O3 Alloys Research Articles

Modulation of electronic structures and transport properties in 2D TM0.5Ga1.5O3 (TM = Al, Ga, In)

In this study, the band structures, density of states and transport properties of two-dimensional (2D) TM0.5Ga1.5O3 (TM=Al, Ga, In) are investigated by the first principle and the deformation potential theory. Both 2D Al0.5Ga1.5O3 and 2D In0.5Ga1.5O3 alloys tend to form under poor-oxygen conditions. Compared to bulk Ga2O3, the bandgap of 2D Ga2O3 is increased by 0.382 eV and its electron mobility is significantly increased from 143.352cm2·V−1·s−1 to 1486.638cm2·V−1·s−1. For 2D Al0.5Ga1.5O3, the bandgap and the electron effective mass are larger than those of 2D Ga2O3, but its electron mobility is reduced by >25%. In contrast, the bandgap of 2D In0.5Ga1.5O3 is narrower than that of 2D Ga2O3 while its electron mobility is increased by >52%, reaching 2262.901cm2·V−1·s−1. Therefore, 2D In0.5Ga1.5O3 has great potential to be used as a transport material for high-speed Ga2O3 electronic devices.

Feeble metallicity and robust semiconducting regime in structurally sensitive Ba(Pb, Sn)O3 alloys

Density functional calculations are carried out to study the symmetry and substitution-driven electronic phase transition in BaPb1−xSnxO3. Two end members, BaSnO3 and BaPbO3, are found to be insulating and metallic, respectively. In the latter case, the metallicity arises with the presence of an electron pocket, formed by Pb-s dominated conduction band edge, and a hole pocket formed by O-p dominated valence bands. While electron carriers are found to be highly mobile, the hole carriers are localized. Our study reveals that an insulating phase can be realized in the metallic cubic BaPbO3 in three ways in order to explore optoelectronic properties: first, by lowering the symmetry of the lattice to monoclinic through rotation and tilting of the PbO6 octahedra; second, by hydrostatic pressure; and third, by alloying with Sn substitution. The presence of soft phonon modes implies the plausibility of symmetry lowering structural transitions. Furthermore, unlike the earlier reports, we find that Sn substituted BaPbO3 cannot exhibit the topological insulator phase due to the absence of the band inversion.

Tensile fracture behavior and texture evolution of a hot-rolled W–Y2(Zr)O3 alloy

• A W–0.4wt% Y 2 (Zr)O 3 alloy with high recrystallization temperature and superior tensile strength have been prepared. • The strength and elongation of the RD and TD orientation samples of the WYZ alloy depend on the length of the free dislocation slip path and the capacity of grains to dislocations. • With the increase of tensile temperature, the α- and γ-fiber texture on the fracture side surface were significantly enhanced. • Larger tensile strain rates overestimate DBTT but exhibit high tensile strength at temperatures above DBTT. A W–0.4wt% Y 2 (Zr)O 3 (WYZ) alloy plate with superior mechanical properties and improved high-temperature stability was prepared by hot rolling. The tensile properties of WYZ alloys with different orientations and recrystallization states were investigated at a temperature range from room temperature (RT) to 500°C. The texture intensity of the original WYZ alloy was weak, but it was significantly enhanced after annealing. The initial recrystallization temperature of the WYZ alloy was approximately 1400°C. After recrystallization annealing, the strength of the WYZ alloy decreased significantly, but it showed an increased ductility. The ultimate tensile strength and fracture elongation of the samples along the rolling direction (RD) were always significantly higher than those of the samples along the transverse direction (TD). Orientation and annealing had no evident effect on ductile-to-brittle transition temperature (DBTT) from tensile data, but increasing strain rate would overestimate DBTT. The DBTT of the WYZ alloy from tensile data was approximately between 200°C and 300°C. With the increase in tensile test temperature, the strengths of α- and γ-fiber textures on the side of fracture surface increased significantly. The increase in α- and γ-fiber texture contents was beneficial to ductility enhancement in the WYZ plate.

First-principles investigation on the structural, electronic, vibrational and magnetic properties of the Co-substituted orthorhombic SrSnO3

Half-metallic (HM) ferromagnets provide an excellent possibility to design spintronic applications, yet a limited number of ferromagnetic materials are available which provide half-metallicity only in one spin channel. Moreover, often theoretically explored HM gap disappears when the material is synthesized experimentally due to small lattice mismatch happens which further hinders the advancement of spin-based applications. In this study, we have systematically explored the structural, electronic, formation, vibrational and magnetic properties of the Co-substituted SrSn(1−x)CoxO3wherex=0,0.25,0.5,0.75 (SSCO) alloys by employing first-principles methods with hybrid functionals. The Co-doping at the Sn-site induced the spontaneous magnetism into the non-magnetic orthorhombic SrSnO3 (SSO) perovskite material. Our calculations show that there are two ferromagnetic semiconductor materials found at the Co doping concentration of x = 0.5 & 0.75 with the large values of energy bandgaps. Furthermore, HM ferromagnetic ground state appears for the Co-doping of SSO at x = 0.25 which is vibrationally stable at the Г-point and has an integral magnetic moment of 1μB. The orthorhombic SrSn0.75Co0.25O3 alloy can be the prospective contender for the spintronic applications due to its thermodynamic and dynamic stability and large value of the HM gap (0.38 eV).

(In,Er)2O3 Alloys and Photoluminescence of Er3+ at Indirect Excitation via the Crystalline Host

(In1−xErx)2O3 ternary alloys are grown on A‐plane sapphire wafers by plasma‐assisted molecular beam epitaxy. The layers crystallize in the cubic bixbyite structure with axis aligned parallel to the substrate normal. The d‐spacing of the {111} lattice planes linearly increases with increasing the Er molar fraction in accordance with Vegard's law. Incorporation of Er in the In2O3 matrix is accompanied by the widening of the optical gap and linear increase of the peak absorption coefficient reaching 380 cm−1 at the telecommunication wavelength of 1.54 µm. The samples exhibit Er3+‐related emission at indirect excitation via the crystalline host at room temperature.

First-principles study of the electronic and optical properties of Li(Nb,Os)O3 alloys

Ferroelectric materials have some unique properties that are promising for photovoltaic applications. However, traditional ferroelectrics usually have a very large bandgap and therefore extremely low absorption in the visible light range. In this work, we study the electronic and optical properties of LiNb1–xOsxO3 alloys via first-principles calculations. We show that doping Os in LiNbO3 can effectively tune the bandgaps of the material. Specifically, less than 10% Os doping in LiNbO3 can reduce the bandgap from 3.78 eV to around 0.7 eV. The optical absorption of LiNb1–xOsxO3 alloys is improved to about two orders of magnitude than that of pure LiNbO3 in the visible light and infrared range. We further show that the alloys can still maintain their ferroelectricity and therefore have the potential for ferroelectric photovoltaic applications.

First – principles calculations on stability and mechanical properties of various ABO3 and their alloys

In this study, we perform first–principle calculations based on density functional theory (DFT) to obtain the ground state structural, elastic and dielectric properties of various ABO3 type ceramics and their {AxA′(1-x)}BO3 and A{BxB′(1-x)}O3 alloys. To represent alloy perovskites, we employ supercells with species A, A’ = Ba, Sr, Pb; B, B’ = Ti, Zr. The effects of composition and atomic configuration/order on lattice structure, thermodynamics, elastic constants and dielectric properties are evaluated. In calculations, we have used linear response and homogeneous field methods and we have also provided an assessment of the performance of these approaches in the determination of aforementioned properties.We have computed dielectric and piezoelectric properties for the cubic form of alloy perovskites. Even though cubic form of alloy perovskites does not have any piezoelectric properties, owing to crystallographic site occupied by different type of atoms, the inversion symmetry breaks down and the structures develop a small tetragonality, in turn a small polarization and non-zero but quite small piezoelectric coefficients emerge as expected. For instance the observed maximum piezoelectric constant for BaZr(1−x)TixO3 is 0.554x10−15C/N. The magnitudes are smaller than the feasible ranges for actual application needs, but they may increase substantially upon phase to lower symmetry tetragonal forms transformation.

Epitaxial growth of γ-(AlxGa1−x)O3 alloy films for band-gap engineering

Defective-spinel-structured γ-(AlxGa1−x)2O3 alloy films were successfully stabilized on MgAl2O4 substrates for the entire composition range using molecular beam epitaxy, though both end members are metastable. The crystallinity of the alloy films (x ≥ 0.22) was considerably better than that of the γ-Ga2O3 film, indicating that the Al2O3 lattice is more flexible and sensitive to the epitaxial force exerted by the substrate than Ga2O3 lattice. The direct and indirect band gaps obtained using deep-UV spectroscopy were in the ranges of 4.96–6.97 and 4.80–6.86 eV, for which the bowing parameters were 0.183 and 0.43 eV, respectively. Our results will be beneficial for further development of γ-(AlxGa1−x)2O3 systems.

Study of surface magnetism, exchange bias effect, and enhanced ferromagnetism in α-Fe1.4Ti0.6O3 alloy

Magnetic properties of mechanical alloyed α-Fe1.4Ti0.6O3 were studied at broad scale of temperatures, starting from room temperature down to 5 K. Morin transition of the base compound α-Fe2O3 was not observed after Ti doping. Enhanced ferromagnetism and exchange bias effect were exhibited in the alloyed compound α-Fe1.4Ti0.6O3. The observations were examined from different aspects of the alloying process, e.g., magnetic solution (spin ordering) at the interfaces of rhombohedral planes and core-shell structure of the nano-grains. Interfacial magnetic modifications were seen to be affected by the factors associated with grain size variation during alloy formation and thermal annealing of the alloy. Detailed analyses were made to understand the surface magnetism and exchange bias effect in the samples. Magnetic features of the present samples in many cases were found to be unconventional in comparison with the simple grain size effect of magnetic particles.

First principles prediction of a morphotropic phase boundary in the Bi(Zn1/2Ti1/2)O3−(Bi1/2Sr1/2)(Zn1/2Nb1/2)O3 alloy

The magnitude and direction of polarization within alloys of the tetragonally distorted Bi(Zn1/2Ti1/2)O3 (BZT) and the rhombohedrally oriented (Bi1/2Sr1/2)(Zn1/2Nb1/2)O3 are explored using density functional theory. For compositions with ≥50% of BZT, we find that the polarization is mainly in the [001] direction. Conversely, for low concentrations of BZT the polarization is rhombohedrally oriented. Based on these results we propose a phase diagram with a monoclinic phase that appears when the BZT concentration is roughly 50%. At this concentration, the material may have a useful piezoelectric response.

High-pressure phases in highly piezoelectricPbZr0.52Ti0.48O3

Two novel room-temperature phase transitions are observed, via synchrotron x-ray diffraction and Raman spectroscopy, in the Pb(Zr0.52Ti0.48)O3 alloy under hydrostatic pressures up to 16 GPa. A monoclinic (M)-to-rhombohedral (R1) phase transition takes place around 2-3 GPa, while this R1 phase transforms into another rhombohedral phase, R2, at about 6-7 GPa. First-principles calculations assign the R3m and R3c symmetry to R1 and R2, respectively, and reveal that R2 acts as a pressure-induced structural bridge between the polar R3m and a predicted antiferrodistortive R-3c phase.

Huge enhancement of electromechanical responses in compositionally modulated Pb(Zr(1-x )Tix)O3.

Monte Carlo simulations based on a first-principles-derived Hamiltonian are conducted to study the properties of Pb(Zr1-xTix)O3 alloys compositionally modulated along the [100] pseudocubic direction near the morphotropic phase boundary. It is shown that compositional modulation causes the polarization to continuously rotate away from the modulation direction, resulting in the unexpected triclinic and C-type monoclinic ground states and huge enhancement of electromechanical responses (the peak of piezoelectric coefficient is as high as 30,000 pC/N). The orientation dependence of dipole-dipole interaction in modulated structure is revealed as the microscopic mechanism to be responsible for these anomalies.

Global and secondary ferroelectric minima in ordered Pb(Sc0.25Nb0.25Ti0.5)O3 alloys

A first-principles-derived approach is developed to study properties of two Pb(Sc0.25Nb0.25Ti0.5)O3 alloys that are both atomically ordered along the [001] direction. Unlike the first alloy, the second structure has differently-charged (001) B-planes. Both systems exhibit a global ferroelectric minimum of orthorhombic symmetry and a secondary ferroelectric minimum of tetragonal symmetry. Some electromechanical responses are enhanced in the secondary minimum, especially for the second structure. Mechanisms allowing a switch from the global to the secondary minimum are discussed.

Planar defects and incommensurate phases in highly ordered perovskite solid solutions.

A first-principles-derived approach is used to study the effects of planar defects on structural properties of a rocksalt-ordered Pb(Sc0.5Nb0.5)O3 alloy. These defects lead to unusual features, including a less symmetrical ground state with respect to the perfectly ordered material. We also propose that a simple and original mechanism, involving these defects, may be responsible for the existence and anomalous characteristics of the incommensurate phases observed in insulating perovskites.

Finite-temperature properties of disordered and ordered Pb(Sc0.5Nb0.5)O3 alloys

A first-principles-derived approach is used to study the properties of rocksalt-ordered and disordered Pb(Sc0.5Nb0.5)O3 (PSN) alloys. The paraelectric-to-ferroelectric transition temperature Tc is strongly dependent on the atomic configuration, while the piezoelectric response versus T/Tc is nearly independent of the chemical order. Our calculations are consistent with the experimental finding of Chu et al. [J. Appl. Phys. 77, 1671 (1995)] that, at T=Tc, ordered PSN undergoes a normal ferroelectric transition, while disordered PSN transforms from a relaxor state to a ferroelectric state.

Finite-temperature properties of Pb(Zr1-xTi(x))O3 alloys from first principles

A first-principles-derived approach is developed to study finite-temperature properties of Pb(Zr1-xTix)O3 (PZT) solid solutions near the morphotropic phase boundary (MPB). Structural and piezoelectric predictions are in excellent agreement with experimental data and direct first-principles results. A low-temperature monoclinic phase is confirmed to exist, and is demonstrated to act as a bridge between the well-known tetragonal and rhombohedral phases delimiting the MPB. A successful explanation for the large piezoelectricity found in PZT ceramics is also provided.

Band states and shallow hole traps in Pb(Zr,Ti)O3 ferroelectrics

Band structure calculations and electron paramagnetic resonance measurements are used to show that Pb states determine many of the electronic properties of Pb(Zr,Ti)O3 ferroelectric materials. The valence-band edge consists of hybridized Pb s and O p states at all compositions. The conduction-band minimum changes from a Ti d-like Γ25′ state to a Pb p-like X1 state with increasing Zr content. The Pb p character accounts for the relatively small 0.2 eV increase in band gap in the Pb(Ti,Zr)O3 alloys with Zr content compared to the large 2 eV increase in band gap in Ba(Ti,Zr)O3 alloys. The paramagnetic Pb3+ hole center is found to become deeper and acquire some p character as the Zr content is raised. This is attributed to the change in conduction-band character combined with a local off-center displacement of the Pb3+ ion.