Random Distribution Of Atoms Research Articles

Destabilizing high-capacity high entropy hydrides via earth abundant substitutions: From predictions to experimental validation

The vast chemical space of high entropy alloys (HEAs) makes trial-and-error experimental approaches for materials discovery intractable and often necessitates data-driven and/or first principles computational insights to successfully target materials with desired properties. In the context of materials discovery for hydrogen storage applications, a theoretical prediction-experimental validation approach can vastly accelerate the search for substitution strategies to destabilize high-capacity hydrides based on benchmark HEAs, e.g. TiVNbCr alloys. Here, machine learning predictions, corroborated by density functional theory calculations, predict substantial hydride destabilization with increasing substitution of earth-abundant Fe content in the (TiVNb)75Cr25-xFex system. The as-prepared alloys crystallize in a single-phase bcc lattice for limited Fe content x < 7, while larger Fe content favors the formation of a secondary C14 Laves phase intermetallic. Short range order for alloys with x < 7 can be well described by a random distribution of atoms within the bcc lattice without lattice distortion. Hydrogen absorption experiments performed on selected alloys validate the predicted thermodynamic destabilization of the corresponding fcc hydrides and demonstrate promising lifecycle performance through reversible absorption/desorption. This demonstrates the potential of computationally expedited hydride discovery and points to further opportunities for optimizing bcc alloy ↔ fcc hydrides for practical hydrogen storage applications.

Constructing Directional Electrostatic Potential Difference via Gradient Nitrogen Doping for Efficient Oxygen Reduction Reaction.

Nitrogen doping has been recognized as an important strategy to enhance the oxygen reduction reaction (ORR) activity of carbon-encapsulated transition metal catalysts (TM@C). However, previous reports on nitrogen doping have tended to result in a random distribution of nitrogen atoms, which leads to disordered electrostatic potential differences on the surface of carbon layers, limiting further control over the materials' electronic structure. Herein, a gradient nitrogen doping strategy to prepare nitrogen-deficient graphene and nitrogen-rich carbon nanotubes encapsulated cobalt nanoparticles catalysts (Co@CNTs@NG) is proposed. The unique gradient nitrogen doping leads to a gradual increase in the electrostatic potential of the carbon layer from the nitrogen-rich region to the nitrogen-deficient region, facilitating the directed electron transfer within these layers and ultimately optimizing the charge distribution of the material. Therefore, this strategy effectively regulates the density of state and work function of the material, further optimizing the adsorption of oxygen-containing intermediates and enhancing ORR activity. Theoretical and experimental results show that under controlled gradient nitrogen doping, Co@CNTs@NG exhibits significantly ORR performance (Eonset=0.96V, E1/2=0.86V). At the same time, Co@CNTs@NG also displays excellent performance as a cathode material for Zn-air batteries, with peak power density of 132.65mAcm-2 and open-circuit voltage (OCV) of 1.51V. This work provides an effective gradient nitrogen doping strategy to optimize the ORR performance.

Well-defined diatomic catalysis for photosynthesis of C2H4 from CO2

Owing to the specific electronic-redistribution and spatial proximity, diatomic catalysts (DACs) have been identified as principal interest for efficient photoconversion of CO2 into C2H4. However, the predominant bottom-up strategy for DACs synthesis has critically constrained the development of highly ordered DACs due to the random distribution of heteronuclear atoms, which hinders the optimization of catalytic performance and the exploration of actual reaction mechanism. Here, an up-bottom ion-cutting architecture is proposed to fabricate the well-defined DACs, and the superior spatial proximity of CuAu diatomics (DAs) decorated TiO2 (CuAu-DAs-TiO2) is successfully constructed due to the compact heteroatomic spacing (2-3 Å). Owing to the profoundly low C-C coupling energy barrier of CuAu-DAs-TiO2, a considerable C2H4 production with superior sustainability is achieved. Our discovery inspires a novel up-bottom strategy for the fabrication of well-defined DACs to motivate optimization of catalytic performance and distinct deduction of heteroatom synergistically catalytic mechanism.

Defects Act in an “Introverted” Manner in FeNiCrCoCu High-Entropy Alloy under Primary Damage

High-entropy alloys (HEAs) attract much attention as possible radiation-resistant materials due to their several unique properties. In this work, the generation and evolution of the radiation damage response of an FeNiCrCoCu HEA and bulk Ni in the early stages were explored using molecular dynamics (MD). The design, concerned with investigating the irradiation tolerance of the FeNiCrCoCu HEA, encompassed the following: (1) The FeNiCrCoCu HEA structure was obtained through a hybrid method that combined Monte Carlo (MC) and MD vs. the random distribution of atoms. (2) Displacement cascades caused by different primary knock-on atom (PKA) energy levels (500 to 5000 eV) of the FeNiCrCoCu HEA vs. bulk Ni were simulated. There was almost no element segregation in bulk FeNiCrCoCu obtained with the MD/MC method by analyzing the Warren–Cowley short-range order (SRO) parameters. In this case, the atom distribution was similar to the random structure that was selected as a substrate to conduct the damage cascade process. A mass of defects (interstitials and vacancies) was generated primarily by PKA departure. The number of adatoms grew, which slightly roughened the surface, and the defects were distributed deeper as the PKA energy increased for both pure Ni and the FeNiCrCoCu HEA. At the time of thermal spike, one fascinating phenomenon occurred where the number of Frenkel pairs for HEA was more than that for pure Ni. However, we obtained the opposite result, that fewer Frenkel pairs survived in the HEA than in pure Ni in the final state of the damage cascade. The number and size of defect clusters grew with increasing PKA energy levels for both materials. Defects were suppressed in the HEA; that is to say, defects were “cowards”, behaving in an introverted manner according to the anthropomorphic rhetorical method.

Crystalline Magnetic Anisotropy in High Entropy (Fe, Co, Ni, Cr, Mn)3O4 Oxide Driven by Single‐Element Orbital Anisotropy

AbstractThe design of multicomponent materials has captured considerable attention due to its extraordinary ability to tailor functional properties. However, how a single element affects the behavior of the overall material has yet to be explored in depth. In this study, the heteroepitaxy of high entropy (Fe, Co, Ni, Cr, Mn)3O4 films with varying strain states are investigated in magnetic performance. It is discovered that the high entropy oxide thin film with compressive strain exhibits an effect of crystalline magnetic anisotropy. Diverse analyses provide a detailed understanding of high entropy magnetic oxide systems, including X‐ray diffraction, reciprocal space mapping, macroscopic magnetic characterization, X‐ray absorption spectroscopy (XAS), etc. Notably, the element‐specific XAS technique proves effective in uncovering the origin of the crystalline magnetic anisotropy. Due to the substrate‐induced epitaxial strain, the eg orbitals of Mn3+ form different energy levels, leading to different preferred electron occupancy. The exploration of magnetic properties in epitaxial high entropy oxide film is then raveled. By navigating the complexities introduced by the random atom distribution and intricate magnetic interactions, this study pioneers novel methodologies for probing the core physics of high entropy oxides.

On the role of vacancy-hydrogen complexes on dislocation nucleation and propagation in metals

New insights are provided into the role of vacancy-hydrogen (VaH) complexes, compared to the hydrogen atoms alone, on hydrogen embrittlement of nickel. The effect of the concentration of hydrogen atoms and VaH complexes is investigated in different crystal orientations on dislocation emission and propagation in single crystal of nickel using atomistic simulations. At first, embrittlement is studied on the basis of unstable and stable stacking fault energies as well as fracture energy to quantify the embrittlement ratio (unstable stacking fault energy/fracture energy). It is found that VaH complexes lead to high embrittlement compared to H atoms alone. Next, dislocation emission and propagation at pre-cracked single crystal crack-tip are investigated under Mode-I loading. Depending upon the elastic interaction energy and misfit volume, high local concentrations at the crack front lead to the formation of nickel-hydride and nickel-hydride with vacancies phases. These phases are shown to cause softening due to earlier and increased dislocation emission from the interface region. On the other hand, dislocation propagation under the random distribution of hydrogen atoms and VaH complexes at the crack front or along the slip plane shows that VaH complexes lead to hardening that corroborates well with the increased shear stresses observed along the slip plane. Further, VaH complexes lead to the disintegration of partial dislocation and a decrease in dislocation travel distance with respect to time. The softening during emission and hardening during propagation and disintegration of partial dislocation loops due to VaH complexes fit the experimental observations of various dislocation structures on fractured surfaces in the presence of hydrogen, as reported in literature.

Lower bound on the spread of valley splitting in Si/SiGe quantum wells induced by atomic rearrangement at the interface

The achievement of universal quantum computing critically relies on scalability. However, ensuring the necessary uniformity for scalable silicon electron spin qubits poses a significant challenge due to the considerable fluctuations in valley splitting energy (E VS) across quantum dot arrays, which impede the initialization of qubit systems comprising multiple spins and give rise to spin–valley entanglement resulting in the loss of spin information. These E VS fluctuations have been attributed to variations in the in-plane averaged alloy concentration along the confinement direction of Si/SiGe quantum wells. In this study, employing atomistic pseudopotential calculations, we unveil a significant spectrum of E VS even in the absence of such concentration fluctuations. This spectrum represents the lower limit of the wide range of E VS observed in numerous Si/SiGe quantum devices. By constructing simplified interface atomic step models, we analytically demonstrate that the lower bound of the E VS spread originates from the in-plane random distribution of Si and Ge atoms within SiGe barriers — an inherent characteristic that has been previously overlooked. Additionally, we propose an interface engineering approach to mitigate the in-plane randomness-induced fluctuations in E VS by inserting a few monolayers of pure Ge barrier at the Si/SiGe interface. Our findings provide valuable insights into the critical role of in-plane randomness in determining E VS in Si/SiGe quantum devices and offer reliable methods to enhance the feasibility of scalable Si-based spin qubits.

Rectifying behavior of inhomogeneous BCN alloy nanotubes

Based on virtual crystal approximation, the electronic properties and rectifying behavior of inhomogeneous BCN alloy nanotubes are investigated using a first-principles study combined with the non-equilibrium Green's function method. Consistent with previous experimental reports, carbon atoms in the BCN alloy nanotubes can randomly replace boron and nitrogen atoms. Hence, the asymmetrical distribution of B, C, and N atoms could result in different charge transmissions under the applied forward and reverse bias voltages, and the I–V characteristics of these inhomogeneous alloy nanotubes exhibit a rectifying behavior. A BCN alloy nanotube exhibits a larger current when more B and N atoms are replaced by C atoms. Consequently, the current depends on the concentration of the carbon atoms. Based on these results, tuning the electronic properties and rectification ratio of inhomogeneous BCN alloy nanotubes with an asymmetrical random distribution of carbon atoms can be applied in the design of nanoelectronic devices.

Improving the Energy Storage Performance of a Chlorinated Poly(vinyl chloride) Film at Elevated Electric Field and Temperature via a Simple Annealing Process

High energy density and efficiency are crucial factors for polymeric dielectrics to satisfy the emerging demand in high-pulse metallized film capacitors. However, achieving high energy density in polar polymeric dielectrics is usually accompanied by a sharp decline in the energy discharge efficiency at elevated electric fields and temperatures. In this work, we investigated the dielectric and energy storage properties of chlorinated poly(vinyl chloride) (CPVC) with moderate polarity. Due to the random distribution of chlorine atoms on the polymer chain, CPVC displays a medium permittivity of about 3 and low loss. Moreover, high-temperature annealing is used to promote the rearrangement of CPVC chain segments, which contributes to the formation of CPVC microcrystals. Consequently, the storage modulus and glass transition temperature (Tg) along with the thermal stability of CPVC are improved due to the microcrystals as physical cross-linking points. Compared to pristine CPVC, heat-treated CPVC shows reduced dielectric loss and elevated breakdown strength. As a result, a high energy density of 9.4 J/cm3 and a discharge efficiency of 82.7% at 625 MV/m are obtained in heat-treated CPVC. This work offers a straightforward approach to improving the energy storage properties of CPVC so that it can be used for high-pulse metallized film capacitors.

Exploring the Hydrogen Sorption Capabilities of a Novel Ti-V-Mn-Zr-Nb High-Entropy Alloy

Hydrogen is considered as a clean energy carrier able to achieve the decarbonization of the economy, but its compact, safe, and efficient storage represents an important challenge. Among many materials forming hydrides, this work reports the study of hydrogen sorption properties of a novel bcc high-entropy alloy, Ti0.30V0.25Mn0.10Zr0.10Nb0.25, synthesized by arc melting. In less than 60 s, the alloy fully absorbs hydrogen at room temperature, reaching a capacity of 2.0 H/M (2.98 wt.%). A two-step reaction with hydrogen is confirmed by pressure-composition isotherms, synchrotron X-ray and neutron diffraction: bcc solid solution ↔ bcc monohydride ↔ fcc dihydride. For the second step transformation, the calculated thermodynamic values indicate the formation of a very stable dihydride, with ΔHabs = −97 kJ/molH2. Moreover, the pair distribution function analysis of high-energy synchrotron X-ray scattering data validates a completely random distribution of metal atoms in the fcc dihydride phase, without noticeable lattice strain nor elemental segregation. In situ synchrotron X-ray and neutron diffraction, performed during hydrogen desorption by heating under vacuum, demonstrated full reversibility of the reaction with hydrogen. On the basis of these results, tuning the chemical composition of high-entropy alloys may have great implications in terms of hydrogen sorption properties.

Nanoscale Clustering in an Additively Manufactured Zr-Based Metallic Glass Evaluated by Atom Probe Tomography.

Composition analysis at the nm-scale, marking the onset of clustering in bulk metallic glasses, can aid the understanding and further optimization of additive manufacturing processes. By atom probe tomography, it is challenging to differentiate nm-scale segregations from random fluctuations. This ambiguity is due to the limited spatial resolution and detection efficiency. Cu and Zr were selected as model systems since the spatial distributions of the isotopes therein constitute ideal solid solutions, as the mixing enthalpy is, by definition, zero. Close agreement is observed between the simulated and measured spatial distributions of the isotopes. Having established the signature of a random distribution of atoms, the elemental distribution in amorphous Zr59.3Cu28.8Al10.4Nb1.5 samples fabricated by laser powder bed fusion is analyzed. By comparison with the length scales of spatial isotope distributions, the probed volume of the bulk metallic glass shows a random distribution of all constitutional elements, and no evidence for clustering is observed. However, heat-treated metallic glass samples clearly exhibit elemental segregation which increases in size with annealing time. Segregations in Zr59.3Cu28.8Al10.4Nb1.5 > 1 nm can be observed and separated from random fluctuations, while accurate determination of segregations < 1 nm in size are limited by spatial resolution and detection efficiency.

Ab-initio study of electronic, mechanical and thermodynamic properties of β-Ti–15Nb–xSi alloys for biomaterials applications

In this paper, we used the first-principles method to investigate the structural, electronic, mechanical and thermodynamic parameters of the ternary [Formula: see text]-Ti–15Nb–xSi alloys with [Formula: see text][Formula: see text]wt.%. We have carried out theoretical computations inside the density functional theory (DFT) utilizing the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) model. The random distribution of Nb atoms in the alloy was described by using both virtual crystal approximation (VCA), special quasirandom structure (SQS) and the coherent potential approximation (CPA) techniques, in combination with first-principles plane-wave pseudopotential (PW-PP) and exact muffin-tin orbital (EMTO) methods. We determined the elastic constants as well as the bulk, shear, Young’s modulus and Poisson’s ratio. Our structural results are in good agreement with the available experimental and theoretical results for the pure structure of the titanium. In addition, we have estimated the band structure and the density of state (DOS) for the electronic computations. Our findings demonstrate that all of the compounds are metallic, stable and meet the requirements for stability. Young’s modulus of Ti–15Nb–0.6Si and Ti–15Nb–1.6Si is 86.5[Formula: see text]GPa and 15.11[Formula: see text]GPa, respectively, which are similar to Young’s moduli of human bone (10–30[Formula: see text]GPa). All calculated parameters of the alloys decreased with increasing of Si concentration except for Poisson’s ratio, anisotropy and B/G ratio. Furthermore, all of the materials investigated showed ductile nature, and Young’s modulus values are needed for further applications. Excitations from the quasi-harmonic Debye approximation’s vibrational part were applied to the 0[Formula: see text]K free energy calculated via ab-initio calculations. The influence of temperatures up to 800 K on phase stability was investigated. These findings can be utilized to help designers create alternative low-modulus alloys for biomedical applications.

Enabling molecular dynamics simulations of helium bubble formation in tritium-containing austenitic stainless steels: An Fe-Ni-Cr-H-He potential

Formation of helium bubbles impacts mechanical properties of materials used in nuclear applications. An Fe-Ni-Cr-H-He quinary potential capable of direct molecular dynamics simulations of nucleation and growth of helium bubbles from randomly-born helium interstitial atoms has been developed. This is accomplished by incorporating helium into an existing Fe-Ni-Cr-H quaternary potential while addressing three challenging paradoxes characterizing helium in austenitic stainless steels: (a) interstitial He atoms form tightly bound dimers and larger clusters in the lattice but He atoms are only bound by weak van de Waals forces in the pure gas phase, (b) He atoms diffuse readily in host metals yet significantly distort the lattice causing large volume expansions, and (c) He atoms prefer tetrahedral interstitial sites as opposed to the larger octahedral sites despite large repulsive interactions with metal atoms within the lattice. We demonstrate that our potential reproduces density functional theory results on important properties relevant to helium bubble nucleation and growth. In addition to molecular statics validation of static properties, molecular dynamics simulation tests establish that our potential leads to the nucleation of helium bubbles from an initial random distribution of He interstitial atoms while at the same time capturing the equation of state in the pure He phase.

Unraveling the effect of hydrogenation on the mechanical properties of coiled carbon nanotubes: a molecular dynamics study.

With the increase in the utilization of nanomaterials in daily life, carbon nanostructures have received the attention of many researchers due to their special physical, chemical, and electrical properties. Chemical functionalization is one of the common methods to improve the thermomechanical properties of carbon nanomaterials used for specific applications. In this research, the effect of functionalization with hydrogen atoms on the mechanical properties of coiled carbon nanotubes with different geometrical dimensions has been examined. In addition, the mechanical properties of CCNTs with random and patterned distributions of hydrogen atoms have been investigated. The random distribution of hydrogen atoms up to 10% causes a sharp decrease in the mechanical properties of CCNTs such as the Young's modulus and spring constant, and increasing the percentage of H-coverage by more than 10% does not cause a significant effect on the mentioned properties. Also, unlike other carbon nanostructures, the stretchability of most CCNTs increases by increasing the percentage of hydrogenation beyond 30 percent. On investigating the effect of temperature on the properties of hydrogenated CCNTs, the temperature increase does not affect the Young's modulus and spring constant, and also there is no explicit relationship between their stretchability and temperature. Exploring the mechanical behavior of hydrogen-functionalized CCNTs via the tensile test and also how their mechanical properties change compared to those of pure CCNTs can help researchers in many applications.

Atomistic simulations of dislocation plasticity in concentrated VCoNi medium entropy alloys: Effects of lattice distortion and short range order

Face-centered cubic (fcc) high and medium entropy alloys (H/MEAs) have been shown to display superior mechanical properties at low temperatures, but significant improvement of their strength at high temperatures is required for industrial applications at extreme conditions. Recently, it has been shown that the breakthrough of the MEAs from equiatomic/near-equiatomic to non-equiatomic ratios leads to strong MEAs with good ductility. To design new H/MEAs, we consider two important factors that may influence strength: the chemical composition and chemical short range order (CSRO). In this study, we investigate the depinning stress (σc) as a criterion of strength of several compositions of VCoNi concentrated solid solution alloys (CSSAs) including V0.33Co0.33Ni0.33, V0.35Co0.2Ni0.45, V0.33Co0.17Ni0.5, and V0.17Co0.33Ni0.5 at 5 K and 300 K, using atomistic simulations. The chosen interatomic potential is shown to be reliable by comparing experimental/ab initio values and calculated parameters such as lattice constant, shear modulus, depinning stress, and temperature variation of stacking fault width for equimolar VCoNi. We find a good agreement between experimental friction stress and the depinning stress extracted from our results for equimolar VCoNi. Also, we find that Vclusters are the main pinning points of dislocations, and With a random distribution of atoms, we find that the alloy composition V0.33Co0.17Ni0.5 displays the largest depinning stress at both 5 and 300 K. Furthermore, to investigate how CSRO affects the strength of these alloys, we design CSRO into the microstructure using two different methods: In the first method, hybrid Molecular-dynamics/Monte-Carlo simulations were employed to simulate annealing at various temperatures. We observe that such simulations create CSRO so that it increases with decreasing annealing temperature. Recently, the CSRO motif and its concentration in an equimolar VCoNi have been determined by experiment. By modeling this experiment, we also implemented the CSRO into microstructure as the second method. By using both methods, the effect of CSRO on the magnitude of the depinning stress is discussed. It was shown that in both methods, CSRO significantly influences the strength of non-equimolar VCoNi alloys.

Stabilization of Dinuclear Rhodium and Iridium Clusters on Layered Titanate and Niobate Supports.

Atomically dispersed organometallic clusters can provide well-defined nuclearity of active sites for both fundamental studies as well as new regimes of activity and selectivity in chemical transformations. More recently, dinuclear clusters adsorbed onto solid surfaces have shown novel catalytic properties resulting from the synergistic effect of two metal centers to anchor different reactant species. Difficulty in synthesizing, stabilizing, and characterizing isolated atoms and clusters without agglomeration challenges allocating catalytic performance to atomic structure. Here, we explore the stability of dinuclear rhodium and iridium clusters adsorbed onto layered titanate and niobate supports using molecular precursors. Both systems maintain their nuclearity when characterized using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Statistical analysis of HAADF-STEM images revealed that rhodium and iridium dimers had mean cluster-to-cluster distances very similar to what is expected from a random distribution of atoms over a large area, indicating that they are dispersed without aggregation. The stability of dinuclear rhodium clusters supported on titanate nanosheets was also investigated by X-ray absorption fine structure (EXAFS), DRIFTS, and first-principles calculations. Both X-ray absorption spectroscopy and HAADF-STEM simulations, guided by density functional theory (DFT)-optimized structure models, suggested that rhodium dimers adsorb onto the nanosheets in an end-on binding mode that is stable up to 100 °C under reducing conditions. This study highlights that crystalline nanosheets derived from layered metal oxides can be used as model supports to selectively stabilize dinuclear clusters, which could have implications for heterogeneous catalysis.

Three-color reflections in one-dimensional ordered and disordered atomic lattices with trapped N-type cold atoms.

Investigating and controlling light propagation in one-dimensional (1D) ordered and disordered atomic lattices is critical both fundamentally and for applications. In this study, cold atoms are trapped in 1D optical lattice and driven to the four-level N configuration. In each period, the atoms exhibit a Gaussian density distribution with the average atomic density N0 (1 + Δk). When the random number Δk = 0 (the atomic density Nk(z)) corresponding to an ordered 1D atomic lattice, there are three reflection regions of high reflectivity located in two EIT windows and one large detuning range. However, the atomic density may increase (N k+(z) with Δk > 0) or decrease (N k-(z) with Δk < 0) owing to the imperfect manufacturing process or random distribution of atoms corresponding to a disordered atomic lattice. The results show that the width and height of reflections can be raised (reduced) by the increased (decreased) ratio of N k+(z)/N k (z) (N k-(z)/N k (z)) with the random distribution of lattice cells with N k+(z) (N k-(z)). When a cluster of disordered lattice cells with N k+(z) and N k-(z) is located at the front or tail of the atomic lattice, reflection symmetry can be broken. However, the symmetry and robustness can be well preserved with the random fluctuation of the average atomic density in each lattice cell.

Local structure in high-entropy transition metal diborides

Studies on high-entropy materials often speculate about the effects of lattice distortion and disorder on characteristics such as hardness, thermal expansion, and electronic properties. Notwithstanding the ongoing race to discover new compositions, investigations of the local structure at the atomic level remain sparse at best. Additionally, assessments of the homogeneity of the distribution of metals within the lattice sites are often restricted to techniques such as energy dispersive spectroscopy which might lead to an inaccurate picture of the bulk material. Herein, we report an extensive and systematic study of a class of emerging high-entropy ceramics that uses a combination of high-resolution synchrotron powder diffraction and extended X-ray absorption fine structure analysis. Our data are consistent with a random distribution of atoms with local strain around the d-metals sites, which describes the bulk structure of these materials. Moreover, a linear trend is observed between the average structure and the first-neighbour distances, regardless the number (from 3 to 5) and type (Ti, Zr, Nb, Hf, Ta, Mo, W) of metals that constitute the high-entropy ceramic, which suggests that any description of properties for such materials need to go beyond the simple dichotomy of long-range order and local structure.

Investigation of magnetic, dielectric, optical, and electrical properties of Fe half-doped PrCrO3 perovskite

Conventional solid-state reaction process was adopted to prepare PrFe0.5Cr0.5O3 polycrystalline material. High-temperature X-ray diffraction (XRD) patterns confirmed the formation of a single-phase crystallizing in a Pbnm orthorhombic system, with a thermally activated expansion of the crystal lattice. SEM images revealed a morphology consisting of irregularly shaped grains with a size of 0.16–0.40 ​μm, while EDX analysis confirms the chemical formula PrFe0.5Cr0.5O3. The infrared (IR) spectrum identified a set of absorption peaks attributed to Cr–O and Fe–O vibrations. Magnetic measurements using Mössbauer spectroscopy revealed the onset of a magnetic frustration phenomenon resulting from the random distribution of Fe and Cr atoms. The frequency-dependent dielectric study showed a large value of ε′ in the low-frequency region explained by the Maxwell-Wagner model. The dielectric response was associated with heterogeneous conduction in the grains and grain boundaries, as highlighted by impedance spectroscopy, indicating that the material consists of conductive grains separated from each other by poorly conducting grain boundaries. Negative temperature coefficient resistance behavior was also found in the system. High-temperature dielectric measurements were also carried out. The Jonscher's power-law fit to the AC conductivity data confirms that the conduction is dominated by the small polaron hopping mechanism. The optical band gap was evaluated using the Tauc plot and found to be around 2.02 ​eV, pointing the multifunctionality of this perovskite material.

Hydrogen absorption/desorption reactions of the (TiVNb)85Cr15 multicomponent alloy

Ti-V-Nb-Cr alloys have been reported as potential candidates for hydrogen storage applications. Investigating the processes of absorption and desorption is paramount to understand the hydrogen storage properties of these novel alloys and to develop more efficient hydrogen storage materials. In this work, we investigated the hydrogen absorption/desorption reactions of the (TiVNb)85Cr15 BCC multicomponent alloy by laboratory and synchrotron X-ray diffraction, Pair Distribution Function (PDF) analyses, Transmission Electron Microscopy (TEM) and thermo-desorption analyses (TDS). Hydrogen absorption behavior was studied by pressure-composition-isotherm (PCI) at room temperature, which demonstrated levels of absorption around 2 H/M (H/M = hydrogen-to-metal ratio) with low equilibrium pressure. The results showed a multi-step hydrogenation process: alloy ↔ BCC solid solution ↔ BCC intermediate hydride ↔ FCC dihydride. PDF analyses showed that the partially hydrogenated alloy at different levels of H/M and the fully hydrogenated alloy can be reasonably described by models with phases presenting random atomic distribution in the metal crystallographic sites. Moreover, the partially hydrogenated sample prior to the complete formation of the intermediate hydride showed evidence of two co-existing BCC phases.