Bond Length Variations Research Articles

Structural, Electronic, and Optical Properties of Cs2SnX4 (X = Cl, Br, and I) Multilayers: A Density Functional Theory Study

The Cs2SnX4 (X = Cl, Br, and I) halide perovskites are currently emerging as a new family of 2D materials and promising candidates for photovoltaic and optoelectronic devices. Herein, the structural, electronic, and optical properties of the Cs2SnX4 multilayers (Ms) with 1–3 layers are investigated by density functional theory (DFT). The structural properties show that the bond length variation of Cs2SnX4 is closely related to surface effects. The Cs2SnCl4 Ms have the lowest formation energies and the best stability, and the formation energies decrease and the stability improves when the number of layers increases. The electronic properties show that the direct‐bandgap semiconductor Cs2SnX4 Ms bandgap values (Cl: 1.513–1.188–0.932, Br: 1.342–1.015–0.862, I: 1.198–0.903–0.729) decrease with the change from 1 to 3 layers and from Cl to I. The p orbitals of X atoms and 5p orbitals of Sn atoms are mainly involved in conduction. The optical properties indicate that Cs2SnX4 Ms achieve coverage and adjustability in the near‐infrared and visible‐light ranges. The calculations provide a fundamental theoretical basis for the research and application of Cs2SnX4.

Experimental and theoretical investigation of structure-magnetic properties relationships in a new heteroleptic one-dimensional triple bridged azido/acetato/DMSO copper (II) coordination polymer

We report the synthesis, structure, and magnetic investigation of a novel one-dimensional copper (II) coordination polymer: formula [Cu(μ1,1N3)(μ1,3OAc)(μ-DMSO)]∞ (1) (OAc=acetate). The connection between neighbouring copper ions arises through a triple bridge involving EO type azide, syn–syn carboxylate and μ-DMSO molecule in apical positions forming an elongated octahedral coordination of copper ions. Magnetic susceptibility measurements show that complex (1) exhibits ferromagnetic intrachain interactions Jchain = +66 cm−1 whereas a metamagnetic behaviour occurs below 2.1 K related with antiferromagnetic inter-chain interactions with a critical field of 600 Oe at 1.8 K. DFT calculations were caried out to rationale the in-chain ferromagnetic interactions between copper ions. We found that the ferromagnetic coupling between copper ions is less sensitive to variation of bond length and/or Cu-N-Cu angle that may be caused by external stimulus such as temperature or pressure.

Structural Behavior Of Garnet-Type Li7La3Zr2O12 Solid Electrolyte Material: A Classical Molecular Dynamics Study

The cubic garnet-type Li7La3Zr2O12 is an eminent candidate for next-generation solid state battery technology due to its thermal stability and high ionic conductivity. As such, its operation mechanisms need to be thoroughly understood, particularly focusing on the structural instability challenge reported to occur at lower temperatures. Herein, the statistical sampling capability of molecular dynamics simulations is employed during the investigation of fundamental structural, kinetic and thermodynamic properties emanating its subjection to pressure and temperature. Systematic induction of pressure yielded transition of the tetragonal phase to the cubic phase at 2 GPa pressure. The lattice parameters for the cubic and tetragonal phases, acquired in the current study are within 0.38 % agreement with literature. Furthermore, the XRD graphs confirm varying phases under different pressure conditions. The temperature phase diagram for 0 GPa structure agrees well with the literature trends and interestingly, the 2 GPa structure retained the cubic phase at various temperatures and confirmed in the XRDs and temperature phase diagram. Interrogatio of LaO8 dodecahedral and ZrO6 octahedra demonstrated no significant variations in bond lengths and bond angles giving a good indication for the regulation of Li+-transport channel size in the 2 GPa structure. Efforts in this study are a preliminary stage to fully understanding the thermodynamic impact as a structural modification avenue pending further investigations.

The intrinsic mechanical properties of NbTaTiZr and the influence of alloying elements Mo and W: A first-principles study

The intrinsic mechanical properties of NbTaTiZr and NbTaTiZrX (X Mo, W) are studied by using a first-principles calculation in combination with special quasi-random structure. For NbTaTiZr RHEA, the ideal tensile strength (ITS) and compressive strength (ICS) along [001] direction is respectively calculated to be 5.64 GPa and −18.4Gpa at the strain 11% and −20%. For shear loading along (211)[111] slip system, the ideal shear strength (ISS) is 3.03 GPa as the strain is about 20%. With addition of Mo into NbTaTiZr, the ITS, ICS and ISS respectively increase up to 9.7, −19.3 and 4.73 GPa, and the addition of W enhances the ITS, ICS and ISS up to 10.5, −21.1 and 5.12 GPa, respectively. Hence addition of Mo or W can significantly improve the ideal strength, especially stronger impact of W element. The calculated elastic moduli E [001] and G [111] are in reasonable agreement with the initial slope of the stress-strain relationship. The derived dimensionless ITS and ISS from first-principle calculations is underestimated the ideal strength in comparison with the prediction by the universal empirical model. Then the microcosmic mechanism is further studied by examination of the detailed bond length variation of three HEAs during the corresponding deformation. Near the critical strain under of tension and shear, the slower descend of stress originates from the gradual breakage of atomic bonds, while the compressive stress decreases rapidly owing to all bond fracturing almost simultaneously. Finally, the ideal strength of three HEAs is further analyzed from electronic structure. • The ideal strength is studied using ab initio calculations based on special quasi-random model. • The ideal strength and critical strain show NbTaTiZr has good strength and ductility. • The dimensionless ideal strength is overestimated in the empirical model. • The slow descend of tensile/shear stress results from the gradual breakage of atomic bonds. • The rapid decrease of compressive stress is due to the almost simultaneous fracture of all bonds.

Boron Nitride- and Graphene-Supported Trimetallic Yolk–Shell and Hollow Nanoparticles

In this study, the stability of yolk–shell and hollow Ni@PdAu and PdAu@Ni nanoparticles in free state and supported on boron nitride and graphene substrates was investigated at 300 K using molecular dynamics simulations. To this end, analyses of several parameters such as relative stability, surface energy, coordination number, number of surface atoms, bond length, displacement vector, and atomic strain were employed. Results show that the yolk–shell and hollow Ni@PdAu nanoparticles are more stable than yolk–shell and hollow PdAu@Ni nanoparticles in free and supported states due to the less surface energy. Furthermore, results show that the hollow nanoparticles are more stable than yolk–shell nanoparticles in all states. In addition, results show that the void and hollow core in yolk–shell and hollow nanoparticles are unstable and collapse due to the contraction and expansion of the core and shell, which lead to the formation of the core–shell and quasi–Janus structures in hollow and yolk–shell nanoparticles, respectively. Moreover, nanoparticles create a quasi–ellipsoid structure with low density and high coordination number on the boron nitride. The yolk–shell and hollow nanoparticles are the most stable on the boron nitride due to the smaller displacement and variation of the atomic bond length and lower atomic strain, which are caused by the strong nonbonded interaction, good atomic match, and placement of the Ni, Au, and Pd atoms on the middle position of the boron nitride hexagonal structure. Generally, this study demonstrated that the stability of yolk–shell and hollow nanoparticles can be tuned by the substrate–nanoparticle interaction.

Theoretical study on BTF-based cocrystals: effect of external electric field.

The external electric field plays an important role in the sensitivity of cocrystal energetic materials. To reveal the influence of external electric field on benzotrifuroxan/2,4,6-trinitroaniline (BTF/TNA), benzotrifuroxan/trinitroazetidine (BTF/TNAZ), benzotrifuroxan/1,3,5-trinitrobenzene (BTF/TNB), and benzotrifuroxan/trinitrotoluene (BTF/TNT) cocrystals' sensitivity, atoms in molecules (AIM), frontier molecular orbitals, nitro group charges (QNO2), electron density values (ρ), electrostatic surface potentials (ESPs), bond dissociation energy (EBDE), and interaction energy (Eint) of the C-NO2 bond were calculated by density functional theory at M062X-D3/ma-def2 TZVPP and B3LYP-D3/6-311 + G (d, p) levels in this article. The results indicate that both negative and positive electric fields reduce the energy gap of the BTF-based cocrystals, and BTF/TNAZ is the most sensitive cocrystal among the four cocrystals. For BTF/TNA and BTF/TNB, the EBDE and the negative charge of the nitro group decreases with increasing positive electric field strength, the Vs max increases with positive electric field strength, and the sensitivity of cocrystal eventually tends to increase under the positive electric field. For BTF/TNAZ and BTF/TNT, the EBDE and the negative charge of the nitro group decrease with increasing negative electric field strength, the Vs max increases with negative electric field strength, and the sensitivity of cocrystal eventually tends to increase under the negative electric field. Finally, the variation in bond length, nitro charge, and AIM electron density values are well correlated with the strengths of the external electric field.

A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

Structural, optical, magnetic, and photoluminescence properties of Sn0.7−xMo0.3 NdxO2+δ (0.0 ≤ x ≤ 0.3)

In this study, the properties of a series of (Sn0.7−xMo0.3 NdxO2+δ) (0.0 ≤ x ≤ 0.3) Nd3+ thin films prepared by sol–gel/spin coating technique were examined. The XRD analysis revealed the formation of all thin films in the form of Cassiterite structure. According to the FTIR investigation, when Nd3+ was substituted for Sn4+ ions in the crystal lattice, the absorption peaks migrated to the lower wavenumber side. This could be related to variations in bond length that occurs when Sn4+ ions in the crystal lattice are replaced with lighter Nd3+ ions. The morphology of the films was examined by using scanning electron microscope (SEM). In terms of Nd content, optical properties such as optical band gap, refractive index (n), and extinction coefficient (k) were investigated. The magnetic characteristics indicated diamagnetic behavior of Sn0.7Mo0.3O2+δ, paramagnetic behavior of Sn0.6Nd0.1Mo0.3O2+δ, and ferromagnetic behavior of samples with a high concentration of Nd, (Sn0.5Nd0.2Mo0.3O2+δ, Sn0.4Nd0.3Mo0.3O2+δ). The presence of active Nd3+ successfully introduced into the Sn:Mo host matrix is confirmed by the excitation dependent (PL) observed in the 350–700 nm range. PL measurements reveal two large bands located at 425 and 466 nm.

Investigations on superconductivity in an equi-atomic disordered Hf-Nb-Ta-Ti-V high entropy alloy

High entropy alloys (HEA) are expected to show unusual superconducting behavior due to chemical disorder along with highly distorted lattice. Herein, we report the synthesis, structure, and detailed superconducting properties of a new and nearly equi-atomic single phase disordered superconducting high entropy alloy with nominal composition Hf20Nb20Ta20Ti20V20. Rietveld refinement of the Synchrotron X-ray diffraction (SXRD) confirms the DFT predicted single-phase BCC structure in the HEA with lattice parameter a = 3.32 Å. Reverse monte carlo (RMC) modelling of the diffraction data evince the local atomic disorder and a considerable lattice distortion overriding the crystal structure due to significant variations in bond length and co-ordination numbers. Microstructural characterization using energy-dispersive X-ray spectroscopy (XEDS) equipped SEM shows dendritic morphology of the as solidified alloy with incomplete partition of the constituent elements. Transport, magnetization and low temperature specific heat measurement established that the current HEA is a moderately coupled type-II superconductor with a Tc of 5.0 K, lower critical field Hc1(0)=19mT and upper critical field Hc2(0) = 6.63T.

Cubine Monolayer as a Super Sensor for NO2 Molecule Detection and Capture

AbstractIn this work, the potential application of cubine monolayer as a highly sensitive sensor for the detection and capture of toxic gas molecule NO2 is investigated by employing density functional theory (DFT) calculations. Eight gas molecules, namely CO, CO2, CH4, H2S, SO2, NH3, NO, and NO2 are considered. After examining the adsorption energy, adsorption distance, variation of bond length and Mulliken charge, the results show that only NO2 molecule present chemical adsorption on cubine monolayer. The calculated work functions and I–V curves indicate that cubine has the highest sensitivity to NO2 molecules. The adsorption energies decrease following the increase of strain in both A and B directions, when the strain along the A direction reaches 12%, the adsorption energy decreases to −0.686 eV, which shows physical adsorption. This provides a desorption strategy for recycling. The calculations employed in this work indicate that the cubine monolayer can be a prominent candidate for the application of a super sensor to detect and capture NO2 toxic gas molecules.

Surface-dependent band structure variations and bond deviations of GaN.

Density functional theory (DFT) calculations on a tunable number of GaN (0001) planes give an invariant band structure, density of states (DOS) diagram, and band gap of the GaN unit cell. Dissimilar band structures and DOS diagrams are obtained for 1, 3, 5, 7, and 9 layers of GaN (101̄0) planes, but the same band structure as that of the (0001) plane returns for 2, 4, 6, and 8 (101̄0) planes. Furthermore, 1 to 4 layers of GaN (101̄1) planes exhibit dissimilar band structures, but the GaN unit cell band structure is obtained for 5 (101̄1) planes. While there are no changes to the Ga-N bond length and bond geometry for the (0001) planes, the (101̄0) planes present bond length variation and bond distortion with odd numbers of layers. Bond length and bond direction deviations are also obtained for 1 to 4 (101̄1) planes. These results suggest that slight structural deviations may be present near the GaN surface to produce facet-dependent properties, and such atomic position deviations in the surface layer can be observed in various semiconductors.

Uniaxially Strained Graphene: Structural Characteristics and G-Mode Splitting.

The potential use of graphene in various strain engineering applications requires an accurate characterization of its properties when the material is under different mechanical loads. In this work, we present the strain dependence of the geometrical characteristics at the atomic level and the Raman active G-band evolution in a uniaxially strained graphene monolayer, using density functional theory methods as well as molecular dynamics atomistic simulations for strains that extend up to the structural failure. The bond length and bond angle variations with strain, applied either along the zigzag or along the armchair direction, are discussed and analytical relations describing this dependence are provided. The G-mode splitting with strain, as obtained by first principles’ methods, is also presented. While for small strains, up to around 1%, the G-band splitting is symmetrical in the two perpendicular directions of tension considered here, this is no longer the case for larger values of strains where the splitting appears to be larger for strains along the zigzag direction. Further, a crossing is observed between the lower frequency split G-mode component and the out-of-plane optical mode at the Γ point for large uniaxial strains (>20%) along the zigzag direction.

Investigation of structural and magnetic properties of Al substituted Ba0.9La0.1Fe12-xAlxO19 hexaferrites prepared by solid-state reaction method

The structural and magnetic properties in the Ba0.9La0.1Fe12-xAlxO19 hexaferrite compound were studied as a function of Al3+ concentration (x = 0.0 to 1.3). The samples were obtained by solid-state reaction synthesis and sintered by the conventional ceramic method. The Rietveld refinement method analysis showed the predominance of barium hexaferrite phase with hexagonal structure characterized by P63/mmc space group. The secondary phase of hematite (α-Fe2O3), with rhombohedral system and R-3c space group, was found. For the principal phase, the lattice parameters a, c, and the cell volume (V) reached a minimum value in the concentration x = 0.7 of Al3+ ion. Variations in the average bond length and distortion index of the oxygen polyhedra were observed at the Ba(2d) site and at the Fe (2b, 4fIV, 4fVI, and 12 k) sites. The Raman spectra showed all bands related to FeO bonds at different crystallographic sites. The saturation magnetization reached the minimum value for the composition x = 0.7. The coercive field increased until reaching the maximum value at x = 1.0 of Al3+ concentration. The magneto-crystalline anisotropy field decreased from x = 0.1 to 0.7 and increased at higher concentrations. At room temperatures the total magnetic moment decreases from μ = 10.74 to 4.33 μB for Al3+ concentration range of 0.1 ≤ x ≤ 0.7 and oscillates with μ = 8.18 and 9.44 μB at compositions of x = 1.0 and 1.3, respectively. The present study is important to understand the impact of Al and La addition on the structural and magnetic properties of barium hexaferrite.

CO2 Adsorption on PtCu Sub-Nanoclusters Deposited on Pyridinic N-Doped Graphene: A DFT Investigation.

To reduce the CO2 concentration in the atmosphere, its conversion to different value-added chemicals plays a very important role. Nevertheless, the stable nature of this molecule limits its conversion. Therefore, the design of highly efficient and selective catalysts for the conversion of CO2 to value-added chemicals is required. Hence, in this work, the CO2 adsorption on Pt4-xCux (x = 0–4) sub-nanoclusters deposited on pyridinic N-doped graphene (PNG) was studied using the density functional theory. First, the stability of Pt4-xCux (x = 0–4) sub-nanoclusters supported on PNG was analyzed. Subsequently, the CO2 adsorption on Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG was computed. According to the binding energies of the Pt4-xCux (x = 0–4) sub-nanoclusters on PNG, it was observed that PNG is a good material to stabilize the Pt4-xCux (x = 0–4) sub-nanoclusters. In addition, charge transfer occurred from Pt4-xCux (x = 0–4) sub-nanoclusters to the PNG. When the CO2 molecule was adsorbed on the Pt4-xCux (x = 0–4) sub-nanoclusters supported on the PNG, the CO2 underwent a bond length elongation and variations in what bending angle is concerned. In addition, the charge transfer from Pt4-xCux (x = 0–4) sub-nanoclusters supported on PNG to the CO2 molecule was observed, which suggests the activation of the CO2 molecule. These results proved that Pt4-xCux (x = 0–4) sub-nanoclusters supported on PNG are adequate candidates for CO2 adsorption and activation.

Effect of sp hybridization and bond-length disorder on magnetism in amorphous carbon- A first-principles study

Magnetism in amorphous carbon system is investigated using first-principle molecular dynamics (MD) and constrained magnetic calculations. The existing literature suggests that sp2-like 3-fold coordinated carbon atoms only play a significant role in obtaining magnetism in amorphous carbon. The present calculations predict the role of the sp1 hybridised atom and the presence of bond-length disorder in sustaining the magnetic moment in the amorphous carbon system. For a low-density amorphous carbon structure with a typical density of 1.0 g/cm3 and below, the sp1-like 2-fold coordinated carbon atoms play an essential role instead of the sp2 atom. In the intermediate density regime, i.e., between 1.0 g/cm3 to 3.0 g/cm3, apart from sp1 or sp2 fraction, the effect of variation in bond length is observed to play a vital role to sustain magnetism in the amorphous carbon system. The origin of magnetism in Q‑carbon in terms of bond-length disorder and change in hybridization ratio is explained.

Theoretical studies on the EPR parameters and local structure of Cu2+ center in [Co(nicotinamide)2(H2O)4](saccharinate)2 crystal

Local structure and electron paramagnetic resonance (EPR) parameters for Cu2+ center in [Co(nicotinamide)2(H2O)4](saccharinate)2 (CoNAS) crystal are theoretically investigated using high-order perturbation formulas for 3d9 ions in rhombically elongated octahedra. In the calculated formulas, the related molecular orbital coefficients are acquired from the superposition model which enables to correlate the crystal field parameters and hence the EPR parameters with the studied Cu2+ center, the admixtures of d-orbitals in the ground state wave function as well as the ligand orbital and spin–orbit coupling contributions are taken into account. Based on the studies, the [CuO4N2]12− cluster of the studied Cu2+ center are found to suffer the axial and perpendicular bond length variations Δz (≈ 0.0612 Å) along z-axis and δr (≈ 0.0360 Å) along x- and y-axes, respectively, due to the Jahn–Teller effect.

In Search of the Perfect Triple BB Bond: Mechanical Tuning of the Host Molecular Trap for the Triple Bond B≡B Fragment.

The coordination of the B2 fragment by two σ-donor ligands L: could lead to a diboryne compound with a formal triple bond L:→B≡B←:L. σ-Type coordination L:→B leads to an excess of electrons around the B2 central fragment, whereas π-back-donation from the B≡B moiety to ligand L has a compensation effect. Coordination of the σ-donor and π-acceptor ligand is accompanied by the lowering of the BB bond order. Here, we propose a new approach to obtain the perfect triple BB bond through the incorporation of the BB unit into a rigid molecular capsule. The idea is the replacement of π-back-donation, as the principal stabilization factor in the linear NBBN structure, with the mechanical stabilization of the BB fragment in the inert molecular capsule, thus preserving the perfect B≡B triple bond. Quantum-chemical calculations show that the rigid molecular capsule provided a linear NBBN structure and an unusually short BB bond of 1.36 Å. Quantum-chemical calculations of the proposed diboryne adducts show a perfect triple bond B≡B without π-back-donation from the B2 unit to the host molecule. Two mechanisms were tested for the molecular design of a diboryne adduct with a perfect B≡B triple bond: the elimination of π-back-donation and the construction of a suitable molecular trap for the encapsulation of the B2 unit. The second factor that could lead to the strengthening or stretching of a selected chemical bond is molecular strain produced by the rigid molecular host capsule, as was shown for B≡B and for C≡C triple bonds. Different derivatives of icosane host molecules exhibited variation in BB bond length and the corresponding frequency of the BB stretch. On the other hand, this group of molecules shows a perfect triple BB bond character and they all possess a similar level of HOMO.

Insights into the Observed trans-Bond Length Variations upon NO Binding to Ferric and Ferrous Porphyrins with Neutral Axial Ligands.

NO is well-known for its trans effect. NO binding to ferrous hemes of the form (por)Fe(L) (L = neutral N-based ligand) to give the {FeNO}7 (por)Fe(NO)(L) product results in a lengthening of the axial trans Fe–L bond. In contrast, NO binding to the ferric center in [(por)Fe(L)]+ to give the {FeNO}6 [(por)Fe(NO)(L)]+ product results in a shortening of the trans Fe–L bond. NO binding to both ferrous and ferric centers involves the lowering of their spin states. Density functional theory (DFT) calculations were used to probe the experimentally observed trans-bond shortening in some NO adducts of ferric porphyrins. We show that the strong σ antibonding interaction of dz2 and the axial (L) ligand p orbitals present in the Fe(II) systems is absent in the Fe(III) systems, as it is now in an unoccupied orbital. This feature, combined with a lowering of spin state upon NO binding, provides a rationale for the observed net trans-bond shortening in the {FeNO}6 but not the {FeNO}7 derivatives.

Atomic Stripe Formation in Infinite-Layer Cuprates.

High-temperature superconductivity appears in cuprate materials that have been tuned in a way where the copper–oxygen bond configuration and coordination is in a state of minimal energy. In competition with the Jahn–Teller effect, which impedes the formation of infinitely connected CuO2 planes, the state of minimal energy persists for planar copper–oxygen bond length variations of up to 10%. We have synthesized the infinite-layer phases of CaCuO2 and SrCuO2 as single-crystalline films using molecular beam epitaxy and performed in-plane scanning transmission electron microscopy mapping. For the infinite-layer phase of CaCuO2 with a short Cu–O bond length, the CuO2 planes maintain their minimal energy by forming distinguished atomic stripes. In contrast, atomic stripe formation does not occur in the infinite-layer phase of SrCuO2, which has a larger Cu–O bond length. The polar field provided by the charge reservoir layer in cuprates with infinitely connected CuO2 planes holds the key over the emergence of superconductivity and is vital to maintain infinitely connected CuO2 planes themselves.

Elucidation and quantification of the factors underlying bond-length variation in inorganic solids for the design of non-oxide materials with superior functional properties

Elucidation and quantification of the factors underlying bond-length variation in inorganic solids for the design of non-oxide materials with superior functional properties