Ideal Solid Solution Research Articles

Ideal Co-NiO solid solution with excellent low-temperature propene catalytic combustion performance

Doping engineering of transitional metal oxides offers a great opportunity to further optimize the performance of corresponding catalysts. Here, we presented a solid-state co-precipitation routine to synthesize the Co-NiO ideal solid solution, where Co atoms achieved nearly atomically isolation in the NiO lattice matrix. Remarkably a clear correlation was established between the reactivity for propene combustion and the Co dopant amounts, with the propene combustion rate increasing proportionally to the Co dopant amounts. The homogeneously dispersed Co single atoms or tiny CoOx nanoclusters ensure this linear relationship. More importantly, due to the emergence of Co single atoms and CoOx nanoclusters with a lower coordination number, significant propene adsorption and subsequent C = C bond dissociation were observed in this Co-doped NiO, which improved the propene combustion reaction rate. Therefore, this solid-state co-precipitation strategy provides an effective doping route for building an ideal doped solid solution.

Diffusion-induced stress in crystals: Implications for timescales of mountain building

Intracrystalline chemical diffusion offers valuable insights into the durations of metamorphic and igneous processes. However, it can yield timescale estimates for orogenic and subduction zone events that are considerably shorter than those obtained via isotopic geochronology. One potential explanation that has been offered for the discrepancy is that the interdiffusion of species with different atomic or ionic radii may generate intracrystalline, compositional stresses that alter or limit diffusional relaxation. In this study we test this idea by developing and applying the compositional stress theory of materials scientists F. Larché and J. Cahn to garnet from the Barrovian sillimanite zone, Scotland. Relaxed contacts from the garnet, independent diffusion chronometers, and thermal modeling all indicate a >100 kyr duration for peak temperature metamorphism. Nonetheless, the garnet records sharp, μm-scale variations in calcium and iron contents that standard diffusion treatments predict should relax in 1–10 kyr at peak temperature conditions. Our results show that the development of compositional stress during diffusional relaxation can explain the preservation of the observed short wavelength compositional oscillations at a >100 kyr timescale. Thus, it may be necessary to account for compositional stress when modeling diffusion in solid solutions with appreciable differences in their endmember molar volumes. This will be particularly relevant when considering sharp, μm-scale chemical gradients involving grossular, the garnet endmember with the largest molar volume relative to pyrope, almandine, and spessartine. Neglecting compositional stress in such cases could result in the underestimation of the timescales of lithospheric processes by potentially orders of magnitude. The effects of compositional stress in garnet are predicted to be the most pronounced under amphibolite and blueschist–eclogite facies conditions. At lower temperatures diffusion is limited, and at higher temperatures both plastic deformation and more ideal solid solution behavior will act to diminish the impact of stress.

A Concept of Local Coordination Number for the Characterization of Solute Clusters within Atom Probe Tomography Data.

Identifying clusters of solute atoms in a matrix of solvent atoms helps to understand precipitation phenomena in alloys, for example, during the age hardening of certain aluminum alloys. Atom probe tomography datasets can deliver such information, provided that appropriate cluster identification routines are available. We investigate algorithms based on the local composition of the neighborhood of solute atoms and compare them with traditional approaches based on the local solute number density, such as the maximum separation distance method. For an ideal solid solution, the pair correlation functions of the kth nearest solute atom in the coordination number representation are derived, and the percolation threshold and the size distribution of clusters are studied. A criterion for selecting optimal control parameters based on maximizing the phase separation by the degree of clustering is proposed for a two-phase system. A map of phase compositions accessible for cluster analysis is constructed. The coordination number approach reduces the influence of density variations commonly observed in atom probe tomography data. Finally, a practical cluster analysis technique applied to the early stages of aluminum alloy aging is described.

Charge transfer effects in (HfNbTiVZr)C—Shown by ab initio calculations and X‐ray photoelectron spectroscopy

AbstractConsidering charge transfer effects and the variability of the bonding between elements with different electronegativity opens up a deeper understanding of the electronic structure and as a result many of the properties in high entropy‐related materials. This study investigates the importance of the diverse bonding and chemical environments when discussing multicomponent carbide materials. A combination of ab initio calculations and X‐ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of multicomponent thin films based on the (HfNbTiVZr)C system. The charge transfer was quantified theoretically using relaxed and nonrelaxed multicomponent as well as binary carbide reference structures, employing a fixed sphere model. High‐resolution XPS spectra from (HfNbTiVZr)C magnetron‐sputtered thin films displayed core‐level binding energy shifts and broadening effects as a result of the complex chemical environment. Charge transfer effects and a changed electronic structure in the multicomponent material, compared with the reference binary carbides, are observed both experimentally and in the density functional theory (DFT) simulations. The observed effects loosely follow electronegativity considerations, leading to a deviation from an ideal solid solution structure assuming nondistinguishable chemically equivalent environments.

Structural Evolution and Photoluminescence Quenching across the FASnI3-xBrx (x = 0-3) Perovskites.

One of the primary methods for band gap tuning in metal halide perovskites has been halide (I/Br) mixing. Despite widespread usage of this type of chemical substitution in perovskite photovoltaics, there is still little understanding of the structural impacts of halide alloying, with the assumption being the formation of ideal solid solutions. The FASnI3-xBrx (x = 0-3) family of compounds provides the first example where the assumption breaks down, as the composition space is broken into two unique regimes (x = 0-2.9; x = 2.9-3) based on their average structure with the former having a 3D and the latter having an extended 3D (pseudo 0D) structure. Pair distribution function (PDF) analyses further suggest a dynamic 5s2 lone pair expression resulting in increasing levels of off-centering of the central Sn as the Br concentration is increased. These antiferroelectric distortions indicate that even the x = 0-2.9 phase space behaves as a nonideal solid-solution on a more local scale. Solid-state NMR confirms the difference in local structure yielding greater insight into the chemical nature and local distributions of the FA+ cation. In contrast to the FAPbI3-xBrx series, a drastic photoluminescence (PL) quenching is observed with x ≥ 1.9 compounds having no observable PL. Our detailed studies attribute this quenching to structural transitions induced by the distortions of the [SnBr6] octahedra in response to stereochemically expressed lone pairs of electrons. This is confirmed through density functional theory, having a direct impact on the electronic structure.

High-throughput approach for investigating interdiffusion in medium- and high-entropy alloys

Interdiffusion experiments are usually time-consuming and tedious since diffusion couples must be annealed at several temperatures for a long time. The efforts required to study interdiffusion in multicomponent alloys increase dramatically as multiple diffusion couples are required to cover broad composition ranges and determine the diffusivities of individual elements in different chemical environments. To circumvent this challenge, we present a high-throughput approach applicable to single-phase and compositionally complex alloys, which are assumed to approximate ideal solid solutions. Here, a simple diffusion-multiple experiment combined with a physically based kinetic model is proposed to efficiently determine the diffusion coefficients of the constituent elements in quaternary CrFeCoNi alloys. Compared with tracer diffusivities reported in the literature, the results, thus, obtained do not differ by more than a factor of 2 and were obtained from a single interdiffusion experiment. In contrast, the diffusivities simulated with commercial mobility and thermodynamic databases are strongly overestimated by a factor ranging from 1 to 16. Therefore, our approach enables high-throughput determination of diffusivities and can help in the design of alloys for high-temperature applications where diffusion plays a key role.

Impact of local chemical ordering on deformation mechanisms in single-crystalline CuNiCoFe high-entropy alloys: a molecular dynamics study

High-entropy alloys (HEAs), composed of multiple constituent elements with concentrations ranging from 5% to 35%, have been considered ideal solid solution of multi-principal elements. However, recent experimental and computational studies have demonstrated that complex enthalpic interactions among constituents lead to a wide variety of local chemical ordering (LCO) at lower temperatures. HEAs containing Cu typically decompose by forming of Cu-rich phases during annealing, thus affecting mechanical properties. In this study, CuNiCoFe HEA was chosen as a model with a tendency for Cu segregation at low temperatures. The formation of LCO and its impact on the deformation behaviors in the single-crystalline CuNiCoFe HEA were studied via molecular dynamics simulations. Our results demonstrate that CuNiCoFe HEA decomposes by Cu clustering, in agreement with prior experimental and computational studies, owing to insufficient configuration entropy to compete against the mixing enthalpy at lower temperatures. A softening in ultimate stress in the LCO models was observed compared to the random solid solution models. The softening is due to the lower unstable stacking fault energy, which determines the nucleation event of dislocations, thereby rationalizing the dislocation nucleation in the Cu-rich regions and the softening of the overall ultimate strength in the LCO models. Additionally, the inhomogeneous FCC–BCC transformation is closely associated with concentration inhomogeneity. CuNiCoFe HEA with LCO can be regarded as composites, consisting of clusters with different properties. Consequently, concentration inhomogeneity induced by LCO profoundly impacts the mechanical properties and deformation behaviors of the HEA. This study provides insights into the effect of LCO on the mechanical properties of CuNiCoFe HEAs, which is crucial for developing HEAs with tailored properties for specific applications.

Multi-principal rare-earth Gd-Tb-Dy-Ho-Er alloys with high magnetocaloric performance near room temperature

Magnetocaloric materials with enhanced magnetocaloric effect and high magnetic transition temperature are crucial for magnetic refrigeration, a novel cooling technology featuring high energy efficiency, low noise and superb environmental friendliness. In this study, a series of Gdx(TbDyHoEr)1-x (x=0.2–0.9) rare-earth alloys with a single-phase HCP (hexgonal-close-packed) structure, were developed utilizing a multi-principal element alloying strategy. It was found that the Gd0.8(TbDyHoEr)0.2 alloy exhibits a high magnetic transition temperature (i.e., ∼268 K), and concurrently enhanced magnetocaloric properties with a maximum magnetic entropy change value of 10.53 J kg−1 K−1 under a field change of 5 T and a refrigerant capacity value of 751.8 J kg−1. Notably, for these multi-principal element alloys, the maximum magnetic entropy change is essentially linearly correlated with the configuration entropy of mixing while the magnetic transition temperature scales with the de Gennes factor, which can be interpreted as a manifestation of ideal solid solutions. This work provides important implications for designing high-performance magnetocaloric materials for magnetic refrigeration.

Determination of short-range order in TiVNbHf(Al)

The presence of short-range chemical order can be a key factor in determining the mechanical behavior of metals, but directly and unambiguously determining its distribution in complex concentrated alloy systems can be challenging. Here, we directly identify and quantify chemical order in the globally single phase BCC-TiVNbHf(Al) system using aberration corrected scanning transmission electron microscopy (STEM) paired with spatial statistics methods. To overcome the difficulties of short-range order (SRO) quantification with STEM when the components of an alloy exhibit large atomic number differences and near equiatomic ratios, “null hypothesis” tests are used to separate experiment from a random chemical distribution. Experiment is found to deviate from both the case of an ideal random solid solution and a fully ordered structure with statistical significance. We also identify local chemical order in TiVNbHf and confirm and quantify the enhancement of SRO with the addition of Al. These results provide insight into local chemical order in the promising TiVNbHf(Al) refractory alloys while highlighting the utility of spatial statistics in characterizing nanoscale SRO in compositionally complex systems.

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.

Spatial inhomogeneity of point defect properties in refractory multi-principal element alloy with short-range order: A first-principles study

Short-range order can be developed in multi-principal element alloys and influences the point defect behavior due to the large variation of the local chemical environment. The effect of short-range order on vacancy and interstitial formation energy and migration behavior was studied in body-centered cubic multi-principal element alloy NbZrTi by first-principles calculations. Two short-range order structures created by density functional theory and Monte Carlo method at 500 and 800 K were compared with the structure of random solid solution. Both vacancy and interstitial formation energies increase with the degree of short-range order. Point defect formation energies tend to be higher in regions enriched in Nb and lower in regions enriched in Zr and Ti. Both vacancies and interstitials prefer to migrate toward Zr,Ti-rich regions and away from Nb-rich regions, suggesting that Zr,Ti-rich regions can potentially act as recombination centers for point defect annihilation. Compared to an ideal random solid solution, the short-range order increases the spatial inhomogeneity of point defect energy landscape. Tuning the degree of short-range order by different processing techniques can be a viable strategy to optimize the point defect behavior to achieve enhanced radiation resistance in multi-principal element alloys.

Simulation of Co-60 uptake on stainless steel and alloy 690 using the OSCAR v1.4 code integrating an advanced dissolution-precipitation model

The contamination of a nuclear cooling system by activated corrosion products (ACPs) is a process that involves many different mechanisms all interacting with each other. One of the most important mechanisms is dissolution-precipitation. This governs the transfer of soluble corrosion products between the circulating water and the immobile oxidized surfaces, and is strongly dependent on the water chemistry. The dissolution-precipitation model was improved in version 1.4 of the OSCAR computer code, which simulates the ACPs transfer in nuclear reactor systems. The OSCAR v1.4 code is now able to better calculate the incorporation of minor species (e.g., a cobalt isotope) into oxides using the chemistry module, PHREEQCEA, which determines the composition of an ideal solid solution and the equilibrium concentrations of elements in the aqueous solution. This model was challenged by comparing the results obtained using OSCAR v1.4 with the experimental results of a test performed in a dedicated loop by Studsvik Nuclear AB. Finally, with this model, the OSCAR v1.4 code accurately reproduces soluble 60Co uptake on stainless steel and alloy 690 under various experimental conditions (pH, Zn injection and flow rate).

The interplay between size, shape, and surface segregation in high-entropy nanoalloys.

The respective influences of particle shape and size on the energetic stability of five-component multimetallic nanoparticles have been computationally investigated for AlCuFeCrNi and AuCuPdNiCo mixtures at equiconcentration. Using available embedded-atom model potentials, exchange Monte Carlo simulations possibly assisted with systematic quenching, we explore tools to approach ideal phase equilibrium in such high-entropy nanoalloys. In particular, we show how deviations to ideal solid solution behaviors can be characterized using percolation analyses, and how the contribution of alloying fluctuations at finite temperature can be inferred to evaluate the entropy of mixing in such nonideal cases. An approximation to the entropy of mixing based on pair correlations only is also found to capture the behavior of the thermodynamical mixing entropy quite well, and can be used as an order parameter of mixing. While the AlCuFeCrNi mixture appears to mix reasonably well in all cases considered, cobalt and nickel segregate significantly in AuCuPdNiCo nanoparticles, deviating strongly from ideal random mixtures. A simple Gaussian regression model applied to a coarse distribution of concentrations is found to correctly predict conditions for optimising the mixing thermodynamical properties of the miscible AlCuFeCrNi nanoparticle.

Single Crystalline, Non‐stoichiometric Cocrystals of Hydrogen‐Bonded Organic Frameworks

AbstractPorous frameworks composed of non‐stoichiometrically mixed multicomponent molecules attract much attention from a functional viewpoint. However, their designed preparation and precise structural characterization remain challenging. Herein, we demonstrate that cocrystallization of tetrakis(4‐carboxyphenyl)hexahydropyrene and pyrene derivatives (CP‐Hp and CP‐Py, respectively) yields non‐stoichiometric mixed frameworks through networking via hydrogen bonding. The composition ratio of CP‐Hp and CP‐Py in the framework was determined by single crystalline X‐ray crystallographic analysis, indicating that the mixed frameworks were formed over a wide range of composition ratios. Furthermore, microscopic Raman spectroscopy on the single crystal indicates that the components are not uniformly distributed such as ideal solid solution, but are done gradationally or inhomogeneously.

Single Crystalline, Non-stoichiometric Cocrystals of Hydrogen-Bonded Organic Frameworks.

Porous frameworks composed of non-stoichiometrically mixed multicomponent molecules attract much attention from a functional viewpoint. However, their designed preparation and precise structural characterization remain challenging. Herein, we demonstrate that cocrystallization of tetrakis(4-carboxyphenyl)hexahydropyrene and pyrene derivatives (CP-Hp and CP-Py, respectively) yields non-stoichiometric mixed frameworks through networking via hydrogen bonding. The composition ratio of CP-Hp and CP-Py in the framework was determined by single crystalline X-ray crystallographic analysis, indicating that the mixed frameworks were formed over a wide range of composition ratios. Furthermore, microscopic Raman spectroscopy on the single crystal indicates that the components are not uniformly distributed such as ideal solid solution, but are done gradationally or inhomogeneously.

Peculiarities of the local structure in new medium- and high-entropy, low-symmetry tungstates

New monoclinic (P2/c) tungstates – a medium-entropy tungstate, (MnNiCuZn)WO4, and a high-entropy tungstate, (MnCoNiCuZn)WO4 – were synthesized and characterized. Their phase purity and solid solution nature were confirmed by powder X-ray diffraction and Raman spectroscopy. X-ray absorption spectroscopy was used to probe the local structure around metal cations. The atomic structures based on the ideal solid solution model were optimized by a simultaneous analysis of the extended X-ray absorption fine structure spectra at multiple metal absorption edges – five for (MnNiCuZn)WO4 and six for (MnCoNiCuZn)WO4 – by means of reverse Monte Carlo simulations. In both compounds, Ni2+ ions have the strongest tendency to organize their local environment and form slightly distorted [NiO6] octahedra, whereas Mn2+, Co2+, and Zn2+ ions have a strongly distorted octahedral coordination. The most intriguing result is that the shape of [CuO6] octahedra in (MnNiCuZn)WO4 and (MnCoNiCuZn)WO4 differs from that found in pure CuWO4, where a strong Jahn–Teller distortion is present: [CuO6] octahedra become more regular with increasing degree of dilution.

Short range ordering improves elastic properties of Mo additive W-Re solid solution: A first principles investigation

Re can significantly improve the ductility of W, while it is prone to form clusters and intermetallic phases, leading to instability and brittleness of W. To explore better ways to inhibit the adverse effects, we explored combined effect of Mo and alloying order on the phase stability and mechanical properties of W-Re alloys by using the first principles method. It is found that the short range ordered (SRO) W-Re structure is softer than that of random and ideal solid solution alloys. While SRO W-Mo-Re structure possesses higher stability and strength than those of random distributed alloys. The SRO W-Mo-Re structure can promote the coherent bonding enhancement between W, Mo and Re. This study shed light on better design of alloy additions and control of alloy orders to modulate the softening and strengthening properties in W and W-Re alloys, especially to reduce the hardening and instability induced by Re rich clusters.

Enthalpies of formation and standard entropies for some potassium Tutton salts

Differential scanning calorimetry (DSC) was used to measure the enthalpies of reaction for the dehydration of the potassium Tutton salts, K2M(SO4)2. 6 H2O with M = Mg, Co, Ni, Cu and Zn. The values determined ranged from 335 kJ mol−1 for K2Mg(SO4)2 6 H2O to 355 kJ mol−1 for K2Ni(SO4)2. 6 H2O with a measured standard deviation of ± 5 kJ mol−1. Although the information needed to obtain precise values for the enthalpies of formation is not available in the literature for all of these salts, values calculated by modeling the amorphous dehydrated compound as an ideal solid solution produced values within 10 kJ mol−1 of the values determined for K2M(SO4)2. 6 H2O (M= Mg, Cu, and Zn) where the information needed for this calculate was available. DSC was also used to determine the entropies of reaction for the dehydration of these salts. Since there is little information about the entropies of these compounds in the literature, the entropies of reaction were used with the ideal solution model for the amorphous compound to estimate the standard molar entropies for these salts. The values determined ranged from 490 J K−1 mol−1 for K2Cu(SO4)2. 6 H2O to 540 J K−1 mol−1 for K2Co(SO4)2. 6 H2O. Since these values are based upon estimated values, they have an estimated error of ± 5 percent.

A Kinetic Monte Carlo Approach to Model Barite Dissolution: The Role of Reactive Site Geometry

Barite (Ba[SO4]) is one of the promising candidates for sequestration of radioactive waste. Barite can incorporate radium (Ra) and form ideal solid solutions, i.e., (Ba,Ra)[SO4]. Together with isostructural celestite (Sr[SO4]), ternary solid solutions, (Ba,Sr,Ra)[SO4], may exist in natural conditions. Our fundamental understanding of the dissolution kinetics of isostructural sulfates is critically important for a better risk assessment of nuclear waste repositories utilizing this mineral for sequestration. So far, the barite-water interface has been studied with experimental methods and atomistic computer simulations. The direct connection between the molecular scale details of the interface structure and experimental observations at the microscopic scale is not yet well understood. Here, we began to investigate this connection by using a kinetic Monte Carlo approach to simulate the barite dissolution process. We constructed a microkinetic model for the dissolution process and identified the reactive sites. Identification of these sites is important for an improved understanding of the dissolution, adsorption, and crystal growth mechanisms at the barite–water interface. We parameterized the molecular detachment rates by using the experimentally observed etch pit morphologies and atomic step velocities. Our parameterization attempts demonstrated that local lattice coordination is not sufficient to differentiate between the kinetically important sites and estimate their detachment rates. We suggest that the water structure and dynamics at identified sites should substantially influence the detachment rates. However, it will require more work to improve the parameterization of the model by means of Molecular Dynamics and ab initio calculations.

Temperature-dependent enthalpy and entropy stabilization of solid solution phases in non-equiatomic CoCrFeNiTi high entropy alloys: computational phase diagrams and thermodynamics

Research interest in multi-principal element high entropy alloys (HEAs) has increased drastically since the field was first formally introduced in 2004. Since then, HEAs have become important candidate materials for many key applications. However, despite the progress made in this field, there remains much ambiguity surrounding HEA phase stabilities. To that end, the calculation of phase diagrams (CALPHAD) method was used to construct extensive temperature-composition phase diagrams of the CoCrFeNi x Ti2−x , Co x CrFeNiTi2−x , CoCrFe x NiTi2−x , and CoCr x FeNiTi2−x HEA systems. Due to its potentially favorable properties, the current work was focused on the single face-centered cubic (fcc) solid solution phase and an extensive thermodynamic analysis was carried out to examine the underlying thermodynamic factors of its stabilization. The mixing enthalpies and entropies of the alloys in the studied systems were calculated, where it was found that the single fcc solid solution phase can be either enthalpy- or entropy-stabilized depending on the temperature. The deviation of these quantities from the ideal solid solution thermodynamic behavior was considered, and it was found that close to and within the single fcc solid solution regions, the deviation is smallest in all systems. Furthermore, a preliminary exploration of the impact of interstitial nonmetals such as C, N, and O showed noticeable alteration of the phase equilibria of the studied systems. This work emphasizes the importance of exploring non-equiatomic compositions of HEAs as well as the necessity of a comprehensive thermodynamic analysis to understand HEAs phase stabilities.