imaging.

Bandgap Control of the Oxygen‐Vacancy‐Induced Two‐Dimensional Electron Gas in SrTiO3

We report very large bandgap enhancement in SrTiO3 (STO) films (fabricated by pulsed laser deposition below 800 {\deg}C), which can be up to 20% greater than the bulk value, depending on the deposition temperature. The origin is comprehensively investigated and finally attributed to Sr/Ti antisite point defects, supported by density functional theory calculations. More importantly, the bandgap enhancement can be utilized to tailor the electronic and magnetic phases of the two-dimensional electron gas (2DEG) in STO-based interface systems. For example, the oxygen-vacancy-induced 2DEG (2DEG-V) at the interface between amorphous LaAlO3 and STO films is more localized and the ferromagnetic order in the STO-film-based 2DEG-V can be clearly seen from low-temperature magnetotransport measurements. This opens an attractive path to tailor electronic, magnetic and optical properties of STO-based oxide interface systems under intensive focus in the oxide electronics community. Meanwhile, our study provides key insight into the origin of the fundamental issue that STO films are difficult to be doped into the fully metallic state by oxygen vacancies.

Thermodynamic stability of Co–Al–W L12γ′

Co-based superalloys in the Co–Al–W system exhibit coherent L12 Co3(Al,W) γ′ precipitates in an face-centered cubic Co γ matrix, analogous to Ni3Al in Ni-based systems. Unlike Ni3Al, however, experimental observations of Co3(Al,W) suggest that it is not a stable phase at 1173K. Here, we perform an extensive series of density functional theory (DFT) calculations of the γ′ Co3(Al,W) phase stability, including point defect energetics and finite-temperature contributions. We first confirm and extend previous DFT calculations of the metastability of L12 Co3(Al0.5W0.5) γ′ at 0K with respect to hexagonal close-packed Co, B2 CoAl and D019 Co3W using the special quasi-random structure (SQS) approach to describe the Al/W solid solution, employing several exchange/correlation functionals, structures with varying degrees of disorder, and newly developed larger SQSs. We expand the validity of this conclusion by considering the formation of antisite and vacancy point defects, predicting defect formation energies similar in magnitude to Ni3Al. However, in contrast to the Ni3Al system, we find that substituting Co on Al sites is thermodynamically favorable at 0K, consistent with experimental observation of Co excess and Al deficiency in γ′ with respect to the Co3(Al0.5W0.5) composition. Lastly, we consider vibrational, electronic and magnetic contributions to the free energy, finding that they promote the stability of γ′, making the phase thermodynamically competitive with the convex hull at elevated temperature. Surprisingly, this is due to the relatively small finite-temperature contributions of one of the γ′ decomposition products, B2 CoAl, effectively destabilizing the Co, CoAl and Co3W three-phase mixture, thus stabilizing the γ′ phase.

Enhanced hole concentration through Ga doping and excess of Mg and thermoelectric properties of p-type Mg2(1+z)(Si0.3Sn0.7)1−yGay

The influence of the doping amount of Ga as well as the excess of Mg on the phase composition and thermoelectric properties of Mg2(1+z)(Si0.3Sn0.7)1−yGay compounds are analyzed in detail. Regarding the content of Mg, a second phase of Mg is detected at grain boundaries whenever its over-stoichiometry exceeds 7%. On the other hand, XRD and EPMA analysis indicate that phase separation occurs when Mg is more than 3.5% deficient with respect to its stoichiometric amount. Thermoelectric property measurements reveal that doping with Ga, along with some over-stoichiometry of Mg, enhances the concentration of holes and electrical conductivity of Mg2(1+z)(Si0.3Sn0.7)1−yGay while it simultaneously reduces the Seebeck coefficient. However, there is little effect on the lattice thermal conductivity. The results also show that, in p-type Mg2Si0.3Sn0.7 based compounds, antisite point defects MgSi form when the content of Mg is over-stoichiometric. This leads to an enhanced concentration of holes. Mg2.10(Si0.3Sn0.7)0.95Ga0.05, having the optimized content of Ga and Mg, possesses the highest ZT value of 0.35 that is achieved at 650 K. This research reveals that both the doping of Ga and the excess of Mg do not have significant influence on the band structure of Mg2Si0.3Sn0.7, and the transport properties of p-type Mg2Si0.3Sn0.7 in the high hole concentration range can be well described by a simple, parabolic band model. The research work also establishes an important basis for further optimization of the figure of merit of p-type Mg2Si1−xSnx solid solutions by making use of over-stoichiometric amounts of Mg.

Native point defect energetics in GaSb: Enablingp-type conductivity of undoped GaSb

The energetics of native point defects in GaSb is studied using the density-functional theory within the hybrid functional scheme (HSE06). Our results indicate that the GaSb antisite has the lowest formation energy and could thus be the acceptor defect responsible for the p-type conductivity of undoped GaSb. We find also that the SbGa antisite has a remarkably low formation energy in Sb-rich growth conditions and it should act as a donor for all Fermi level positions in the band gap. However, we suggest that the structural metastability of the SbGa antisite or extrinsic point defects, namely carbon and in particular oxygen, may neutralize its compensating character.

Effects of proton irradiation on indium zinc oxide-based thin-film transistors

In this research, we performed a high-dose proton-beam irradiation process to investigate the effect of irradiation on the performance of indium-doped zinc oxide-based thin-film transistors (IZO-TFTs) by controlling their electrical and structural properties. The resistivity of IZO thin films dramatically increased with increasing proton-irradiation doses up to ∼ 10 14 cm − 2, after which the resistivity recovered to a level similar to as-deposited IZO thin film. After proton irradiation, the crystallinity of IZO thin films was reduced due to defect-isolation and/or chemical-isolation. The field effect mobility of IZO-TFTs decreased from 2.20 cm 2 V − 1 s − 1 to 1.22 cm 2 V − 1 s − 1, turn-on voltage shifted −7 V to −21 V and subthreshold swing value increased after proton irradiation with a 10 12 cm − 2 dose. With proton-irradiation doses over 10 14 cm − 2, IZO-TFTs device performance recovered to a level similar to that of a pristine device. Moreover, irradiated thin films and devices have low sensitivities to post-thermal annealing due to the creation of antisite oxygen point defects.

Antisite defects and traps in perovskiteYAlO3andLuAlO3: Density functional calculations

Density functional calculations in supercells are used to investigate antisite point defects in $\mathrm{Y}\mathrm{Al}{\mathrm{O}}_{3}$ and $\mathrm{Lu}\mathrm{Al}{\mathrm{O}}_{3}$. It is found that antisites consisting of an Al on the Y(Lu) site and those consisting of an Y(Lu) on the Al site both have low energy with respect to binary oxide reservoirs. Defects containing Al on the $A$ site yield electron traps at $0.4--0.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the conduction band edge, while Y(Lu) ions on the Al site do not. Based on this it is suggested that growth conditions that avoid Al on the $A$ site may reduce the number of electron traps in these materials.

Study on the Energy-Band Structure of Indium-Rich InGaN/GaN Quantum Dot System

The ground-state energy levels and energy band structures of the In-rich InGaN/GaN quantumdot (QD) system were analyzed by performing the capacitance-voltage (C-V), deep-level transient spectroscopy (DLTS), and photoluminescence (PL) measurements. The InGaN QD system was grown by using a metal-organic chemical vapor deposition method. The lateral size, the height, and the density of a disc-shaped InGaN QDs were 47.3 nm, 1.2 nm and 9.0 ◊ 10 9 cm 2 , respectively. The ground states of the InGaN QDs were located at 0.44 eV from the top of the barrier, which correspond to the conduction band edge of GaN. The QDs had various levels, except for the ground-state energy. A band-to-band transition energy of 2.92 eV was found by using PL measurements. Moreover, antisite point defects were also founded in GaN buer layer, and their activation energy and emission cross-section were 0.70 eV and 1.19 ◊ 10 13 cm 2 , respectively.

X-ray diffraction analysis of the structure of antisite arsenic point defects in low-temperature-grown GaAs layer

A method for the structure analysis of point defects in a semiconductor layer is developed by combining x-ray diffraction and growth of a superlattice where the concentration of point defects is periodically varied in the growth direction. Intensities of satellite reflections from the superlattice depend predominantly on the atomic structure of point defects, and hence this method can be applicable to the case of a low concentration of point defects. By using this method, the atomic structure of antisite As point defects in GaAs layers grown by molecular-beam epitaxy at low temperatures has been analyzed. Measured intensity ratios of the first-order satellite reflection in the lower angle side to that in the higher angle side for a number of (hkl) reflections are compared with those calculated based on structure models. The analysis has shown that experimental intensity ratios cannot be reproduced by models which include only a uniform tetragonal lattice distortion and local atomic displacements around an antisite As atom. A fairly good agreement between measured and calculated intensity ratios is obtained with a model which account for both gradual change in the tetragonal lattice distortion in the (0 0 1) plane and displacements of neighboring atoms away from the antisite As atom.