Effect Of Compositional Disorder Research Articles

High Entropy Protected Sharp Magnetic Transitions in Highly Disordered Spinel Ferrites.

How disorder affects magnetic ordering is always an intriguing question, and it becomes even more interesting in the recently rising high entropy oxides due to the extremely high disorder density. However, due to the lack of high-quality single crystal samples, the strong compositional disorder effect on magnetic transition has not been deeply investigated. In this work, we have successfully synthesized high-quality single crystalline high entropy spinel ferrites (Mg0.2Mn0.2Fe0.2Co0.2Ni0.2)xFe3-xO4. Our findings from high-temperature magnetization and neutron diffraction experiments showed ferrimagnetic transitions at 748, 694, and 674 K for x values of 1, 1.5, and 1.8, respectively. Notably, the magnetic transition almost showed no broadening for x values of 1 and 1.5, compared to Fe3O4. Extended X-ray absorption fine structure measurements provided insights into the elemental distribution among the octahedral and tetrahedral sites. The random distribution of elements across these sites reduced the formation of local clusters and short-range orders, enhancing sample homogeneity and preserving the sharpness of the magnetic transition, despite bond length variation. Our study not only marks the first successful synthesis of an HEO bulk single crystal exhibiting long-range magnetic order but also sheds light on the interaction between high configurational entropy and magnetic orderings. This opens new avenues for future research and applications of magnetic high entropy oxides.

Mössbauer and XAFS studies of the effect of compositional disorder on the local structure of Pb2FeSbO6

X-ray absorption spectroscopic (XAS) and Mössbauer investigations have been carried out to obtain information on the local environment of Fe ions in Pb2FeSbO6 (PFS) samples with different degree of the Fe3+ and Sb5+ ordering. Analysis of Mössbauer spectra shows, that the fraction of doublet component in the spectrum of the disordered PFS sample is significantly larger, than that of the ordered one, however singlet component still persists, indicating the presence of the compositionally ordered regions. The Fe K-spectrum absorption for the ordered PFS sample exhibits the lowest pre-edge intensities, indicating that Fe position is close to the oxygen octahedron center. The higher amplitude of this peak for the disordered sample provides evidence for structural distortions, involving Fe displacement off the octahedra centers. Additional support to this conclusion provides the structure of absorption spectra, which gave substantially higher Debye-Waller factor of Fe–O shell for disordered sample as compared to the ordered one.

The discovery of the effect of compositional disorder on the opto-electronic properties of the deformed InGaAsP quaternary

This article presents a theoretical study of the optoelectronic properties of InGaAsP alloys fabricated on three different substrates: InP, GaA, and ZnSe. Particular attention has been paid to the study of bandgap energy along points of high symmetry in the Brillouin zone, absorption coefficient, and refractive index of materials without compositional disorder and subjected to deformation. Our goal is to expand the range of optoelectronic properties. We found that the absorption coefficient of the deformed InGaAsP/InP material in the absence of disorder varies between 9.66×104cm−1 and 1.35×105cm−1 changing the content of phosphorus and gallium, making it possible to provide a wide range of refractive indices [3.06, 3.84] compared to GaAs substrates [2.67, 3.02] and ZnSe [2.69, 3.05]. This refractive index contrast can be exploited in the design of optical waveguides.

Influence of pressure and composition on electronic properties, phonon frequencies, and sound velocity for the zinc-blende GaAs1-xNx alloy

We investigated the electronic, phonon frequencies, and sound velocity of GaAs1-xNx ternary semiconductor alloys with the zinc-blende crystal structure over the entire nitrogen concentration range (with x from 0 to 1) using the empirical pseudo-potential model within the virtual crystal approximation including the compositional disorder effect. The pressure-dependent electronic, phonon frequencies and sound velocity of GaAs1-xNx ternary alloy have been studied. Our findings and the existing experimental data are found to be in good agreement. According to the dependence on pressure, a rising bandgap is predicted for GaAs1-xNx alloys at high-pressure values. According to the findings of this study, the GaAs1-xNx characteristics could have substantial optoelectronic applications in the infrared and mid-infrared spectral ranges.

Magnetic properties of 1D spin systems with compositional disorder of three-spin structural units

Abstract The exact diagonalization (ED) approach and Quantum Monte-Carlo (QMC) method were used for the study of the lowest energy states and low-temperature magnetic properties of some disordered 1D Heisenberg spin-1/2 systems formed by two types of three-spin structural units. For the system with a singlet ground state and the random distribution of structural units along the chain system, a significant decrease of the size of the intermediate magnetization plateau in comparison to the corresponding uniform spin system was found. For the “polyallyl” spin chain with a macroscopic value of the ground state spin, a transition to the singlet ground state due to the effect of compositional disorder was observed.

Acoustic velocity and phonon frequencies of GaxIn1−xSb alloys under pressure, temperature, and compositions

In the present work, we have reported the empirical pseudo-potential method (EPM) based on the virtual crystal approximation (VCA) incorporated the compositional disorder effect (CDE) to study some physical properties of GaxIn1−xSb alloys. The dependence of the acoustic velocity and phonon frequencies for the studied alloys with the Ga content has been analyzed. The phonon frequencies and the sound velocity under the effect of pressure and temperature have been determined. The obtained outcomes were generally in very good agreement with the accessible experimental and theoretical data. The acoustic velocity and phonon frequencies of GaxIn1−xSb under pressure and temperature have not been fully studied, so, we highlighted the study of this topic. The alloy under investigation could be useful in optoelectronic and photonic materials.

Energy band structure, acoustic velocities, optical phonon frequencies and mechanical properties of InP1-xSbx alloys under temperature and pressure

The dependence of electronic band structure, optical phonon frequencies, acoustic velocities, and mechanical properties of InP1-xSbx alloys on temperature and pressure were analyzed. The empirical pseudo-potential method (EPM) is performed to study the electronic structure and the mechanical properties for the considered alloys. The virtual crystal approximation (VCA) with the compositional disorder effect (CDE) is taken into account in this work. The influences of temperature and pressure on the optical phonon frequencies and on the acoustic velocities of the studied alloys have not been fully studied. So, we concerned on these properties under the influence of pressure and temperature. Good accord was obtained between our results and the available published values. Our findings may serve as reference for the experimental work. The studied properties of the considered alloys may give a helpful for the optoelectronic applications.

An Innovative Model for Electronic Band Structure Analysis of Doped and Un-Doped ZnO

An empirical pseudopotential method, in conjunction with virtual crystal approximation and the compositional disorder effect, is exploited to extract the full electronic band structure of various ZnO thin films to ensure excellent agreement with the experimental results, and to compare with the theoretical results obtained on the basis of various density functional theories. The lattice parameters obtained through x-ray diffraction (XRD) studies have been used to characterize the full electronic band structure of various ZnO thin films. The whole work reported here has been carried out using the TNL FullBandTM simulator (Tech Next Lab). The impact of intrinsic and extrinsic doping and the formation of polycrystalline planes in thin film samples on the band gap parameter have been analyzed in terms of the internal structure factor, u. The rigorous analysis of crystalline and polycrystalline samples show that the band gap value is strongly dependent on the internal structure parameter, u. The u value has been found to be significantly affected by lattice disorder generated by the formation of various defects and polycrystalline planes in the thin films. An innovative model demonstrating the relationship between the alloy disorder effect and the internal parameter is reported. With the help of an innovative model, the value of alloy disorder parameters, P, have been extracted for undoped and 1, 2, and 3 at% Cd-, Sr-, and Fe-doped ZnO thin film samples, respectively. Reasonable agreement has been obtained between the reported and experimental optical band gap results. The results reported in the current article show superiority against previously theoretical results based on first-principles methods.

The band gap bowing of CdS x Te1−x alloys beyond the virtual crystal approximation

The effect of alloy disorder on direct and indirect band gap energies in CdS x Te1−x semiconducting alloys has been investigated. The calculations are performed using a pseudopotential approach within the virtual crystal approximation (VCA). In the simplest approximation based on the VCA, the fundamental band gap bowing of CdS x Te1−x is found to be in disagreement with that of the experiment. The value of this bowing is highly improved by taking into account the compositional disorder effect (i.e. going beyond the VCA).

Disorder effects in nitride semiconductors: impact on fundamental and device properties

AbstractSemiconductor structures used for fundamental or device applications most often incorporate alloy materials. In “usual” or “common” III–V alloys, based on the InGaAsP or InGaAlAs material systems, the effects of compositional disorder on the electronic properties can be treated in a perturbative approach. This is not the case in the more recent nitride-based GaInAlN alloys, where the potential changes associated with the various atoms induce strong localization effects, which cannot be described perturbatively. Since the early studies of these materials and devices, disorder effects have indeed been identified to play a major role in their properties. Although many studies have been performed on the structural characterization of materials, on intrinsic electronic localization properties, and on the impact of disorder on device operation, there are still many open questions on all these topics. Taking disorder into account also leads to unmanageable problems in simulations. As a prerequisite to address material and device simulations, a critical examination of experiments must be considered to ensure that one measures intrinsic parameters as these materials are difficult to grow with low defect densities. A specific property of nitride semiconductors that can obscure intrinsic properties is the strong spontaneous and piezoelectric fields. We outline in this review the remaining challenges faced when attempting to fully describe nitride-based material systems, taking the examples of LEDs. The objectives of a better understanding of disorder phenomena are to explain the hidden phenomena often forcing one to use ad hoc parameters, or additional poorly defined concepts, to make simulations agree with experiments. Finally, we describe a novel simulation tool based on a mathematical breakthrough to solve the Schrödinger equation in disordered potentials that facilitates 3D simulations that include alloy disorder.

An innovative technique for electronic transport model of group-III nitrides

An optimized empirical pseudopotential method (EPM) in conjunction with virtual crystal approximation (VCA) and the compositional disorder effect is used for simulation to extract the electronic material parameters of wurtzite nitride alloys to ensure excellent agreement with the experiments. The proposed direct bandgap results of group-III nitride alloys are also compared with the different density functional theories (DFT) based theoretical results. The model developed in current work, significantly improves the accuracy of calculated band gaps as compared to the ab-initio method based results. The physics of carrier transport in binary and ternary nitride materials is investigated with the help of in-house developed Monte Carlo algorithms for solution of Boltzmann transport equation (BTE) including nonlinear scattering mechanisms. Carrier–carrier scattering mechanisms defined through Coulomb-, piezoelectric-, ionized impurity-, surface roughness-scattering with acoustic and intervalley scatterings, all have been given due consideration in present model. The direct and indirect energy bandgap results have been calibrated with the experimental data and use of symmetric and asymmetric form factors associated with respective materials. The electron mobility results of each binary nitride material have been compared and contrasted with experimental results under appropriate conditions and good agreement has been found between simulated and experimental results.

Microhardness and mechanical stability of CdSexTe1-x: A pseudopotential approach

The composition dependence of the elastic constants and their related mechanical properties of CdSexTe1-x semiconducting ternary alloys have been investigated. The calculations are performed using a pseudopotential approach under a modified virtual crystal approximation that takes into account the effect of compositional disorder. A reasonably good accord is found between the present results and those available in the literature. The present findings predict that by incorporating more Se atoms into CdTe, the material system of interest becomes harder and more resistant to the transverse deformations. For all Se concentrations ranging from 0 to 1, CdSexTe1-x is predicted to be mechanically stable, to have a brittle nature and to be stiffest along the ˂111˃ body diagonals.

Negative Thermal Expansion Near the Precipice of Structural Stability in Open Perovskites.

Negative thermal expansion (NTE) describes the anomalous propensity of materials to shrink when heated. Since its discovery, the NTE effect has been found in a wide variety of materials with an array of magnetic, electronic and structural properties. In some cases, the NTE originates from phase competition arising from the electronic or magnetic degrees of freedom but we here focus on a particular class of NTE which originates from intrinsic dynamical origins related to the lattice degrees of freedom, a property we term structural negative thermal expansion (SNTE). Here we review some select cases of NTE which strictly arise from anharmonic phonon dynamics, with a focus on open perovskite lattices. We find that NTE is often present close in proximity to competing structural phases, with structural phase transition lines terminating near T=0 K yielding the most prominent displays of the SNTE effect. We further provide a theoretical model to make precise the proposed relationship among the signature behavior of SNTE, the proximity of these systems to structural quantum phase transitions and the effects of phase fluctuations near these unique regions of the structural phase diagram. The effects of compositional disorder on NTE and structural phase stability in perovskites are discussed.

Microhardness, phase transition, acoustic wave velocities and melting temperature of AlxGa1−xSb

The band structure of Al[Formula: see text]Ga[Formula: see text]Sb semiconducting ternary alloys and their related properties such as elastic constants, microhardness, transition pressure to the first phase, acoustic wave velocities and melting temperature have been investigated. The calculations are performed using a pseudopotential approach within the virtual crystal approximation which includes the effect of compositional disorder as an effective potential. Generally, our results are found to be in good accord with the experimental results. The composition dependence of all features of interest showed a monotonic behavior and suggests that the stiffness, hardness and structural stability becomes better in Al[Formula: see text]Ga[Formula: see text]Sb for higher Al concentrations. The bulk sound speeds and melting temperature are found to become larger when increasing the Al content.

Effect of Composition Disorder in a System of Superconducting Granules on the Superconducting Properties of La2–xSrxCuO4 Ceramic Samples

The effect of the degree of doping with strontium has on the superconducting properties of granular superconductor La2–xSrxCuO4 is considered. It is found that reducing the concentration of strontium under conditions of compositional disorder broadens the superconducting transition above the temperature of intergranular Josephson coupling and raises the temperature of the completion of the global superconducting transition.

Mechanical properties of AlPxSb1-x semiconductor alloys under the effect of temperature and pressure

Using the empirical pseudo-potential method (EPM) combined with the virtual crystal approximation (VCA) included the compositional disorder effect, some of optoelectronic and mechanical properties of AlPxSb1-x ternary alloys under the effect of pressure and temperature have been studied. Our results for the energy band gaps, refractive index, dielectric constants, elastic constants and some related parameters like elastic moduli, Poisson's ratio, bond stretching and bond bending force constants, internal strain parameter, linear compressibility, Cauchy ratio and anisotropy factor are successfully calculated for different values of temperature and pressure. A good agreement is noticed between our results and the available experimental and theoretical published data.

Influence of composition, temperature and pressure on the optoelectronic and mechanical properties of InPxSb1-x alloys

A detailed study of the effect of composition, temperature and pressure on the mechanical and optoelectronic properties of InPxSb1-x alloys has been reported. The calculations have been done using pseudo-potential method (EPM) within the improved virtual crystal approximation (VCA), where the compositional disorder effect is taken into account. Our results for the elastic constants, elastic moduli, Poisson's ratio, bond stretching force constant, bond bending force constant, internal strain parameter, linear compressibility, Cauchy ratio, and anisotropy factor are successfully calculated and compared with the available published data.

Energy gaps, valence and conduction charge densities and optical properties of GaAs1−xPx

The electronic structure and its derived valence and conduction charge distributions along with the optical properties of zinc-blende GaAs[Formula: see text]P[Formula: see text] ternary alloys have been studied. The calculations are performed using a pseudopotential approach under the virtual crystal approximation (VCA) which takes into account the compositional disorder effect. Our findings are found to be generally in good accord with experiment. The composition dependence of direct and indirect bandgaps showed a clear bandgap bowing. The nature of the gap is found to depend on phosphorous content. The bonding and ionicity of the material of interest have been examined in terms of the anti-symmetric gap and charge densities. The variation in the optical constants versus phosphorous concentration has been discussed. The present investigation may give a useful applications in infrared and visible spectrum light emitters.

Optical phonon modes and polaron related parameters in GaxIn1−xP

Based on a pseudopotential approach under the virtual crystal approximation that includes the effect of compositional disorder, the optical lattice vibration frequencies and polaron related parameters in zinc-blende GaxIn1−xP have been studied. Our findings showed generally reasonably good accord with data in the literature. Other case, our results are predictions. The composition dependence of longitudinal optical (LO) and transverse optical (TO) phonon modes, LO-TO splittings, Frӧhlich coupling parameter, Debye temperature of LO phonon frequency, and polaron effective mass has been analyzed and discussed. While a non-monotonic behavior has been noticed for the LO and TO phonon frequencies versus Ga concentration x, a monotonic behavior has been observed for the rest of the features of interest. The information derived from this investigation may be useful for optoelectronic technological applications.

Random electric field instabilities of relaxor ferroelectrics

Relaxor ferroelectrics are complex oxide materials which are rather unique to study the effects of compositional disorder on phase transitions. Here, we study the effects of quenched cubic random electric fields on the lattice instabilities that lead to a ferroelectric transition and show that, within a microscopic model and a statistical mechanical solution, even weak compositional disorder can prohibit the development of long-range order and that a random field state with anisotropic and power-law correlations of polarization emerges from the combined effect of their characteristic dipole forces and their inherent charge disorder. We compare and reproduce several key experimental observations in the well-studied relaxor PbMg1/3Nb2/3O3–PbTiO3.