First-principles Density Functional Theory Research Articles

Structural, magnetic, optical, and electronic properties of vanadium-doped barium hexaferrite nanoparticles: Experimental and DFT approaches

Vanadium-doped barium hexaferrite with a nanostructure is a highly valuable material in various technological fields, such as electronics, permanent magnets, and sensors. The Ba1-xFe12-xVxO19 (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) nanoparticles derived from the aqueous solutions containing Fe:Ba molar ratio of 10:1 through the sol-gel auto-combustion method. Computational study was also performed using the first-principles density functional theory (DFT) approach. Crystal structure optimization, band structure, and density of states (DOS) calculations were conducted by CASTEP code. The variation of the structural, magnetic, optical, morphological, and electronic properties of V5+-doped barium hexaferrite was investigated. The XRD analysis combined with the Rietveld refinement showed the hexagonal structure of M-type barium ferrite (BaM), confirmed by the FT-IR analysis. The morphology of BaM nanoparticles was studied by the FE-SEM and TEM micrographs. In addition, magnetic and optical properties were analyzed through VSM and UV–Vis analysis. Crystallite size was found to be highly effective in tuning the coercivity and optical band gap of barium hexaferrites, which varied, respectively, from 3.24 to 4.83 kOe, and 2.69–3.69 eV. Magnetic results showed that several variables like cations distribution, lattice strain, and hematite secondary phase affected the nanoparticles’ magnetization. The DFT simulation results showed a sharp reduction of electronic band gap energy whether V takes the position of Ba or Fe (from 1.10 eV in undoped to 0.64 and 0.14 eV in doped structures). The projected density of states (PDOS) calculations demonstrated that the d orbitals of V and Ba mainly contribute to the valence band maximum (VBM) and conduction band minimum (CBM), respectively.

Solid-liquid phase boundary of oxide solid solutions using neural network potentials

We propose a cost-effective computational approach for predicting the phase boundary of oxide solid solutions, i.e., melting point, by identifying the point where the free energies of the solid and liquid phases are equivalent. To evaluate the melting point, we computed the temperature-dependent free energies of both phases using the thermodynamic integration (TI) method. This was combined with the reverse-scaling technique, employing the Einstein model for the solid and the scaled Uhlenbeck–Ford model for the liquid as the reference systems. The change in free energy between the real and reference states in the present TI method was calculated as the ensemble average of the configurations sampled from molecular dynamics (MD) simulations using neural network potentials (NNPs) trained on first-principles density functional theory (DFT) data. Initially, we benchmarked copper (Cu) as a typical metal. Tungsten (W) and magnesium oxide (MgO) were then tested as typical high melting-point metals and oxides, respectively, achieving good agreement with both ab initio MD (AIMD) results and experimental data. We then applied our established approach to a BaO–CaO oxide solid solution, observing that the computed phase boundary aligns well with Calphad predictions from previous studies. The integration of NNPs into phase boundary prediction is essential for reducing computational costs while ensuring accuracy, compared with AIMD-based approaches.

Unraveling the Origin of the Repulsive Interaction between Hydrogen Adsorbates on Platinum Single-Crystal Electrodes.

Hydrogen adsorption on platinum (Pt) single-crystal electrodes has been studied intensively in both experiments and computations. Yet, the precise origin and nature of the repulsive interactions observed between hydrogen adsorbates (Hads) have remained elusive. Here, we use first-principles density functional theory calculations to investigate in detail the interactions between Hads on Pt(111), Pt(100), and Pt(110) surfaces. The repulsive interaction between Hads on Pt(111) is deconvoluted into three different physical contributions, namely, (i) electrostatic interactions, (ii) surface distortion effect, and (iii) surface coordination effect. The long-range electrostatic interaction, which is generally considered the most important source of repulsive interactions in surface adsorption, was found to contribute less than 30% of the overall repulsive interaction. The remaining >70% arises from the other two contributions, underscoring the critical influence of surface-mediated interactions on the adsorption process. Surface distortion and coordination effects are found to strongly depend on the coverage and adsorption geometry: the effect of surface distortion dominates when adsorbates reside two or more Pt atoms apart; the effect of surface coordination dominates if hydrogen is adsorbed on neighboring adsorption sites. The above effects are considerably less pronounced on Pt(100) and Pt(110), therefore resulting in weaker interactions between Hads on these two surfaces. Overall, the study highlights the relevance of surface-mediated effects on adsorbate-adsorbate interactions, such as the often-overlooked surface distortion. The effect of these interactions on the hotly debated adsorption site for the adsorbed hydrogen intermediate in the hydrogen evolution reaction is also discussed.

Designing Polychromatic Er3+/Yb3+-Codoped Bi2WO6 Upconverting Nanoparticles with Enhanced Visible-Light-Triggered Photocatalytic Properties toward Tetracycline Degradation.

To address the unsatisfactory photodegradation capacities of photocatalysts, Er3+/Yb3+-codoped Bi2WO6 (Bi2WO6:Er3+/xYb3+) nanoparticles (NPs) with polychromatic upconversion (UC) emission and boosted visible-light-triggered photocatalytic abilities were designed. First-principles density functional theory was employed to study the impact of Er3+ and Yb3+ codoping on the electronic structure of Bi2WO6. Upon 980 nm excitation, the resultant NPs emitted polychromatic UC emissions caused by energy back transfer from Er3+ to Yb3+. Moreover, the involved UC emission mechanism was clarified through examining the pump power related to UC emission spectra. By investigating the visible-light-induced tetracycline (TC) decomposition, the photocatalytic activities of developed NPs were explored, where Bi2WO6:Er3+/0.07Yb3+ NP can degrade 81.76% of TC within 30 min, with a k value of 0.0552 min-1. Both the theoretical calculation and trapping results reveal that the •O2-, h+, and •OH were formed during the pollutant removal process. Additionally, the toxic TC can be photodegraded to nontoxic products via the synthesized photocatalysts, leading to wastewater purification. These achievements manifest that Bi2WO6:Er3+/xYb3+ NPs are promising visible-light-triggered photocatalysts to degrade pollutants, and our findings also propose a facile approach to regulate the photocatalytic activities of photocatalysts via utilizing doping and UC emission strategies.

Manganese doping in zinc based hybrid metal halides to realize highly stable efficient green emission and flexible radiation detection

Extensive studies have been conducted on hybrid metal halides due to their application in radiation detection, solid-state lighting, and solar cells. Here, we present an environmentally friendly zero-dimensional halide, (C9H15N3)ZnBr4, which crystallizes in the P21/c space group. This compound is highly thermally stable, and doping Mn2+ results in a bright green light emission, with an impressive internal quantum efficiency of 52.9 % and an external quantum efficiency of 45.9 % for (C9H15N3)Mn0.3Zn0.7Br4. Combining spectroscopic analysis with first-principles density functional theory (DFT), it is concluded that the high external quantum efficiency arises from efficient energy transfer from the organic component to the [MnBr4]2-. Notably, the (C9H15N3)Mn0.3Zn0.7Br4 luminescence intensity maintains 50 % of its room temperature level even at 400 K. Moreover, these doped powders display exceptional scintillation performance, higher than Bi4Ge3O12. Finally, the radioluminescence intensity of (C9H15N3)Mn0.3Zn0.7Br4@polydimethylsiloxane flexible films is about three times that of Bi4Ge3O12. These features position Mn:(C9H15N3)ZnBr4 as an ideal material for X-ray detection and an efficient green photoluminescent material.

Effect of Lattice Misfit on the Stability of the Misfit Layer Compound (SnS)1+xNbS2

The prototype misfit layer compound (SnS)1.17NbS2 consists alternatingly of a metallic triatomic NbS2 layer, in which Nb atoms are sandwiched by S atoms, and an insulating SnS double layer featuring a NaCl-type structure. Here we investigate the effect of lattice misfit on the stability and chemical bonding in the misfit layer compound using a first-principles density functional theory approach. The calculations show that for the (SnS)1+xNbS2 approximants, the most stable one has x = 0.167, close to the experimental observations. Charge analysis finds a moderate charge transfer from SnS to NbS2. Sn or S vacancies in the SnS part affect the electronic properties and interlayer interactions. The obtained information here helps in understanding the mechanism of formation and stability of misfit layer compounds and ferecrystals and further contributes to the design of novel multilayer compounds and emerging van der Waals heterostructures.

The Electrochemical Peroxydisulfate-Oxalate Autocatalytic Reaction.

Aqueous solutions containing both the strong oxidant, peroxydisulfate (S2O82-), and the strong reductant, oxalate (C2O42-), are thermodynamically unstable due to the highly exothermic homogeneous redox reaction: S2O82- + C2O42- → 2 SO42- + 2 CO2 (ΔG0 = -490 kJ/mol). However, at room temperature, this reaction does not occur to a significant extent over the time scale of a day due to its inherently slow kinetics. We demonstrate that the S2O82-/C2O42- redox reaction occurs rapidly, once initiated by the Ru(NH3)62+-mediated 1e- reduction of S2O82- to form S2O83•-, which rapidly undergoes bond cleavage to form SO42- and the highly oxidizing radical SO4•-. Theoretically, the mediated electrochemical generation of a single molecule of S2O83•- can initiate an autocatalytic cycle that consumes both S2O82- and C2O42- in bulk solution. Several experimental demonstrations of S2O82-/C2O42- autocatalysis are presented. Differential electrochemical mass spectrometry measurements demonstrate that CO2 is generated in solution for at least 10 min following a 30-s initiation step. Quantitative bulk electrolysis of S2O82- in solutions containing excess C2O42- is initiated by electrogeneration of immeasurably small quantities of S2O83•-. Capture of CO2 as BaCO3 during electrolysis additionally confirms the autocatalytic generation of CO2. First-principles density functional theory calculations, ab initio molecular dynamics simulations, and finite difference simulations of cyclic voltammetric responses are presented that support and provide additional insights into the initiation and mechanism of S2O82-/C2O42- autocatalysis. Preliminary evidence indicates that autocatalysis also results in a chemical traveling reaction front that propagates into the solution normal to the planar electrode surface.

Stacking-dependent Van Hove singularity shifts in three-dimensional charge density waves of kagome metals AV3Sb5 (A = K, Rb, Cs)

Vanadium-based kagomé systems AV3Sb5 (A = K, Rb, Cs) have emerged as paradigmatic examples exhibiting unconventional charge density waves (CDWs) and superconductivity linked to van Hove singularities (VHSs). Despite extensive studies, the three-dimensional (3D) nature of CDW states in these systems remains elusive. This study employs first-principles density functional theory and a tight-binding model to investigate the stacking-dependent electronic structures of 3D CDWs in AV3Sb5, emphasizing the significant role of interlayer coupling in behaviors of the VHSs associated with diverse 3D CDW orders. We develop a minimal 3D tight-binding model and present a detailed analysis of band structures and density of states for various 3D CDW stacking configurations, including those with and without a π-phase shift stacking of the inverse star of David, as well as alternating stacking of the inverse star of David and the star of David. We find that VHSs exist below the Fermi level even in 3D CDWs without π-phase shift stackings, and that these VHSs shift downward in the π-phase shift stacking CDW structure, stabilizing the 2×2×2π-shifted inverse star of David distortions in alternating vanadium layers as the ground state 3D CDW order of AV3Sb5. Our work provides the electronic origin of 3D CDW orders, paving the way for a deeper understanding of CDWs and superconductivity in AV3Sb5 kagomé metals.

Neural network potential for Zr-H

The introduction of Hydrogen (H) into Zirconium (Zr) influences many mechanical properties, especially due to low H solubility and easy formation of Zirconium hydride phases. Understanding the various effects of H requires studies with atomistic resolution but at scales that incorporate defects such as cracks, interfaces, and dislocations. Such studies thus demand accurate interatomic potentials. Here, a neural network potential (NNP) for the Zr-H system is developed within the Behler-Parrinello framework. The Zr-H NNP retains the accuracy of a recent NNP for hcp Zr and exhibits excellent agreement with first-principles density functional theory (DFT) for (i) H interstitials and their diffusion in hcp Zr, (ii) formation energies, elastic constants, and surface energies of relevant Zr hydrides, and (iii) energetics of a common Zr/Zr-H interface. The Zr-H NNP shows physical behavior for many different crack orientations in the most-stable ε-hydride, and structures and reasonable relative energetics for the 〈a〉 screw dislocation in pure Zr. This Zr-H NNP should thus be very powerful for future study of many phenomena driving H degradation in Zr that require atomistic detail at scales far above those accessible by first-principles.

Unveiling the mass-loading effect on the electrochemical performance of Mn3O4 thin film electrodes: A combined computational and experimental study

Abstract The remarkable storage performance of manganese oxide (Mn3O4) makes it an appealing option for use as electrodes in electrochemical capacitors. However, the storage kinetics were significantly influenced by the mass loading of the electrode. Herein, we have inspected the dependency of mass loading on the storage performance of the spray pyrolyzed Mn3O4 thin film electrodes along with the correlation of structural and morphological characteristics. X-ray diffraction and Raman spectroscopic studies proven the formation of spinel Mn3O4 with a tetragonal structure. Morphological analysis revealed that all films exhibited fibrous structures with interconnected patterns at higher mass loadings. Moreover, the surface roughness and wettability of the electrode surface were influenced by variations in mass loading. Notably, thin-film electrode with a mass loading of 0.4 mg/cm2 exhibited the highest specific capacitance value of 168 F/g at 5 mV/s in a three-electrode system. Further, electrochemical impedance spectroscopic studies showed that there were noticeable changes in the capacitive behaviour of the electrode with respect to variations in mass loading. Moreover, the Dunn approach was employed to differentiate the underlying storage mechanism of the Mn3O4 electrode. Additionally, first-principles Density Functional Theory (DFT) studies were carried out in connection with the experimental study to comprehend the structure and electronic band structure of Mn3O4. This study underscores the critical importance of mass loading for enhancing the storage performance of Mn3O4 thin-film electrodes.

Diaminopropane-Functionalized Metal-Organic Frameworks with Controllable Diamine Loss and One-Channel Flipped CO2 Adsorption Mode.

Diamine-functionalized metal-organic frameworks (MOFs) based on Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobihyenyl-3,3'-dicarboxylate) have been frequently reported as promising CO2 adsorbents due to their characteristic step-shaped adsorption behavior. However, high CO2 desorption temperatures for diamine-Mg2(dobpdc)-based adsorbents led to gradual diamine loss while the existence of an exotic CO2 adsorption mode remains experimentally unanswered. Herein, we present CO2 adsorbents obtained by functionalizing Mn2(dobpdc) with a diaminopropane series. These adsorbents offer low regeneration energies, allowing CO2 desorption at lower temperatures than the reported Mg-based analogs. Our first-principles density functional theory calculations showed that the binding strength between the diamine and Mn ions in Mn2(dobpdc) was stronger than that between the diamine and Mg ions in Mg2(dobpdc), preventing diamine loss even at high temperatures and enabling efficient regeneration. Additionally, the computational and experimental data demonstrated that primary-tertiary diamine-functionalized MOFs exhibit one-channel flipped adsorption structures that have not been experimentally revealed. Our findings provide insights into the role of metal ions in diamine loss for the future development of efficient amine-based CO2 adsorbents.

Deeply Reduced Chiral MoIV6-Polyoxometalates with Highly Enhanced Electronic Conductivity.

A series of deeply reduced MoIV6-polyoxometalates (POMs), [MoIV6(bpy)6MoVI2O16S2]·10H2O (1) (bpy = 2,2'-bipyridine), [MoIV6(bpy)4(L-TrA)2MoV4(bpy)2O14S4]2·36.5H2O (2), and [MoIV6(bpy)4(D-TrA)2MoV4(bpy)2O14S4]2 (3) (TrA = tartaric acid), were obtained from one-pot hydrothermal syntheses. They feature the bioxo-bridged [MoIV3O3S]4+ incomplete cuboidal cores stabilized by strong triangular metal-metal bonds and datively chelated bpy π-ligands. The two apical MoVIO4 in 12e-Mo8 (1) and two [MoV2O3STrAbpy] in 16e-Mo10 (2, 3) complete the octahedral coordination geometry of the deeply reduced MoIV, not counting the MoIV-MoIV d2-d2 bonds. The aromatic planar bpy π ligands form a two-dimensional face-to-face π stacking supramolecular structure, which are further strengthened by point-to-face C-H···π interactions in the π stacked layers. 16e-Mo10 (2, 3) has stronger intramolecular face-to-face π stacking interactions, which are interconnected by intermolecular C-H···π into a three-dimensional framework structure. These structural features account for their highly enhanced electronic conductivities (1, 6.19 × 10-8 S cm-1; 2, 2.18 × 10-7 S cm-1), providing a new way of thinking to improve electronic conductivity of POMs. 1-3 have been described by first-principles density function theory (DFT) calculations and also characterized by elemental analyses, powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), solid UV-visible spectra, and circular dichroic (CD) spectra.

Gate-Switchable Molecular Diffusion on a Graphene Field-Effect Transistor.

Controlling the surface diffusion of particles on 2D devices creates opportunities for advancing microscopic processes such as nanoassembly, thin-film growth, and catalysis. Here, we demonstrate the ability to control the diffusion of F4TCNQ molecules at the surface of clean graphene field-effect transistors (FETs) via electrostatic gating. Tuning the back-gate voltage (VG) of a graphene FET switches molecular adsorbates between negative and neutral charge states, leading to dramatic changes in their diffusion properties. Scanning tunneling microscopy measurements reveal that the diffusivity of neutral molecules decreases rapidly with a decreasing VG and involves rotational diffusion processes. The molecular diffusivity of negatively charged molecules, on the other hand, remains nearly constant over a wide range of applied VG values and is dominated by purely translational processes. First-principles density functional theory calculations confirm that the energy landscapes experienced by neutral vs charged molecules lead to diffusion behavior consistent with experiment. Gate-tunability of the diffusion barrier for F4TCNQ molecules on graphene enables graphene FETs to act as diffusion switches.

Solute Segregation and Pinning Effect on Lateral Twin Boundary in Magnesium

Deformation twinning creates a three-dimensional twin domain via the migration of forward, normal and lateral twin boundaries (TBs) with respect to twin shear direction, normal to twin plane and twin lateral direction. Solute segregation and pinning effect on the forward and normal TBs have been experimentally observed and demonstrated via atomistic simulations. Here, we conducted a comprehensive study of solute segregation and the pinning effect on the lateral TBs in Mg. First-principles density functional theory calculations were used to obtain the segregation and formation energies of 19 alloying elements in coherent regions of lateral TBs. Alloying elements with greater difference in atomic radius from Mg generally show more negative segregation energy. Moreover, alloying elements with good solubility are selected to demonstrate the pinning effect on a coherent interface. Ge, Ga, Y, Gd, La and Ca show negative segregation energy and solubility energy, indicating that these elements can form stable segregation and have a strong pinning effect at the lateral boundary. Molecular dynamics simulations revealed that solutes in coherent regions are more effective in pinning lateral TBs than those in misfit regions. The results provide insight into the selection of solute atoms for tailoring twinning behavior.

Optoelectronic and structural properties of bulk cubic perovskite RbTaO3 and surface (0 0 1) for optoelectronic and spintronics applications

We used first-principles density functional theory (DFT) to study the optical, electronic, and structural properties of RbTaO3 cubic perovskite in both bulk and surface (0 0 1) forms. The RbTaO3 bulk system exhibited semiconducting behaviour with a band gap (Eg) of 2.20 eV. It showed weak reflectivity, excellent optical conductivity, and high absorption of incident energetic photons, indicating its potential for optoelectronic applications. For the surface (0 0 1), two RbO and TaO2 terminations were constructed. We calculate the atomic displacement (ΔZ) and surface energy (Esurf) for both terminations. The RbO- and TaO2-(0 0 1) terminated surfaces appeared half-metallic ferromagnetic (HMF) with a spin magnetic moment of integer value 1.00 μβ and fully spin polarization (SP) 100 %. The Eg for RbO- and TaO2-terminations were 2.60 eV and 2.30 eV, respectively, suggesting their potential for spintronics. We also analysed and compared the optical properties and atomic populations (Q) and their variation (ΔQ) for (0 0 1)-RbTaO3 terminations with the bulk system.

Enhanced dehydrogenation of MgH2 modified by Ti and S: A first-principles investigation

The present work gives the structural, dehydriding and electronic characteristics of MgH2 with Ti or/and S additives by first-principles density functional theory calculations. It is found that Ti and S prefer to occupy Mg site and interstitial site, respectively, in terms of occupation energy (Eocc). Furthermore, Ti + S incorporation into MgH2 lattice is more energetically favorable (Eocc = −0.051 eV–0.944 eV) relative to single Ti incorporation (Eocc = 1.314 eV–1.784 eV), for which the promotion of adequate Mg site by S addition is chiefly responsible. Ti, S, and especially Ti + S doping not only reduce the hydrogen dissociation energy from 1.619 eV/2.317 eV (Mg16H32) to 0.350 eV/0.967 eV (Mg15TiSH32) for H-atom/H-molecule desorption, but also decrease the hydrogen diffusion energy barrier from 0.706 eV/0.704 eV (Mg16H32) to 0.342 eV/0.169 eV (Mg15TiSH32) along path I/II, respectively. As a result, a significant improvement on the thermodynamical and kinetical performance of MgH2 can be achieved using Ti or/and S. This improvement is attributed to the higher electronegativity of Ti (relative to Mg), the decrease of Mg–H interaction and the formation of Ti–H and Mg–S bonds. Our studies provide new insight into the enhanced dehydrogenation of MgH2 modified by Ti and S, which is useful for designing advanced magnesium hydrides with favorable H-desorption performance.

Electronic and optical properties of all-inorganic TiX3 transition metal halide nanowires

Transition metal halides are promising for use in photovoltaics and optoelectronics. This research systematically investigated the composition-dependent electronic and optical properties of all-inorganic TiX3 (X = Cl/Br/I) transition metal halide nanowires using first-principles density functional theory (DFT) and time-dependent DFT calculations. The findings emphasize the significant impact of the specific halide type on the electronic and optical characteristics of TiX3 nanowires. Particularly, the type of halide significantly influences the electronic states near the Fermi level and the infrared photoabsorption properties. An important discovery is the exceptional photoabsorption strength observed in the TiCl3 nanowire, reaching an impressive value of 26000 cm−1. The study also offers insights into exciton generation, aided by Transition Contribution Maps. Apart from its theoretical implications, we expect that the insights gained from this research will contribute to the advancement of active optical devices utilizing all-inorganic halide perovskites.

Superior selectivity for NH3 (NO2) gas molecules in In2SSe (Ga2SSe) Janus materials: a first-principles study

Anthropogenic gasses are very detrimental, requiring superior sensitive and selective materials to sense and segregate them. Using first-principles density functional theory (DFT) tools we have explored the sensitivity and selectivity of CO2, CO, NH3, NO2, H2S, and SO2 gases in the promising group-III Janus Ga2SSe and In2SSe nanostructured materials. We have explored all the possible adsorption sites in the Ga2SSe and In2SSe monolayer for sensing the gases and found that all the gasses are physisorbed in the sites with the lowest adsorption energy of −0.392 eV (−0.167 eV) for NH3 (NO2) on top of Indium (on the bridge-3 site) site of In2SSe (Ga2SSe). All adsorbed gasses significantly alter the bandgap of Ga2SSe and In2SSe from their pristine value and NO2-adsorbed M2SSe (M = Ga, In) structure exhibits significant bandgap changes: ∼0.16 eV reduction in Ga2SSe and ∼0.3 eV reduction in In2SSe from the pristine value, signifying substantial increase in conductivity. Additionally, analyzing the total density of states (TDOS), it can be concluded that NH3 at the Indium site of In2SSe and NO2 at the Bridge-3 site of Ga2SSe exhibit the most significant conductivity changes. Considering charge transfer, it is determined that 0.727 e/Å−3 of charge is transferred from In2SSe to NH3, while 1.05 e/Å−3 of charge is transferred from Ga2SSe to NO2 gas molecules, inferring that both NH3 and NO2 act as electron acceptors. Through this analysis, we found that NH3 is very selective on In2SSe while NO2 is selective on Ga2SSe Janus materials among the control gasses. This selectivity toward NH3 (NO2) gas on In2SSe (Ga2SSe) Janus material can open the new possibility of these materials for noxious gas sensing as well as NO2 utilization applications.

Magnetic splitting induced ferromagnetism in chromium-doped HfSe2

Two-dimensional (2D) magnetic layered materials have gained significant interest in spintronic for memory storage devices. Herein, the electronic and magnetic properties of high-quality single crystals of chromium (Cr)-doped hafnium diselenide were studied. The electronic study indicated that the Cr-doped HfSe2 reveals the semiconducting nature with a proper bandgap as identified by angle-resolved photoemission spectroscopy and first-principle density functional theory (DFT) calculations. The magnetic results indicate that the HfSe₂ exhibited ferromagnetic characteristics with the substitution of Cr atoms in a perpendicular direction. The Cr-doped HfSe2 sample shows a ferromagnetic phase below ∼58 K, indicating the critical temperature (Tc) of magnetic order, while above this temperature, it shows a paramagnetic phase that can follow the Curie-Weiss Law. It can be seen that the magnetic moment is approximately 0.16 emu/g at 5 K, which further decreases with increasing temperature until 60 K and then transforms to a paramagnetic phase. The sample can clearly show a weak out-of-plane magnetic anisotropy with a coercivity of around 50.56 Oe and a saturation magnetization value of 0.029 emu/g. Convincingly, it was observed from the ARPES results that the valence band is split due to the spin-orbit interaction of Cr atoms, which can induce ferromagnetic order. At the same time, the temperature-dependent ARPES data shows that the splitting is higher below the Tc, while it is weaker above the Tc. The DFT results revealed that near the Fermi level, the spin-up and spin-down are spin-polarized with asymmetric trends. It can be observed that substituting Cr on the octahedral sites of Hf atoms influences the electron spin order created by Hf vacancies (as validated by the magnetic coupling measured by electron paramagnetic resonance spectroscopy experiments), which can change the magnetic alignment to induce the magnetism in HfSe2. This study highlights that doping transition metal in HfSe2 has promising potential for future applications, especially in memory devices and spintronic technologies.

First-Principles Analysis of the Effects of Halogen Variation on the Properties of Lead-Free Novel Perovskites AlGeX3 (X = F, Cl, Br, and I).

To commercialize optoelectronic products and perovskite-based solar cells, nontoxic inorganic cubic metal halide perovskites have gained popularity. This study presents the structural, mechanical, electronic, and optical properties of novel lead-free metal cubic halide perovskites AlGeX3 (X = F, Cl, Br, and I) using the first-principles Density Functional Theory (DFT) approach. The verification of the mechanical stability of all compounds is conducted through Born stability criteria and formation energy. All of the compounds in the elastic investigations exhibit anisotropy, ductility, and elastic stability. The electronic band structures estimated by HSE06 and GGA-PBE functional demonstrate indirect to direct band gap transformation after substituting the halide F with the halides Cl, Br, and I. The tunability of halide-dependent energy bandgaps and the underestimation of energy band gaps are also noticed for the studied compounds by comparing the results obtained from the GGA-PBE functional and HSE06 investigations. The origins of bandgap transformation as well as halide-dependent energy gap modulation are explained by both the partial and total density of states (PDOS and TDOS). The results presented here suggest that all the compounds show low reflectivity, a high absorption coefficient, and also high optical conductivity in the visible and UV regions, making these materials suitable for multijunctional solar cells. Also, these materials have a greater impact on other optoelectronic device applications. Compared to other compounds, the optical investigation reported here demonstrates that AlGeI3 exhibits excellent optical conductivity and absorption in the visible region.