Sublattice Sites Research Articles

Investigation of annealing temperature effect on structural, morphological, optical, magnetic, and dielectric properties of cobalt-doped manganese ferrites

Spinel ferrite Mn0.95Co0.05Fe2O4 was prepared via chemical co-precipitation routes and annealed at four distinct temperatures of 600 °C, 650 °C, 700 °C, and 750 °C. The XRD peak confirmed a single-phase cubic spinel structure, with an increasing crystallite size of 21.64–23.98 nm by the annealing effect. The TEM images show an increase in particle size with rising annealing temperature and indicate well-defined nanocrystalline Mn-Co ferrite. FT-IR spectra detected two vibrational modes at interstitial sub-lattice sites, identifying Co-O, Mn-O, and Fe-O bonds with the spinel structure of Mn-Co ferrite. UV–vis–NIR reflectance revealed an optimal direct band gap of 3.76–4.70 eV, indicating a scope of semiconducting behaviors in the nanocatalysts. In the magnetic hysteresis curve, saturation magnetization decreased while coercivity increased with rising annealing and crystallite size. The M-T loop reveals the Curie temperature of Mn-Co ferrite decreases from 533.84 °C to 382.74 °C. Frequency-dependent complex permeability at different annealing temperatures shows stable initial permeability over a wide frequency range (1 kHz to 100 MHz) and a quality factor several times higher due to reduced magnetic tangent loss. The real dielectric constant and losses decline in rising alternating fields (100 Hz to 1 MHz) and stabilize at higher frequencies following Maxwell-Wagner-type polarization. Eventually, structural, morphological, and magnetic properties are performed relatively well at higher annealing temperatures, and optical and dielectric properties are performed at lower annealing temperatures.

Vanadium embedded in monolayer silicene: Energetics and proximity-induced magnetism

Using the density-functional theory approach, including Hubbard U correction, we investigate the defect structures consisting of vanadium (V) atoms embedded in a monolayer silicene. Specifically, we consider V–V atom pairs in antiferromagnetic (AFM), ferromagnetic (FM), and non-magnetic states, which are embedded in substitutional and interstitial sites. We determine the ground-state structures, formation and binding energies, electronic structures, induced magnetization, as well as the spin-exchange coupling between the V–V pair. For the substitutional vanadium atom pair, the stability of the AFM and FM spin configurations depends on the sublattice sites in which the V atoms are sited. When the V pair is located on a similar sublattice site type, the AFM spin alignment is more energetically favored, whereas when the pair is located in a different sublattice site, the FM interactions are more stable. However, the relative stability of the AFM or FM configurations changes rapidly as the separation between the V pair increases. Regarding the interstitial-hole V–V pair configurations, the most stable structure is when the pair is at the nearest-neighbor hole sites and is in an FM alignment. Also, at larger separations, the AFM or FM hole configurations are approximately degenerate in energy. Furthermore, we elucidate on the Ruderman–Kittel–Kasuya–Yosida, direct-exchange, and the superexchange interaction mechanisms in the vanadium-embedded silicene. In addition, we estimate a Curie temperature (Tc) of up to ∼500 K for a silicene structure containing a V pair in the FM spin alignment. Such a high Tc, in addition to the stability of the material, suggests that vanadium-embedded silicene is a potential candidate material for spintronic device applications.

Prediction of the catalytic mechanism of hydrogen evolution reaction enhanced by surface oxidation on FCC_CoCrFeNi and Co0.35Cr0.15Fe0.2Mo0.1Ni0.2 multi-principal element alloys based on site preference

Some multi-principal element alloy (MPEA) catalysts may exhibit the ability to promote the hydrogen evolution reaction (HER) and maintain stability under acidic conditions. Here, we investigated the impact of ordered behavior and surface oxidation of FCC_CoCrFeNi and Co0.35Cr0.15Fe0.2Mo0.1Ni0.2 MPEAs on HER performance through first-principles calculations. The ordering behaviors of MPEAs were described using L12_AuCu3 sublattice model and the predicted site occupying fractions (SOFs). And we found that the catalytic activity of single intermediate *H at top or bridge adsorption sites surpassed that at the hollow site. However, the hollow site exhibits strong adsorption towards *H, resulting in the transfer of *H from other sites to hollow site. Meanwhile, compared to those containing only hydrogen, MPEAs with both hydrogen and oxygen exhibit lower overpotentials, primarily due to oxygen facilitating hydrogen adsorption and subsequent desorption from MPEA surfaces, thus, such fundamental finding highlights the beneficial role of alloy surface oxidation in promoting HER performance. Furthermore, the overpotential values of the three slab models based on SOFs are similar, demonstrating considerably consistent atomic distribution behaviors, thereby better reflecting the overall catalytic performance of ordered MPEAs. These explorations bring new insights into the ordered behavior and surface oxidation of MPEAs in HER catalysis.

Unveiling the Mo effect on the microstructural stability of L12-strengthened chemically complex alloys

The influence of Mo on the coarsening behavior of L12-precipitates in chemically complex alloys (CCAs) was thoroughly investigated. Mo addition significantly impacted the morphology, misfit, and microstructural stability of L12-precipitates during aging treatment. Mo incorporation into the B sublattice sites of L12-precipitates (along with Al and Ti) increased their volume fraction and solvus temperature. It also weakened the segregation of solutes between the FCC-matrix and L12-precipitates, resulting in reduced misfit. However, during the aging processing, the misfits in all three CCAs continuously increased. Quantitative analysis revealed that a misfit of approximately 0.5 % yielded cubic morphology of L12-precipitate for CCAs with positive misfits. Further coarsening kinetics studies unveiled a transition from initial matrix-diffusion controlled coarsening to interface-reaction controlled coarsening. Notably, Mo addition prolonged the matrix-diffusion controlled stage and delayed the transition to interface-reaction controlled coarsening. These findings provide valuable insights that can guide future alloy design and heat treatment strategies to optimize the high-temperature structural performance of L12-precipitate CCAs.

High-Entropy La(FeCuMnMgTi)O3 Nanoparticles as Heterogeneous Catalyst for CO2 Electroreduction Reaction.

In this work, La(FeCuMnMgTi)O3 HEO nanoparticles with a perovskite-type structure are synthesized and used in the electrocatalytic CO2 reduction reaction (CO2RR). The catalyst demonstrates high performance as an electrocatalyst for the CO2RR, with a Faradaic efficiency (FE) of 92.5% at a current density of 21.9 mA cm-2 under -0.75 V vs a saturated calomel electrode (SCE). Particularly, an FE above 54% is obtained for methyl isopropyl ketone (C5H10O, MIPK) at a partial current density of 16 mA cm-2, overcoming all previous works. Besides, the as-prepared HEO catalyst displays robust stability in the CO2RR. The excellent catalytic performance of La(FeCuMnMgTi)O3 is ascribed to the synergistic effect between the electronic effects associated with five cations occupying the high-entropy sublattice sites and the oxygen vacancies within the perovskite structure of the HEO. Finally, DFT calculations indicate that Cu plays a vital role in the catalytic activity of the La(FeCuMnMgTi)O3 HEO nanoparticles toward C2+ products.

Thermal stability of refined Al6(Fe, Mn) phase formed in laser powder bed fusion process

The addition of Mn in Al–Fe alloys stabilizes the Al6Fe metastable phase (orthorhombic structure, oC28) owing to the partial occupation of Fe sublattice sites by Mn atoms (formation of Al6(Fe, Mn) phase), thereby achieving superior high-temperature strength of Al–Fe–Mn ternary alloys that are manufactured via laser powder bed fusion (L-PBF) process. This study aimed to fundamentally understand the thermal stability of the refined Al6(Fe, Mn) phase formed in the L-PBF fabricated Al–2.5%Fe–2%Mn alloy in terms of a thermodynamic approach and the kinetics of morphological changes in the Al6(Fe, Mn) phase at elevated temperatures ranging from 200 to 500 ℃. The L-PBF processed samples showed a high hardness of approximately 125 HV due to the formation of numerous dispersoids of the Al6(Fe, Mn) phase with sizes of several tens of nanometers in the α-Al matrix containing concentrated solute elements of Fe and Mn. The hardness and microstructure were almost unchanged even after exposure to a high temperature of 200 ℃ for an extended period of 1000 h. Upon exposure to 300 ℃, nucleation and growth of the Al6(Fe, Mn) phase occurred locally, particularly at grain boundaries in the α-Al matrix. Such a local growth contributed to a reduction in hardness upon thermal exposure. Simultaneously, numerous nanoscale precipitates were formed in the α-Al matrix, resulting in a suppressed reduction in hardness. Contrary to the result of the thermodynamic calculations, the θ-Al13Fe4 phase was scarcely found even after long-term exposure to 500 ℃. The θ-phase formation was accompanied by the dissolution of the refined Al6Fe metastable phase for strengthening, which significantly reduced the hardness during thermal exposure. The suppressed θ-phase formation could be associated with the thermodynamically stable Al6(Fe, Mn) phase developed during the L-PBF process, which predominantly contributed to the high microstructural stability of the L-PBF fabricated Al–Fe–Mn alloy at elevated temperatures.

Identifying the charge states of carbon vacancies in 4H-SiC by ab initio metadynamics

4H Silicon carbide (4H-SiC) is widely recognized as a highly promising material for high-voltage and high-power electronic applications due to its exceptional properties. The performance of devices based on 4H-SiC is often weakened by the presence of carbon-related point defects, particularly carbon vacancies (VC). The defects of VC introduce deep-level traps (e.g., Z1/2 and EH6/7) that deteriorate device functionality. Experimental and theoretical studies on VC have led to some conflicting results about the charge states of VC, especially for the charge state ordering of EH6/7. We now employ ab initio metadynamics (META) to systematically investigate configuration space including the direction and magnitude of bond distortion and identify the most stable structures of VC. Eventually, the charge states of VC in 4H-SiC are identified. The Z1 (EH6) and Z2 (EH7) indicate transitions from acceptor (donor) levels of VC, located on the h and k sublattice sites, respectively. Z1 and Z2 demonstrate negative-U ordering, characterized by U values of −0.16 and −0.37 eV, respectively. Conversely, EH6 and EH7 display positive-U ordering, with U values of 0.16 and 0.08 eV, respectively. The current results provide insights into the properties of VC in 4H-SiC, highlighting the effectiveness of META in the exploration of complex potential energy surfaces associated with point defects in solids.

Phase equilibria of the Co-Cr-Mn ternary system at 700 ℃

The isothermal phase diagram of the Co-Cr-Mn ternary system at 700 ℃ was experimentally determined via field-emission electron probe microanalysis/wavelength dispersive X-ray spectrometry and electron backscatter diffraction analyses. Three-phase regions of αCo(para)+ βMn+ σ and αMn+ βMn+ σ were confirmed via equilibrated microstructures; however, three-phase microstructures related to αCo(para)+ εCo+ σ and α’+ (Cr)+ σ were not observed in our study. The three-phase region of αCo(para)+ εCo+ σ was estimated to be very narrow because the phase boundaries between the αCo(para) and εCo phases were different within 2 at% for the Co, Cr, and Mn compositions. Furthermore, the α’ phase, presumed to be a nitride, was not observed in the microstructure equilibrated under a strictly controlled high-purity Ar gas (99.999%) atmosphere. The σ phase indicated a wide single-phase region similar to a previous study reported at temperatures between 900 ℃ and 1200 ℃. The αMn phase, which is stable below 727 ℃ in the Cr-Mn binary system and below 900 ℃ in the Co-Mn binary system, had a large Co solubility of 9.8 at% in the Co-Cr-Mn ternary system. The composition dependences on the lattice constants of the σ phase in the Co-Cr-Mn ternary system were examined via X-ray diffraction analysis; this analysis showed that the lattice constants a and c changed significantly with increasing Mn composition in the Co and Mn substitution, while the lattice constant of a was constant and c was slightly increased by increasing Mn composition in Cr and Mn substitution. From first-principles calculations, it was deduced that increases or decreases in bonding lengths between the nearest atoms occupying the five sublattice sites were associated with small changes in the lattice constant when Mn replaced Cr. The hardness of the σ phase decreased with increasing c/a, and the hardness in the crystal orientation of (100) was more dependent on the Mn composition than that in the crystal orientation of (001).

Carbon monoxide adsorption on different sub-lattice sites of nitrogen and phosphorous doped and Co-doped germanene- a first principle study

Carbon monoxide adsorption on different sub-lattice sites of nitrogen and phosphorous doped and Co-doped germanene- a first principle study

Atomic-scale investigation of H-trapping by fine NbC precipitates in a low C ferritic steel

The origin of hydrogen trapping by the Nb-carbide precipitates in a ferritic steel was studied at the atomic scale with the help of HRTEM (high resolution transmission electron microscopy) and 3D-APT (atom probe tomography). A Nb-added ferritic steel was prepared in the laboratory and subjected to aging after solutionizing treatment to have fine Nb-carbide precipitates in ferrite matrix. The precipitation kinetics were simulated beforehand using TC-Prisma in Thermocalc software. A large number of nanometer-sized Nb-carbide precipitates were observed in the ferrite matrix using TEM. 3D-APT studies on the steel sample electrolytically charged with D (deuterium) showed that the D atoms are trapped more within the precipitates than at the precipitate-matrix interfaces. It has been argued that H atoms occupy the vacant lattice sites on carbon sub-lattice sites in non-stoichiometric NbC0.83.

Observation of coherently coupled cation spin dynamics in an insulating ferrimagnetic oxide

Many technologically useful magnetic oxides are ferrimagnetic insulators, which consist of chemically distinct cations. Here, we examine the spin dynamics of different magnetic cations in ferrimagnetic NiZnAl-ferrite (Ni0.65Zn0.35Al0.8Fe1.2O4) under continuous microwave excitation. Specifically, we employ time-resolved x-ray ferromagnetic resonance to separately probe Fe2+/3+ and Ni2+ cations on different sublattice sites. Our results show that the precessing cation moments retain a rigid, collinear configuration to within ≈2°. Moreover, the effective spin relaxation is identical to within <10% for all magnetic cations in the ferrite. Thus, we validate the oft-assumed “ferromagnetic-like” dynamics in the resonantly driven ferrimagnetic oxide: the magnetic moments from different cations precess as a coherent, collective magnetization, despite the high contents of nonmagnetic Zn2+ and Al3+ diluting the exchange interactions.

Synthesis and characterization of intermetallic compounds with one medium- or high-entropy sublattice occupied by p-block elements

The use of p-block elements to form a high-entropy sublattice in the chemically complex intermetallic compounds (CCICs) is justified. The results suggest that the possibility of mutual compensation of the characteristics (i.e. electronegativity and atomic radius) should be considered when developing new criteria to predict the stability of CCICs. The unraveled Cu3(InSnSb), Cu3(InSnSbGa), Cu3(InSnSbGe), Cu3(InSnSbGeSi) and Cu3(InSnSbGaGe) CCICs with one medium- or high-entropy sublattice occupied by p-elements were synthesized for the first time. Cu3(InSnSb) crystallized into a orthorhombic Cu3Sn type structure (space group Pmmn), while samples with four or five elements occupying the high entropy sublattice site crystallized into a hexagonal Cu10Sn3 type structure (space group P63/m), except Cu3(InSnSbGeSi) that crystallized into a mixture of Cu3Sn and Cu15Si4 type structures. The change in the crystal structure upon adding Ge and Ga could be attributed to the intrinsic nature and partial deviation from the stoichiometric ratios of the elements occupying the low- and high-entropy sites.

Comparative analysis of Boron, nitrogen, and phosphorous doping in monolayer of semi-metallic Xenes (Graphene, Silicene, and Germanene) - A first principle calculation based approach

In this work, for the first time, a density functional theory (DFT) based detailed theoretical study has been performed on the comparative effects of Boron (B), Nitrogen (N), and Phosphorous (P) on the structural stability, structural and electronic properties of semimetallic Xene (Graphene, Silicene, and Germanene). The B, N, and P doping reduces the structural stability in Graphene whereas the exact opposite trend has been observed in Germanene. In contrast, B and N doping increase the structural stability, and P doping slightly reduces the structural stability in Silicene. This work also systematically demonstrates the impact of introducing doping species in the same and different sub-lattice sites of Xenes, which distinctly influences cohesive energy, bond length, lattice vector, and average buckling distance. A small compressive strain for N doping and small tensile strains for both B and P doping have been observed in Graphene. However, a notably larger change in the lattice structures and thereby relatively higher tensile strains has been observed in doped Silicene and Germanene. Next, the impact of doping on the electronic properties of Xenes has been assessed from the energy band gap, effective masses, and spin-orbital splitting at the band edges. The substitutional doping in Xene breaks the sub-lattice symmetry and subsequently introduces finite band gaps and effective masses at the band edges. The introduction of dopants in the same sub-lattice site leads to larger energy bandgap and effective masses compared to doping at different sub-lattice sites of Xenes with the exception of P doping in Silicene and Germanene, in which an exact opposite trend has been observed. The doping at the same sub-lattice sites demonstrates significant spin-orbit splitting at the band edges of Silicene and Germanene. This work presents an extensive theoretical and design-level insight into the non-metallic doping approach in Xenes to realize semi-metallic to semiconducting transformation, which can facilitate electronic, optoelectronic, and energy storage applications of doped Xenes in the future.

Elemental Sub-Lattice Occupation and Microstructural Evolution in γ/γ' Co-12Ti-4Mo-Cr Alloys.

We report on comparative atom probe tomography investigations of γ/γ′-forming Co–12Ti–4Mo–Cr alloys. Moderate additions of Cr (2 and 4 at%) reduced the γ/γ′ lattice misfit and increased the γ′ volume fraction of a Co–12Ti–4Mo alloy significantly. These microstructural changes were accompanied by changes in the elemental partitioning between γ and γ′ and site-occupancy in γ′. Spatial distribution maps revealed that Mo occupied both Co and Ti sub-lattice sites in γ′. In agreement with the experimental data, thermodynamic calculations predicted a stronger tendency for Mo to occupy the Co-sites than for Cr and an increase in Cr fraction on the Ti-sites with increasing Cr content.

Majorana corner states in an attractive quantum spin Hall insulator with opposite in-plane Zeeman energy at two sublattice sites

Higher-order topological superconductors and superfluids (SFs) host lower-dimensional Majorana corner and hinge states since novel topology exhibitions on boundaries. While such topological nontrivial phases have been explored extensively, more possible schemes are necessary for engineering Majorana states. In this paper we propose Majorana corner states could be realized in a two-dimensional attractive quantum spin-Hall insulator with opposite in-plane Zeeman energy at two sublattice sites. The appropriate Zeeman field leads to the opposite Dirac mass for adjacent edges of a square sample, and naturally induce Majorana corner states. This topological phase can be characterized by Majorana edge polarizations, and it is robust against perturbations on random potentials and random phase fluctuations as long as the edge gap remains open. Our work provides a new possibility to realize a second-order topological SF in two dimensions and engineer Majorana corner states.

Studies on the abnormal effect of tensile strain on the MC→M23C6 in-situ transformation in Ni-based superalloy

Experiment and first-principles calculations were utilized to study the effect of tensile strain on the MC→M23C6 in-situ transformation in MC-M23C6 complex carbide as well as its micro-mechanism. The experiment indicates that strain inhibits the transformation. According to the calculations, Cr atoms will occupy M sublattice sites and diffusion through the VM-(VC)6 vacancy clusters in MC. Nevertheless, a single VM mechanism works during the diffusion of Cr atoms across the MC/M23C6 interface. Tensile strain as well as its direction can seriously influence the diffusion behavior of atoms by expanding or compressing the lattice. Generally, tensile strain will facilitate the diffusion of Cr atoms in MC and the flipping of diagonals. However, tensile strain decreases the lattice mismatch at the MC/M23C6 interface. Thus, the activation energy of the diffusion of Cr across the interface increases and the diffusion coefficient decreases. Strain inhibits the transformation mainly by restraining the diffusion of Cr atoms across the MC/M23C6 interface. The demonstrated concept enables a deeper understanding of superalloys and shed light on the rational design of higher property superalloys for various potential applications.

Topological edge states in dipolar zig-zag stripes

We study the magnon spectrum of stacked zig-zag chains of point magnetic dipoles with an easy axis. The anisotropy due to the dipolar interactions and the two-point basis of the zig-zag chain unit cell combine to give rise to topologically non-trivial magnon bands in 2D zig-zag lattices. Adjusting the distance between the two sublattice sites in the unit cell causes a band touching, which triggers the exchange of the Chern numbers of volume bands switching the sign of the thermal conductivity and the sense of motion of edges modes in zig-zag stripes. We show that these topological features survive when the range of the dipolar interactions is truncated up to the second nearest neighbors.

Electronic structure and spin-lattice relaxation in superconducting vortex states on the kagome lattice near van Hove filling

Starting from a tight-binding model on the kagome lattice near the van Hove filling, the superconducting (SC) properties are investigated self-consistently by using the Bogoliubov--de Gennes equations with the consideration of the inequivalent third-neighbor (TN) bonds. Near the van Hove filling, the most favorable SC pairings are found to derive from the electrons belonging to the same sublattice sites, including the on-site $s$-wave and the spin-singlet/spin-triplet TN pairings. The inequivalent TN bonds will result in multiple SC components with different orbital angular momenta for the TN SC pairings. Whereas the density of states and the temperature ($T$) dependence of the spin-lattice relaxation rate (${T}_{1}^{\ensuremath{-}1}$) exhibit distinct line shapes in the SC state for the three cases, a peak structure in the $T$ dependence of ${T}_{1}^{\ensuremath{-}1}$ can be found for all of them just below ${T}_{c}$ as a result of the van Hove singularity, even though the SC gap has nodes. The effects of magnetic vortices on the low-energy excitations and on the $T$ dependence of ${T}_{1}^{\ensuremath{-}1}$ with the implications of the results are also discussed for all the cases.

Theoretical analysis of the NH3, NO, and NO2 adsorption on boron-nitrogen and boron-phosphorous co-doped monolayer graphene - A comparative study

In this work, a density functional theory (DFT) based detailed theoretical study has been performed on the Boron/Nitrogen (B/N) and Boron/Phosphorous (B/P) codoped monolayer Graphene for molecular adsorptions and co-adsorptions of three Nitrogen-based environmental pollutant gasses, namely Ammonia (NH3), Nitrogen Di-Oxide (NO2) and Nitric Oxide (NO). The B acts as a localized electron deficit site in the codoped lattice and subsequently offers a suitable adsorption site for electron-enriched gas molecules like NH3 and NOx. The work systematically explores the influence of introducing Nitrogen or Phosphorus impurities in the same and different sub-lattice sites of boron-doped Graphene in the context of molecular adsorption. In this context, six doping configurations are identified that demonstrate distinct electronic properties and characteristic responses towards individual gas molecule adsorption. The molecular adsorption strongly influences the spatial distribution of electronic states near the band edges due to orbital overlaps from the adsorbed gas molecules, which leads to significant modulations in the energy band gaps and effective masses of the codoped lattices. The co-doping strategies appear highly advantageous for electrochemical gas sensing as it leads to appreciable semiconducting bandgap opening while retaining the intrinsic nature of the Graphene. The subsequent molecular adsorptions and co-adsorptions result in notable charge transfer between the gas molecules and the host lattice and considerably enhance the density of electronic states around the Fermi-level for codoped lattices.

Topology of multipartite non-Hermitian one-dimensional systems

The multipartite non-Hermitian Su-Schrieffer-Heeger model is explored as a prototypical example of one-dimensional systems with several sublattice sites for unveiling intriguing insulating and metallic phases with no Hermitian counterparts. These phases are characterized by composite cyclic loops of multiple complex-energy bands encircling single or multiple exceptional points (EPs) on the parametric space of real and imaginary energy. We show the topology of these composite loops is similar to well-known topological objects like M\"obius strips and Penrose triangles, and can be quantified by a nonadiabatic cyclic geometric phase which includes contributions only from the participating bands. We analytically derive a complete phase diagram with the phase boundaries of the model. We further examine the connection between encircling of multiple EPs by complex-energy bands on parametric space and associated topology.