Sc Atoms Research Articles

Sc-Centered Octahedron Enables High-Speed Phase Change Memory with Improved Data Retention and Reduced Power Consumption.

Phase change memory (PCM) with advantages of high operation speed, multilevel storage capability, spiking-time-dependent plasticity, etc., has wide application scenarios in both Von Neumann systems and neuromorphic systems. In the automotive application, intelligent system not only needs high efficiency to handle massive data processing but also good robustness to retain the existing data against high working temperature. In this work, Sc-doped GeTe is developed for PCM, which has achieved 120 °C data retention for 10 years, 6 ns operation speed, and 7 nJ low power consumption. The high data retention is attributed to the high coordination number of Sc and its strong bonds with Te atoms in the amorphous phase, which enhances the robustness of the atomic matrices. Sc-centered octahedrons in amorphous state provide a nucleation center, leading to fast crystallization. In the crystalline phase, Sc atoms occupy Ge vacancies to form a homogenous GeTe-like rhombohedral phase. The strong covalent-like Sc-Te bonds weaken the neighboring Ge-Te bonds, lowering energy for melting. Together with the increased energy efficiency originated from confined grain size, the reduced power consumption has been achieved. The improvements in data retention, speed, and power efficiency have made Sc-doped GeTe a promising candidate for high-performance automobile electronics application.

Segregation behaviors of Sc and unique primary Al3Sc in Al-Sc alloys prepared by molten salt electrolysis

This work mainly deals with the segregating behaviors of Sc and the growth of unique primary Al3Sc in Al-Sc alloys prepared by molten salt electrolysis. The alloys contain 0.23–1.38 wt%Sc where Sc segregation is observed. It is found that a high current density and long electrolysis time are in favor of high Sc content, and so do the high temperature and the addition level of Sc2O3. Sc content at the edge of Al based alloy (average Sc content: 0.75 wt%) can be as high as 1.09 wt%, while it is merely 0.24 wt% at the central area. The cooling rates have a strong impact on the morphology and particle size of primary Al3Sc, but a weak influence on Sc segregation. The cusped cubic and dendritic primary Al3Sc can precipitate in the prepared Al-Sc alloys. In a slightly hypereutectic Al-0.67 wt%Sc alloy, a large and cusped dendrite grows from the edge into the center. The primary and secondary dendritic arms can be as long as 600 and 250 μm, respectively. The Sc segregating behaviors in Al-Sc alloys is due to the mechanism controlled by the limited diffusion rate of Sc in liquid Al. This can involve the establishment of a near spherical discharge interface between liquid Al and the electrolyte. The Sc rich layer near Al-molten salt interface may provide the potential primary nuclei and sufficient Sc atoms for the growth of large dendritic primary Al3Sc.

Sc and Ti-functionalized 4-tert-butylcalix[4]arene as reversible hydrogen storage material

Abstract Using the idea of metal functionalized material for H2 storage, 4-tert-butylcalix[4]arene (CA) functionalized with Sc and Ti atoms are explored. The first principles density functional theory (DFT) with M06 functional and 6-311G(d,p) basis set is used to explore the hydrogen storage properties of metal functionalized CA. Sc and Ti strongly binds with CA by Dewar coordination with high binding energy. It is found that maximum four hydrogen molecules are adsorbed on each metal site in Sc and Ti functionalized CA. Hydrogen molecules are adsorbed on metals by Kubas and Niu-Rao-Jena mechanism. In Sc functionalized CA system all 4 hydrogen molecules on each Sc bind in molecular fashion while on each Ti in Ti functionalized CA, the first hydrogen molecule binds in dissociative fashion and remaining three hydrogen molecules bind in a molecular form. The stability of Sc and Ti functionalized CA is studied by computing conceptual DFT parameters, which obeys maximum hardness and minimum electrophilicity principle. Hirshfeld charge analysis and electrostatic potential map explore the charge transfer mechanism during the hydrogen adsorption. Born-Oppenheimer molecular dynamics simulations are performed at temperature range 200–473 K to study the stability of the system and the reversibility of adsorbed hydrogen from the system. The calculated H wt% is found to be 10.3 and 10.1, respectively for Sc and Ti functionalized CA systems on complete H2 saturation. This study explores that Sc and Ti functionalized CA systems are efficient reversible hydrogen storage material.

Local structure origin of ultrafast crystallization driven by high-fidelity octahedral clusters in amorphous Sc0.2Sb2Te3

Phase-change material Sc0.2Sb2Te3 (SST) can remarkably boost the writing speed of memory devices due to the extremely fast crystallization. It was demonstrated that the fast crystallization is because Sc stabilizes the 4-fold rings which act as precursors of nuclei in the amorphous phase. In this work, by using first-principles molecular dynamics simulations, we studied the local structures in the liquid and amorphous SST at various temperatures. The results reveal that Sc-centered configurations are almost in the form of stable octahedral clusters, which enhances the local order of the amorphous phase. Different from Sb- and Te-centered clusters which have lower coordination numbers, Sc-centered clusters are mainly in high-coordinated octahedral structures. These Sc-centered octahedral clusters present a high stability in supercooled liquid and amorphous states, remarkably reducing the incubation time of nucleation and speeding up the crystallization. Our study reveals the role of Sc atoms in the liquid and amorphous structure, paving the way for the application of Sc-based phase-change memory.

Crystallochemical Design of Huntite-Family Compounds

Huntite-family nominally-pure and activated/co-activated LnM3(BO3)4 (Ln = La–Lu, Y; M = Al, Fe, Cr, Ga, Sc) compounds and their-based solid solutions are promising materials for lasers, nonlinear optics, spintronics, and photonics, which are characterized by multifunctional properties depending on a composition and crystal structure. The purpose of the work is to establish stability regions for the rare-earth orthoborates in crystallochemical coordinates (sizes of Ln and M ions) based on their real compositions and space symmetry depending on thermodynamic, kinetic, and crystallochemical factors. The use of diffraction structural techniques to study single crystals with a detailed analysis of diffraction patterns, refinement of crystallographic site occupancies (real composition), and determination of structure–composition correlations is the most efficient and effective option to achieve the purpose. This approach is applied and shown primarily for the rare-earth scandium borates having interesting structural features compared with the other orthoborates. Visualization of structures allowed to establish features of formation of phases with different compositions, to classify and systematize huntite-family compounds using crystallochemical concepts (structure and superstructure, ordering and disordering, isostructural and isotype compounds) and phenomena (isomorphism, morphotropism, polymorphism, polytypism). Particular attention is paid to methods and conditions for crystal growth, affecting a crystal real composition and symmetry. A critical analysis of literature data made it possible to formulate unsolved problems in materials science of rare-earth orthoborates, mainly scandium borates, which are distinguished by an ability to form internal and substitutional (Ln and Sc atoms), unlimited and limited solid solutions depending on the geometric factor.

BN-analogue of [2,2]paracyclophane functionalized with Sc and Ti for hydrogen storage

Use of metal functionalized molecules for reversible hydrogen storage is well established due to their high gravimetric hydrogen storage capacity. We study BN-analogue of [2,2]paracyclophane (BNP22) containing two borazine rings and functionalized with Sc and Ti atoms to interpret its hydrogen binding capacity. First principles calculations based on density functional theory suggest that BNP22-2Sc and BNP222Ti systems can have a gravimetric density as high as 8.9 and 9.9 %, respectively. The BNP22-2Sc system can be operated under ambient thermodynamic conditions within the allowable pressure range of 3–30 atm and van ‘t Hoff desorption temperature range of 219–438 K as confirmed from molecular dynamics and occupation number studies. Both the systems establish Brønsted-Evans-Polanyi relation with BNP22-2Sc having significantly less activation energy of 0.38 eV. We discuss the energetic stability of these systems with respect to vibrational frequency, absolute hardness, and transition states. Consistent with 2020 targets set by the DOE, BNP22-2Sc proves to be budding hydrogen storage material.

First principle study of reversible hydrogen storage in Sc grafted Calix[4]arene and Octamethylcalix[4]arene

The use of hydrogen as a sustainable clean energy source has several benefits, such as reduction in dependency on petroleum fuel and emission of green house gases, and enhanced energy security. The H2 storage properties of Sc grafted calix[4]arene (CX) and octamethylcalix[4]arene (MCX) are investigated by using density functional theory with M06/6-311G(d,p) level of theory. It is observed that Sc strongly binds with benzene rings of CX and MCX through Dewar coordination with average Sc binding energy of 1.09 and 1.25 eV, respectively for CXSc4 and MCXSc4. Each Sc atom adsorbs 4 H2 molecules on both the Sc grafted systems and H2 molecules are bound by Kubas interaction with H2 interaction energy in the range of 0.2–0.5 eV. The calculated conceptual reactivity index shows the stability of the systems increases with number of hydrogen molecules. Hirshfeld charge analysis shows the charge transfer mechanism during H2 adsorption. Born-Oppenheimer molecular dynamics simulations of CXSc4-16H2 and MCXSc4-16H2 systems, show that these systems are stable up to 273 K and all the adsorbed H2 releases at 373 K. The hydrogen storage capacity of Sc grafted CX system is found to be 8.9 wt % and for MCX system is 9.7 wt %. The energy and storage capacity meets the US Department of Energy target, which makes them a propitious hydrogen storage material.

Characteristics of Electron Transport in Molecular Single-Atom Transistors Based on Atoms of Sc, Cr, Ru, Rh, and Pt

The transport characteristics of single-atom transistors with charge centers based on Sc, Cr, Ru, Rh, and Pt atoms are studied. Single-particle electron spectra of molecules of the coordination compound of these atoms with an aurophilic terpyridine derivative in different charge states are calculated and their features are analyzed. The effect the length of linkers have on the properties of effective tunnel barriers and transport characteristics of a single-atom transistor is studied.

Attachment of CO to a (6, 6) CNT with a Sc adsorbate atom

We have comparatively investigated the adsorption behaviors of Sc atom on the external and internal surfaces of the pristine, Stone-Wales (SW), and single-vacancy (SV) defect nanotubes based on the density functional theory. Our results reveal that the stability of Sc atom onto the external and internal surfaces of the carbon nanotubes is in following orders by defect: SV>SW1>SW2>pristine (external surfaces), SV>SW2>SW1>pristine (internal surfaces). To explore the reactivity of Sc@CNT/SW/SV complexes which can serve as a gas sensor or a catalyst in direct methanol fuel cell (DMFC), the interaction between a CO molecule and Sc@CNT/SW/SV is also examined. The SV defect in carbon nanotubes can make the confined Sc atom with good CO adsorption property, while the confined Sc in the SW defect carbon nanotube has good anti-CO poisoning. By our studies, it plays an important guidance role in improving the activity of metal catalyst and the sensitivity and stability of metal particles to functional molecules, and provides the theoretical basis for designing new carbon nanotube-based metal catalyst.

DFT simulation on hydrogen storage property over Sc decorated B38 fullerene

In this study, the structural stability and hydrogen adsorption of Sc@B38 are investigated using density functional theory (DFT). Sc atoms are found to be sufficiently bonded to the B38 without clustering. In 1Sc@B38, each Sc atom can bind up to 6H2 with average adsorption energy of 0.358 eV. 4Sc@B38 systems could effectively absorb 24H2, the corresponding gravimetric density of the complexes is 7.57 wt% with an average adsorption energy of 0.224 eV. The results show that Sc decorated B38 is a kind of potential hydrogen storage material.

Unexpected diverseness on electronic density and bonding behaviours for Sc2X@C2v(63751)-C86 and Sc2X@C1(63755)-C86 (X = S and O)

Decisive investigations on stabilities of Sc2O@C86 and Sc2S@C86 isomers were carried out via density functional theory with statistical thermodynamic method. Sc2X@C2v(63751)-C86 and Sc2X@C1(63755)-C86 (X = O and S), observing isolated pentagon rules, were confirmed as thermodynamically preferred isomers within fullerene-formation temperature. Particularly, it is the first time to acknowledge C1(63755)-C86 as host for encapsulating Sc2X (X = O and S), herein. Interactions between Sc and non-metal atoms in Sc2X@C2v(63751)-C86 and Sc2X@C1(63755)-C86 (X = O and S) were specifically explored. Importantly, characteristics and electron density ellipticity for independent X-Sc (X = O and S) bonds and their bonding paths performed clear distinction due to weak intramolecular interaction.

Tautomerization of Phenol at the External Lewis Acid Sites of Scandium-, Iron- and Gallium-Substituted Zeolite MFI

We have employed density functional theory calculations to analyze the possible tautomerization of phenol mediated by three different Lewis acid sites at the external (010) surface of zeolite MFI. A silicon atom of the silanol group was substituted by Sc, Fe, and Ga metal atoms, which adopted a formal charge of 3+. This substituted silanol was dehydrated in order to form three-coordinated Lewis acid sites. The tautomerization of phenol involves the adsorption of the molecule on the Lewis site, the dissociation of the phenolic O–H bond and the transfer of the proton to the zeolite framework. This proton is transferred from the zeolite to the C atom at ortho positions to the phenolic O atom, thus generating the tautomer. The acidity of the substituted Lewis sites follows a strength order of Ga < Fe < Sc, where the Sc substitution provides the lowest energy barriers: 33 kJ/mol for the dissociation of the O–H bond, and 32 kJ/mol for the formation of the C–H bond, calculated with the GGA functional PBE including Grimme’s dispersion corrections. We observed that the GGA functional PBE produces binding energies and energy barriers with a difference of less than 13 kJ/mol compared to the meta-GGA TPSS and rev-TPSS and the hybrid-GGA HSE06.

The adsorption and diffusion properties of scandium atom on the surfaces of tungsten and noble metals

As for the scandate cathode, the performance of Sc atom, including the adsorbability and diffusivity, plays an important role in the emission property and emission stability of the scandate cathode. W is the most widely used matrix metal in thermionic scandate cathodes, however, the cathode suffers from the recovery of scandium after ion bombardment. Herein, the adsorption properties and diffusion performance of Sc atom on the surfaces of W, Ir, Os, Re and Ru are systematically studied with the density functional theory (DFT). It is found that the adsorbabilities and diffusivities of Sc atom on the surface of W, Ir, Os, Re and Ru are in the order as W(0 1 1) < Re(0 0 0 1) < Ru(0 0 0 1) < Os(0 0 0 1) < Ir(112¯1) and Ir(112¯1) < W(0 1 1) ∼ Os(0 0 0 1) < Ru(0 0 0 1) < Re(0 0 0 1), respectively. The adsorbabilities and the diffusivities of Sc atom on the surface of Re and Ru are higher than those on the surface of W, which suggests that Re and Ru are the potential materials to be the matrices or the coatings of scandate cathodes to improve the ion bombarding insensitivity and electron emission recovery of the cathode.

First-principles study of adsorption of 3d and 4d transition metal atoms on aluminene

Using the first principles calculations based on density functional theory, we study the structural, electronic, and magnetic properties of composite systems of first and second row transition metal (TM) atoms adsorbed aluminene. This study aims to probe the stability of the system as a result of adsorption of TM atoms on the two-dimensional (2D) surface of planar aluminene. We also study the same for two other well-known 2D single-atom-monolayer (SAML) structures, namely, silicene and arsenene, and we compare the results with those of aluminene. The TM atoms are adsorbed on these 2DSAML structures in three different crystallographically inequivalent sites, namely, the hollow (H), the top (T) and the bridge (Br) sites. The stability of the composite systems, which are a combination of a 2D monolayer structure and a TM atom each, is systematically analyzed in terms of binding energy per atom, for eighteen different TM atoms, with 3 d or 4 d electrons in their valence shell. We compare the results of optimized geometry and stability of the above-mentioned 2D structures when the TM atoms are adsorbed at the H, T and Br sites. We further analyze the interaction energy to discuss about and compare the ease of adsorption of various TM atoms on the three different 2DSAML structures. Interestingly, in most of the cases, adsorption of TM atoms typically yields higher interaction energy in case of aluminene when compared with silicene and arsenene systems. Among all the TM adatoms, few atoms have been identified to have higher interaction energy compared to the others. From our calculations of the density of states and band structure of the composite systems, almost all the composite systems have been found to be metallic, except the composite system of Sc atom and silicene, which exhibits a half-metallic state. Further, though all the three 2DSAML structures are non-magnetic in pristine form, a few composite systems are found to be magnetic in nature.

The effect of doping with rare earth elements (Sc, Y, and La) on the stability, structural, electronic and photocatalytic properties of the O-terminated ZnO surface; A first-principles study

The effect of doping with rare earth atoms (Sc, Y, and La) on the stability, structural, electronic, and photocatalytic properties of the O-terminated ZnO surface was investigated by using the first-principles method. The obtained results show that all these elements have negative formation energies for all possible values of the oxygen chemical potential, and this means that the doping process with RE atoms (RE = Sc, Y, and La) enhances the stability of this surface. Our results show also that, among all considered elements, the Sc atoms have the lowest formation energy followed by Y and La, respectively. We have also investigated the effect of the formation of a RE-VO complex on the properties of the ZnO(0001¯) surfaces. We find that the formation of a RE-VO complex is energetically more favored than the formation of isolated REZn defect under Zn-rich conditions. The effect of the formation of both REZn and RE-VO defects on the electronic and photocatalytic properties of the O-terminated ZnO surface was studied in details and the obtained results show that the RE-VO complex may be the origin of the photocatalytic properties enhancement of the doped surfaces. Moreover, we found that the Sc-doped surface prepared under Zn rich conditions may have the best photocatalytic properties followed by Y- and La-doped surfaces, respectively.

Synthesis of Two-Dimensional Nb1.33C (MXene) with Randomly Distributed Vacancies by Etching of the Quaternary Solid Solution (Nb2/3Sc1/3)2AlC MAX Phase

Introducing point defects in two-dimensional (2D) materials can alter or enhance their properties. Here, we demonstrate how etching a laminated (Nb2/3Sc1/3)2AlC MAX phase (solid solution) of both the Sc and Al atoms results in a 2D Nb1.33C material (MXene) with a large number of vacancies and vacancy clusters. This method is applicable to any quaternary, or higher, MAX phase, wherein one of the transition metals is more reactive than the other and could be of vital importance in applications such as catalysis and energy storage. We also report, for the first time, on the existence of solid solution (Nb2/3Sc1/3)3AlC2 and (Nb2/3Sc1/3)4AlC3 phases.

Intermetallic Growth and Interfacial Properties of the Grain Refiners in Al Alloys.

Al3TM(TM = Ti, Zr, Hf, Sc) particles acting as effective grain refiners for Al alloys have been receiving extensive attention these days. In order to judge their nucleation behaviors, first-principles calculations are used to investigate their intermetallic and interfacial properties. Based on energy analysis, Al3Zr and Al3Sc are more suitable for use as grain refiners than the other two intermetallic compounds. Interfacial properties show that Al/Al3TM(TM = Ti, Zr, Hf, Sc) interfaces in I-ter interfacial mode exhibit better interface wetting effects due to larger Griffith rupture work and a smaller interface energy. Among these, Al/Al3Sc achieves the lowest interfacial energy, which shows that Sc atoms should get priority for occupying interfacial sites. Additionally, Sc-doped Al/Al3(Zr, Sc) interfacial properties show that Sc can effectively improve the Al/Al3(Zr, Sc) binding strength with the Al matrix. By combining the characteristics of interfaces with the properties of intermetallics, the core-shell structure with Al3Zr-core or Al3Zr(Sc1-1)-core encircled with an Sc-rich shell forms.

Computational Studies on the ScnNm (n + m=10) Clusters: Structure, Electronic and Vibrational Properties

Besides the size and structure, compositions also dramatically affect the properties of clusters. In fact, the increased degree of freedom poses much more challenges to determine the global minimum structure of multi-component clusters. In this thesis, based on the CALYPSO structure searching method, the global minimum structures are obtained for ScnNm (n + m=10) clusters at PW91/6-311+G(d) level. The growth behavior indicates that the cage unit tends to arrange into the compact configurations, and the occupied positions of N atoms shift from the surface towards the center of coordination site with the increasing number of Sc atoms. The relative stabilities have been discussed by analyzing the average binding energies and HOMO–LUMO gaps. In addition, the molecular orbitals, dipole moments, polarizability, hyperpolarizabilities, natural population, natural electron configuration, and Infared and Raman spectra calculations allow complete characterization of the electronic and vibrational properties for the global minimum structural clusters.

A New Effective Approach to Prevent the Degradation of Black Phosphorus: The Scandium Transition Metal Doping

Density functional calculations are used to investigate the effective approach to prevent the degradation of black phosphorus (BP). For the few-layer BP, it is found that the Sc atom monodoped the 3 × 3 unit cell with a doping ratio of 1.4%, which can retain the semiconductor character with a moderate band gap (Egap) of 0.84 eV. Obviously, the two layer BP(2MLBP) with one Sc atom binding to three P atoms on the upmost layer has the lowest formation energy (−4.57 eV), exploring its greatest stability. Moreover, the calculated binding energy of Sc above the three P atoms is the largest (4.24 eV) energy, which is larger than the experimental cohesive energy of bulk Sc (3.90 eV), indicating that the clustering of Sc atoms on the surface of BP can be avoided. Importantly, the Sc atom adsorbed 2MLBP structure (2MLBP-Sc) has a conduction band minimum (CBM) value of −4.18 eV, which is below the redox potential of O2/O2– (−4.11 eV). When the O2 molecule is adsorbed on the 2MLBP-Sc, although the Sc–O and O–P bonds ...

Effects of Alloying Atoms on Antiphase Boundary Energy and Yield Stress Anomaly of L12 Intermetallics: First-Principles Study

The antiphase boundary energies of {111} and {010} planes in L1 2 intermetallics (Ni 3 Ge, Ni 3 Si, Al 3 Sc, Ni 3 Al, Ni 3 Ga and Al 3 Ti) under different pressure are presented using first-principle methods. The yield stress anomaly is predicted by the energy criterion p-factor based on the anisotropy of antiphase boundary energies and elasticity. These L1 2 intermetallics exhibit anomalous yield stress behavior except Al 3 Sc. It is found that pressure cannot introduce the transition between anomalous and normal behavior. In order to investigate the transition, Al 3 Sc, Ni 3 Si and Ni 3 Ge with substituting atoms are investigated in detail due to p-factors of them are close to the critical value p c = 3 . Al 3 Sc can change to anomalous when Sc atoms in {010} planes are substituted by Ti with plane concentration 25%. When Li substitutes Al in {111} planes, anomalous Al 3 Sc will change to normal. Ni 3 Si and Ni 3 Ge can exhibit normal yield stress behavior when Ge and Si in {111} planes are substituted by alloying atoms with plane concentrations 12.5% and 25%. When Ga and Al substitute in {010} planes, normal Ni 3 Si and Ni 3 Ge will revert to anomalous behavior. Therefore, transparent transition between normal and anomalous yield stress behavior in L1 2 intermetallics can be introduced by alloying atoms.