The tungsten-assisted vacuum distillation technique has emerged as a critical method for high-purity rare-earth metal (e.g., Sc) purification. Nevertheless, the mechanism underlying W-facilitated impurity (e.g., Al) removal remains controversial. Here, by combined first-principles calculations and chemical bonding analyses, the interactions between Sc/W and impurity Al were quantitatively investigated employing the B2-type ScAl and WAl models that were established to approximate local coordination environments during distillation. Results show that both the Sc-Al and W-Al bonds exhibit hybrid covalent-ionic characteristics, in which the ionicity of the Sc-Al bond is higher than that of the W-Al bond. The strength of the W-Al bond, governed by the p-d covalent hybridization, exhibits a 1.6-fold enhancement compared to the Sc-Al interactions, and the W-W bond in the WAl model is also stronger than the Sc-Sc bond in the ScAl model. The higher strengths of both W-Al and W-W bonds make the WAl model a higher cohesive energy against the ScAl model. Nevertheless, unlike the inherent stability of the ScAl model, the WAl model is unstable in thermodynamics and dynamics due to the prominent antibonding states of the Al-Al and W-W bonds near the Fermi level, highlighting that enhanced bonding strength does not necessarily guarantee stability. These quantitative bonding and stability evaluations suggest that the established purification models are not suitable for this study. A novel atomic-scale adsorption-mediated purification model where W preferentially binds Al through strong covalent interactions is proposed. This work provides fundamental insights into the W-facilitated purification process and demonstrates the critical role of orbital-level bonding analysis in high-purity metal purification.

Heusler alloys are renowned for their outstanding functional properties but suffer from inherent brittleness that limits wide applications. Addressing this limitation demands a fundamental understanding of the origins of inherent brittleness. In this study, by a combined first-principles calculations and quantum chemical bonding analysis, the electronic structure origin of the brittleness of Heusler alloys was systematically investigated, taking Ni2MGa (M = Cr, Mn, Fe, and Co) as representative systems. Contrary to conventional understanding, we find that the interatomic chemical bonding is not the dominant factor governing brittleness in Heusler alloys. Instead, our analysis reveals pronounced electron localization at tetrahedral interstitial sites, which exhibits a strong correlation with the brittleness-ductility. By further incorporating the localization of interatomic bonding electrons, a robust linear relation is established. Based on these insights, we introduce a mean valence electron localization (MVEL) descriptor that integrates both interatomic and interstitial electronic localization, which effectively captures the brittleness-ductility of the Ni2MGa alloys. The validity of MVEL can be well expanded other Ni-, Co-, and Mn-based Heusler alloys. In addition, beyond valence electron concentration, we reveal that electronegativity difference between constituent elements, intricately linked to ionicity of chemical bonding, also critically influences global electron localization. This work provides a solid theoretical foundation for understanding the origin of brittleness and for guiding the design of advanced Heusler alloys with enhanced ductility.

Phase engineering is crucial for tailoring the properties of transition-metal alloys, yet the stability competition between close-packed fcc and hcp phases remains unclear. Here, by combining first-principles calculations with chemical-bonding analysis, we reveal that their competition is governed by a valence electron concentration (VEC)-mediated band-filling mechanism. Unlike the unimodal electronic density of state (DOS) of fcc, hcp exhibits a shallow-pseudogap that lowers band energy at medium VEC (similar to 8), thereby stabilizing hcp. Orbital analysis attributes this pseudogap to the redistribution of e(g) states arising from the change in the close-packed plane stacking sequence from ABCABC (fcc) to ABAB (hcp).

Martensitic transformation from high-temperature B2 phase to the low-temperature B19 ' phase in NiTi shape-memory alloys exhibits anomalous lattice expansion-a long-standing puzzle whose underlying mechanism has remained elusive. Here, we resolve this issue by introducing quantum-chemical bonding analysis, revealing that on-site electron interactions govern the phase transition. Quantitative analysis shows reduced bonding strength in the B19 ' phase, accounting for the volume anomaly. Crucially, the enhanced stability of the B19 ' phase arises from the contribution of on-site electrons near the Fermi level to the band-structure energy due to symmetry-breaking crystal-field effect, suggesting a cooperative Jahn-Teller-like distortion. Our findings uncover a "weakened-yet-stabilized" picture for martensitic transformations, providing novel electronic-scale insights into anomalous phase transitions.

Generating a pre-strain by mechanical loading during martensitic transformation stands as a crucial strategy to obtain memory effect in shape memory alloys (SMAs). As martensitic transformation is realized by an anisotropic lattice deformation, the formation of martensite variants is always governed by strain accommodation. In a stress-free state, the orientation variants are organized hierarchically into colonies with a fixed number of variants. Under an external load, the transformation becomes selective. Although variant selection has long been a subject of interest, knowledge on selection via the activation of the transformation shear system under a load and by local strain mitigation is limited. Here, by a combined in-situ neutron diffraction and exhaustive EBSD crystallographic examination, the variant selection under a compressive load during martensitic transformation was thoroughly investigated using Ni51Mn34In15 as an example alloy. Remarkably, a dual-scale selection mechanism, i.e., colony and intra-colony variants, was revealed, which is in stark contrast to the stress-free scenario. For colonies, those containing variants receiving the highest resolved shear stress on their dominant transformation shear system were selected. Within the colonies, the selection is on variant volume fraction. Those making the maximum contribution to the external compression strain were majorly selected. Nevertheless, due to local incompatible strains created by the favorable variants, the variants with deformation opposite to the external compression were also selected to mitigate local incompatible strain and promote further formation of the favorable variants. This study provides useful experimental evidence and analysis data for related crystal plasticity modeling and simulation.

Designing the high-performance magneto- or elasto-caloric effect in NiMnIn-based alloys with spin-lattice coupling in a deep-freezing temperature range of similar to 200 K to similar to 255 K is challenging due to the limited lattice entropy change |Delta S-lat| and large negative contribution of magnetic entropy change |Delta S-mag| during phase transitions. In this work, we systematically study the first-order magneto-structural transition in NiMnIn-based alloys by in-situ microstructural characterizations, physical property measurements, and first-principles calculations. A multi-element alloying strategy involving Cu and Ga co-doping is proposed to manipulate the phase transition. The co-doping reduces the lattice anharmonicity and thermal expansion coefficient of the martensitic phase, leading to an increase in the unit cell volume change Delta V and |Delta S-lat|. It also modifies the electronic density of states, causing a decrease in the magnetization change Delta M and |Delta S-mag|. The relief of the lattice mismatch reduces hysteresis losses in the refrigeration cycle. These synergetic effects yield excellent magneto- and elasto-caloric effects, with the effective magnetocaloric refrigeration capacity reaching up to similar to 182 J kg(-1) under the magnetic field of 5 T or an adiabatic temperature change of -4 K under a low field of 1.5 T and an adiabatic temperature change of -7 K with the strain of 5 % at similar to 230 K for the first field application. In addition, a cyclic MCE |Delta T-ad| of 1.4 K and a cyclic eCE |Delta T-ad| of 3.1 K are obtained. The excellent properties of co-doping alloy offer a potential solution for solid-state deep-freezing refrigeration.

The first-order magneto-structural transition with large adiabatic magnetic entropy change shows great potential for magnetocaloric refrigeration but is hindered by large thermal and magnetic hysteresis effects, such as in the MM'X alloys (M, M' = 3d transition element, X = Si, Ge). While progress has been made in reducing thermal hysteresis through improved geometric compatibility, the joint effects of thermal and magnetic hysteresis are not well understood, which is a crucial issue in the practical operation of magnetocaloric refrigeration. In this work, we present an in-situ study of the temperature effects on the magnetic-field-driven magneto-structural transition in MnCoGe-based compounds. Our results show that the metamagnetic transition from paramagnetic hexagonal to ferromagnetic orthorhombic structures is more easily driven by an external magnetic field during cooling compared to heating. The magnetization loop exhibits larger hysteresis losses during cooling, while the magneto-structural transition temperature determined from iso-field magnetizations is more affected by the magnetic field. Additionally, the negative thermal expansion behavior displays a more uniform distribution within the entire phase transition temperature window during cooling. In-situ X-ray diffraction measurements and geometric nonlinear theory analysis suggest that those discrepancies during heating and cooling originate from smaller lattice geometric incompatibilities, particularly along the a-axis of the orthorhombic structure. Furthermore, it is found that the transformation stretch tensor eigenvalues exhibit linear behavior, converging towards 1 and indicating improved compatibility, potentially leading to reduced thermal hysteresis effects at lower temperatures. These findings offer insights into magneto-structural transitions and hysteresis behaviors, informing the development of high-performance magnetocaloric materials.

Lattice volume dependence of phase stability, i.e., the contracted lattice of crystal tends to elevate the temperature of phase transition, is a well-known criterion to guide materials design in the Ni-Mn(Fe)-based magnetic shape memory alloys. However, there exist some confusing exceptions, such as Ni2Mn(Ga, Al), which are not well understood. In this work, the structural stabilities, phonon curves and electronic structures of the isoelectronically substituted Ni2MnGa1-xAlx (x = 0-1) alloys were studied by first-principles calculations. We find that the abnormal structural stability in the examined alloys is an intrinsic effect resulting from the variation of chemical composition. Al element has an intrinsically stronger ability to stabilize the austenite compared with Ga. In terms of electronic structures, both the Fermi surface nesting and the minority-spin electronic states near the Fermi level, the usual driving forces of structural transition, are insensitive to Al substitution. Bonding analyses indicate that the reduced covalent hybridization between Ni and sp elements (Al, Ga) governs the structural transformation. The ionic interactions between Ni and sp element also affect the phase stability. These covalent and ionic interactions could be the origin of the abnormal dependence of structural stability on lattice volume in the studied alloys. The results of this work clarify the abnormal lattice-volume-related phase stability and thus lay a theoretical foundation for the design of advanced magnetic shape memory alloys.

Precipitation is one of a few effective ways enabling to reduce the brittleness of NiFeGa magnetic shape memory alloys. However, the physical origin behind precipitation and the key factors deciding precipitation remain unknown. There is still a lack of available methods to control the precipitation of NiFeGa. To this end, the precipitation of Ni2FeGa is systemically studied by combining first-principles calculations and experimental examinations from the following four aspects: the stability of the matrix, the crystal structure and stability of precipitates, the key factors dominating precipitation and the strategy of tailoring precipitation. Results show that the L21 matrix phase possesses weak elastic and dynamic stabilities, which may be linked to the occurrence of precipitation. It is identified theoretically and experimentally that the precipitate has L12 rather than the previously reported FCC structure. The underlying mechanism was clarified from electron density of state and chemical bond. Valence electron concentration e/a and magnetism are found to be the dominant factors affecting the relative stability of precipitate against matrix with sensitivity coefficients of -175.1 and 67.4 meV, respectively. The increase of e/a prefers stabilizing the precipitate, but the enhanced magnetism favors the matrix. Conversely, the influence of lattice volume on precipitation is weak from the aspects of both sensitivity and adjustable range. Lastly, an effective strategy of tuning precipitation, i.e., by tailoring e/a and magnetism, is proposed and verified. This work is expected to lay a theoretical foundation for tailoring precipitation and further optimizing the mechanical and functional performances of ferromagnetic Heusler alloys.

Finite-temperature ductility-brittleness and electronic structures of Al$_3$Sc, Al$_2$Sc and AlSc are studied comparatively by first-principles calculations and ab-initio molecular dynamics. Results show that Al$_3$Sc and Al$_2$Sc are inherently brittle at both ground state and finite temperatures. By contrast, AlSc possesses a significantly superior ductility evaluated from all Pugh's, Pettifor's and Poisson's ductility-brittleness criteria. At ground state, AlSc meets the criteria of ductile according to Pugh's and Poisson's theories, while it is categorized as the brittle in the frame of Pettifor's picture. With the increasing temperature, the ductility of all the studied compounds exhibits a noticeable improvement. In particular, as the temperature rises, the Cauchy pressure of AlSc undergoes a transition from negative to positive. Thus, at high temperatures (T > 600 K), AlSc is unequivocally classified as the ductile from all criteria considered. In all Al$_3$Sc, Al$_2$Sc and AlSc, the Al-Al bond, originated from s-p and p-p orbital hybridizations, and the Al-Sc bond, dominated by p-d covalent hybridization, are the first and second strongest chemical bonds, respectively. To explain the difference in mechanical properties of the studied compounds, the mean bond strength (MBS) is evaluated. The weaker Al-Al bond in AlSc, leading to a smaller MBS, could be the origin for the softer elastic stiffness and superior intrinsic ductility. The longer length of the Al-Al bond in AlSc is responsible for its weaker bond strength. Furthermore, the enhanced metallicity of the Al-Al bond in AlSc would also contribute to its exceptional ductility.

Exploring novel magnetic Heusler alloys is of great significance for the development of a new generation of smart sensing materials. The A(2)BC type magnetic alloy, which comprises transition magnetic metal elements A and B and III-V main group element C (p-block element), has gained significant attention due to its various physical and chemical properties, including semimetallic magnetism, ferromagnetic shape memory effect, multicaloric effect, and superconductive effect. In this study, eight new A(2)BTi type magnetic functional alloys, including three Co-based alloys (Co2MnTi, Co2FeTi, and Co2NiTi), three Fe-based alloys (Fe2MnTi, Fe2CoTi, and Fe2NiTi), and two Ni-based alloys (Ni2FeTi and Ni2CoTi), were investigated for their phase stability against tetragonal distortion using first-principles calculation. The underlying mechanism for the stability of the L2(1) phase was discussed. The results show that valence electron concentration and magnetism are the key parameters in determining the structural stability of L2(1) phase in A(2)BTi type alloys. Co2NiTi, Fe2NiTi, and Ni2CoTi alloy samples, whose L2(1) structure is an unstable phase, were prepared, and their crystal structure, phase transformation, magnetic properties, electrical resistance, and mechanical properties were investigated experimentally. The results show that at 298 K, Co2NiTi is composed of an ordered face-centered cubic L1(2) structured matrix phase and a hexagonal Co3Ti-type second phase, Fe2NiTi is composed of a hexagonal Fe2Ti-type matrix phase and a tetragonal FeNi-type second phase, and Ni2CoTi has a single hexagonal Ni3Ti-type structure. The fact that no compound undergoes a first-order structural phase transition may be due to the weak stabilities of their L2(1) phases. Fe2NiTi and Ni2CoTi have strong magnetic properties and undergo a second-order Curie magnetic transition during cooling. Fe2NiTi has high compressive strength (1280 MPa), moderate compressive strain (5%), and large resistance (120 mu Omega.cm), while Co2NiTi and Ni2CoTi have excellent compressive plasticity and small resistance. This phenomenon may be related to the different proportions of metallic and covalent bonding caused by the difference in valence electron concentration of the three alloys.

Covalent organic frameworks (COFs) have emerged as a new kind of crystalline porous material constructed from organic building blocks through dynamic covalent bonds. The desired COFs can be synthesized through the condensation of pre-designed building blocks. With multitudinously unique characteristics, such as tunable aperture, high specific surface area, excellent stability, enzyme-like activities, and fluorescent properties, COFs have found widespread applications in the field of analytical chemistry. This review introduces the structure and properties of COFs to illustrate how they can be purposefully constructed according to specific application requirements. Subsequently, we detail the analytical applications and advancements of COFs in sample pretreatment, separation science, and chemical sensing. The enzyme-like activities of COFs and the sensing strategies of COFs-based nanozymes are also summarized. Furthermore, potential challenges and further development prospects are also discussed for the further advancement of COFs in analytical applications.

Tetragonal ratio (c/a) is a critical structural parameter that heavily decides plenty of performances of tetragonal martensitic materials. Nevertheless, the knowledge about c/a remains limited and there is still no definitive strategy to tailor it. In this work, the machine learning method, combined with high-throughput ab-initio calculation, was introduced as an attempt to investigate c/a. After training, a high-precision random forest model of predicting c/a was established. Combining a five-step descriptor screening and visualization analyses, the most pertinent parameters deciding c/a, i.e., mean melting point (TM) and mean volume (V) of constituent elements, were identified. Furthermore, a simple relation between c/a and these two parameters, i.e., c/a = m TM/V + b where the m and b are constants, was proposed. Surprisingly, this relation exhibits an exceptional generalization performance in a wide range of untrained materials. This work is expected to provide an explicit, efficient and general route to tailor c/a and hence promote the design of advanced martensitic materials.

Impacts of Mn alloying on lattice stabilities, magnetic properties, electronic structures of the bcc and fcc phases and the fcc→bcc phase transition in Fe16−xMnx (x = 0, 1 and 2) alloys are studied by first-principles calculations. Results show that the doped Mn atom prefers ferromagnetic and antiferromagnetic interaction with the host Fe atoms in the bcc and fcc phases, respectively. In these two phases, the magnetic moment of Mn is smaller and larger than Fe, respectively. The local moment of Fe is decided by the Fe-Mn distance in the bcc phase, whereas in the fcc phase, it is determined by spatial orientation with Mn. In the different phases, Mn prefers different site occupations, which can be understood from the electronic density of states near Fermi energy, implying a possibility of element redistribution during phase transition. The driving force of phase transition decreases with Mn alloying. Both destabilized bcc phase and stabilized fcc phase contribute to the inhibited phase transition, but the latter plays a dominant role. Antiferromagnetism is recognized as the key reason for the enhanced stability of the fcc phase by Mn alloying.

The poor mechanical properties will result in low fatigue life and limited working temperature window of elastocaloric effect (eCE) for ferromagnetic shape memory alloys with lower critical stress. In this work, a microstructure design strategy, i.e., simultaneously introducing strong texture and ductile interfacial.-phase, was adopted to reduce the strain discontinuity near the grain boundary and enhance the grain boundary cohesion, further improving the mechanical properties and eCE performance. With this strategy, the compressive strength and strain of the prototype Ni54Fe19Ga27 alloy can exceed 1.5 GPa and 30.0% at room temperature, respectively. Furthermore, a wide working temperature window of 120 K with an adiabatic temperature change of -8.5 to -1.2 K and superior eCE fatigue life of 2 x 10(4) cycles with an adiabatic temperature change of similar to 3 K have been achieved. This work is expected to provide an effective strategy to enhance the eCE performance of ferromagnetic shape memory alloys.

A large adiabatic temperature change (Delta T-ad) is a prerequisite for the application of elastocaloric refrigeration. Theoretically, a large volume change ratio (Delta V/V-0) during martensitic transformation is favorable to enhance Delta T-ad. However, the design or prediction of Delta V/V-0 in experiments is a complex task because the structure of martensite changes simultaneously when the lattice parameter of austenite is tuned by modifying chemical composition. So far, the solid strategy to tailor Delta V/V-0 is still urgently desirable. In this work, a first-principles-based method was proposed to estimate Delta V/V-0 for Ni-Mn-based alloys. With this method, the substitution of Ga for In is found to be an effective method to increase the value of Delta V/V-0 for Ni-Mn-In alloys. Combined with the strategies of reducing the negative contribution of magnetic entropy change (via the substitution of Cu for Mn) and introducing strong crystallographic texture (through directional solidification), an outstanding elastocaloric prototype alloy of Ni-50(Mn28.5Cu4.5)(In14Ga3) was fabricated experimentally. At room temperature, a huge Delta T-ad of -19 K and a large specific adiabatic temperature change of 67.8 K/GPa are obtained. The proposed first-principle-assisted framework opens up the possibility of efficiently tailoring Delta V/V-0 to promote the design of advanced elastocaloric refrigerants. (C) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Microalloying by introducing small atoms into the interstitial sites of crystal represents an important strategy in composition design, usually enabling a leap in material performance under a tiny doping concentration. However, for the Ni–Mn-based magnetic shape memory alloys, plenty of critical scientific issues related to interstitial alloying remains ambiguous. In this work, by first-principles calculations, the occupation preferences, and the impacts and the underlying mechanisms of H, C, N, and O on magnetism, phase stability, and electronic structures of Ni2MnGa, were systemically investigated. By using a two-stage relaxation strategy, it is confirmed that all the studied interstitial atoms prefer to occupy the octahedral interstice, although the undistorted octahedral interstice possesses a smaller size than that of the tetragonal interstice. The magnetic moments of Ni and Mn around the interstitial element are highly modified, which is attributed to the decreased concentration of conduction electrons, resultant from the formation of covalent bonds between Ni and the interstitial atoms, and the revised distances between Mn–Ni(Mn) caused by the local lattice distortion. Interstitial alloying can highly tailor the phase stability and the c/a ratio of martensite. The doping of C has a great potential to destabilize the austenite owing to the reduced ferromagnetism, which is opposite to the case in steel. The atom radius of an interstitial atom may be the critical factor dominating the elastic stability of the alloyed systems. This work is expected to provide fundamental information for interstitial alloying to promote the design of advanced magnetic shape memory alloys.

Ni-Mn-Z (Z = In, Sn, and, Sb) metamagnetic shape memory alloys exhibit plenty of state-of-the-art magnetoresponsive effects owing to the magnetic-field-induced reversible martensitic transformation. For this compound, achieving high magnetization difference, large transformation entropy change, strong field dependence of transformation temperature, and low thermal hysteresis is essential to obtain significant magnetoresponsive functional properties. In this work, starting from the Ni45Co5Mn36In14 alloy, the substitution of Cu for In is introduced to optimize these properties simultaneously, giving rise to the substantial improvement in the magnetoresponsive effects. By composition design, a second-phase freed Ni45Co5Mn36In13.2Cu0.8 alloy with the phase temperature just below room temperature was selected. Using the directionally solidified technique, a strong < 001 > -textured sample is prepared to enhance magnetoresponsive behaviors. Measurements show that the reversible magnetoresistance and magnetic-field-induced strain of the prepared sample reach -79% and 0.24%, respectively. Moreover, a maximum isothermal magnetic entropy change of 16.0 J kg(-1) K-1 and the maximum adiabatic temperature change of -3.4 K are measured under a field change of 1.5 T, showing excellent low-field magnetocaloric response. The work is expected to supply a prototype alloy of NiMn-based alloy to promote the development of magnetic shape memory alloy. (C) 2021 Elsevier B.V. All rights reserved.

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Brittleness is a bottleneck hindering the applications of fruitful functional properties of Ni–Mn-based multiferroic alloys. Recently, experimental studies on B alloying shed new light on this issue. However, the knowledge related to B alloying is limited until now. More importantly, the mechanism of the improved ductility, which is intrinsically related to the chemical bond that is difficult to reveal by routine experiments, is still unclear. In this context, by first-principles calculations, the impact and the correlated mechanism of B alloying were systemically studied by investigating four alloying systems, i.e., (Ni2-xBx)MnGa, Ni2(Mn1-xBx)Ga, Ni2Mn(Ga1-xBx) and (Ni2MnGa)1-xBx. Results show that B prefers the direct occupation manner when it replaces Ni, Mn and Ga. For interstitial doping, B tends to locate at octahedral rather than tetrahedral interstice. Calculations show that the replacement of B for Ga can effectively improve (reduce) the inherent ductility (inherent strength) due to the weaker covalent strength of Ni(Mn)–B compared with Ni(Mn)–Ga. In contrast, B staying at octahedral interstice will lead to the formation of new chemical bonds between Ni(Mn) and B, bringing about a significantly improved strength and a greatly reduced ductility. Upon the substitutions for Ni and Mn, they affect both the inherent ductility and strength insignificantly. For phase transition, the replacement of B for Ga tends to destabilize the austenite, which can be understood in the picture of the band Jahn–Teller effect. Besides, the substitution for Ga would not lead to an obvious reduction of magnetization.