Synthesis and Properties of Pr1-xRbxMnO3 (0.05≤x≤0.08) with Perovskite-Type Structure

Rb-substitued Pr_1-xRb_xMnO₃ (0.05≤x≤0.08) was successfully synthesized by solid state reaction. Powder X-ray diffraction showed that all the compounds were orthorhombic with the space group of Pnma. Spin glass behaviors were observed for all the compounds at low temperature, suggesting the competition between ferromagnetic and antiferromagnetic. The temperature dependence of the resistivity for the compound Pr_0.92Rb_0.08MnO_3.02 at 0 and 2 T magnetic field was also investigated. The compound shows semiconducting behavior, and the band gap is 0.3 eV. The maximum magnetoresistance is about 30% at 2 T magnetic field near 116 K.     

External electric field effect on electronic properties and charge transfer in CoI2/NiI2 spinterface

Electronic structure and spin-related properties of CoI₂/NiI₂ heterostructure were studied by means of density functional theory. It was shown that the electronic structure at the Fermi level can be characterized by a band gap. The effect of the external electric field on charge transfer and electronic properties of the CoI₂/NiI₂ interface was investigated, and it was found that band gap width depends on the strength of the applied electric field, switching its nature from semiconducting to a half-metallic one. An easy control of the electronic properties and promising spin-polarized nature of the CoI₂/NiI₂ spinterface allows the heterostructure to be used in spin-related applications.     

Ab initio electronic structure of NiCoCrGa half-metallic quaternary Heusler compound

In this study, ab initio calculation results of electronic structure and elastic properties of NiCoCrGa quaternary Heusler compound are presented. Plane wave pseudopotential method is used with spin-polarized Generalized Gradient Approximation (σ-GGA) scheme of the Density Functional Theory (DFT). Static elastic constants of the cubic system satisfy mechanical stability criteria. The cubic phase of the system remains stable under tetragonal distortion. The spin-polarized electronic band structures and density of electronic states indicate a metallic band structure for majority spins, while minority spin structure has semiconducting character. This situation displays a slightly disturbed half-metallic behavior with high-spin polarization ratio (P = 0.961) at Fermi level E_F. Two electronic bands of minority spins resulting from d-states of cobalt atom cross Fermi level at Γ-point. This situation gives a finite but very low density of states at E_F. The material can be classified as a new half-metallic ferromagnet for spintronic applications.     

Half-metallicity,magnetism, mechanical,thermal, and phonon properties of FeCrTe and FeCrSe half-Heusler alloys under pressure

The half-metallicity of Heusler alloys is quite sensitive to high pressure and disorder. To understand this phenomenon better, we systematically studied the half-metallic nature, magnetism, phonon, and thermomechanical properties of FeCrTe and FeCrSe Heusler alloys under high pressure using ab initio calculations based on density functional theory. The ground-state lattice constants for FeCrTe and FeCrSe alloys are 5.93 and 5.57 Å, respectively, consistent with available theoretical results. Formation energy, cohesive energy, elastic constants, and phonon dispersion confirmed that both compounds are thermodynamically and mechanically stable. The FeCrTe and FeCrSe alloys showed a half-metallic character with a band gap of 0.68 and 0.58 eV at 0 GPa pressure, respectively, and magnetic moments of 2.01 μ_B for both alloys, using generalized gradient approximation (GGA) approximation. FeCrTe alloy changes from metallic to half-metallic above 30 GPa pressure using GGA + U. The elastic properties were scrutinized, and it was found that, at 0 GPa pressure, FeCrTe is ductile, and FeCrSe is brittle. Under pressure, FeCrSe becomes brittle above 10 GPa pressure. Average sound velocity V_m, Debye temperature Ɵ_D, and heat capacity C_V were predicted under pressure. These outcomes will improve the integration of Fe-based half-Heusler alloys in spintronic devices.     

Ternary K2Zn5As4‐type pnictides Rb2Cd5As4 and Rb2Zn5Sb4, and the solid solution Rb2Cd5(As,Sb)4

Dirubidium pentacadmium tetraarsenide, Rb₂Cd₅As₄, dirubidium pentazinc tetraantimonide, Rb₂Zn₅Sb₄, and the solid‐solution phase dirubidium pentacadmium tetra(arsenide/antimonide), Rb₂Cd₅(As,Sb)₄ [or Rb₂Cd₅As_3.00(1)Sb_1.00(1)], have been prepared by direct reaction of the component elements at high temperature. These compounds are charge‐balanced Zintl phases and adopt the orthorhombic K₂Zn₅As₄‐type structure (Pearson symbol oC44), featuring a three‐dimensional [M₅Pn₄]²⁻ framework [M = Zn or Cd; Pn is a pnicogen or Group 15 (Group V) element] built of linked MPn₄ tetrahedra, and large channels extending along the b axis which host Rb⁺ cations. The As and Sb atoms in Rb₂Cd₅(As,Sb)₄ are randomly disordered over the two available pnicogen sites. Band‐structure calculations predict that Rb₂Cd₅As₄ is a small‐band‐gap semiconductor and Rb₂Zn₅Sb₄ is a semimetal.     

Quantum-Chemical Simulation of the Proton Transport in Mono- and Disubstituted Salts with Octahedral Anions

Salts Rb₂H₃IO₆, Rb₄H₆I₂O₁₂, and Rb₄H₂I₂O₁₀ and adducts CsHSO₄· H₆TeO₆ and Cs₂SO₄· H₆TeO₆ of the salt · acid type are calculated within density functional theory B3LYP. Calculations for Te, I, Rb, and Cs atoms make use of basis set LanL2DZ complemented by polarization d,p-functions and pseudopotential LanL2; for Li, O, and H atoms, basis set 6-31G** is used. The activation energy for the proton migration is commensurate with that for the water molecule abstraction in the salts and is smaller in rubidium salts than in cesium salts.     

On‐surface Synthesis of a Semiconducting 2D Metal–Organic Framework Cu3(C6O6) Exhibiting Dispersive Electronic Bands

A 2D metal–organic framework (2D‐MOF) was formed on a Cu(111) substrate using benzenehexol molecules. By means of a combination of scanning tunneling microscopy and spectroscopy, X‐ray photoelectron spectroscopy and density‐functional theory, the structure of the 2D‐MOF is determined to be Cu₃(C₆O₆), which is stabilized by O–Cu–O bonding motifs. We find that upon adsorption on Cu(111), the 2D‐MOF features a semiconductor band structure with a direct band gap of 1.5 eV. The O–Cu–O bonds offer efficient charge delocalization, which gives rise to a highly dispersive conduction band with an effective mass of 0.45 m_e at the band bottom, implying a high electron mobility in this material.     

Understanding the synergistic effects,optical and electronic properties of ternary Fe/C/S‐doped TiO2 anatase within the DFT + U approach

Although TiO₂ is an efficient photocatalyst, its large band gap limits its photocatalytic activity only to the ultraviolet region. An experimentally synthesized ternary Fe/C/S‐doped TiO₂ anatase showed improved visible light photocatalytic activity. However, a theoretical study of the underlying mechanism of the enhanced photocatalytic activity and the interaction of ternary Fe/C/S‐doped TiO₂ has not yet been investigated. In this study, the defect formation energy, electronic structure and optical property of TiO₂ doped with Fe, C, and S are investigated in detail using the density functional theory + U method. The calculated band gap (3.21 eV) of TiO₂ anatase agree well with the experimental band gap (3.20 eV). The defect formation energy shows that the co‐ and ternary‐doped systems are thermodynamically favorable under oxygen‐rich condition. Compared to the undoped TiO₂, the absorption edge of the mono‐, co‐, and ternary‐doped TiO₂ is significantly enhanced in the visible light region. We have shown that ternary doping with C, S, and Fe induces a clean band structure without any impurity states. Moreover, the ternary Fe/C/S‐doped TiO₂ exhibit an enhanced photocatalytic activity, a smaller band gap and negative formation energy compared to the mono‐ and co‐doped systems. Moreover, the band edges of Fe/C/S‐doped TiO₂ align well with the redox potentials of water, which shows that the ternary Fe/C/S‐doped TiO₂ is promising photocatalysts to split water into hydrogen and oxygen. These findings rationalize the available experimental results and can assist the design of TiO₂‐based photocatalyst materials.     

Ab initio calculations of structural, elastic, and electronic properties of silver nitrides

The all-electron approach implemented in the CRYSTAL06 program is used along with a pseudopotential method in the pseudo-atomic orbital basis set to study the crystal structure, elastic constants and bulk moduli, the band structure and density of states for the family of silver nitrides. Calculations are performed within density functional theory with the use of local and gradient functionals to describe exchange and correlation. For the general type of the cubic lattice, all considered compounds can be put in the following order of their relative stability: AgN (rock salt structure), AgN₂ (fluorite structure), Ag₂N (cuprite structure), and Ag₃N (anti-ReO₂). It is shown that AgN, AgN₂, and Ag₂N are metals, whereas Ag₃N is a semiconductor with a band gap of 0.25 eV. Chemical bonding in these compounds has ionic and covalent components, apart from the metal one.     

Effect of electron-withdrawing groups on photovoltaic performance of thiophene-vinyl-thiophene derivative and benzochalcogenadiazole based copolymers: A computational study

We report a density functional theory study of the effect of electron-withdrawing groups such as –F, –CN, –NO₂ on the geometrical, optoelectronic, intramolecular charge transfer (ICT), and photovoltaic properties of (E)-1,2-bis(5-alkyl-[2,3′-bithiophene]-2′-yl)ethene (TVT-T) based donor-acceptor (D-A) copolymers with different acceptor units, that is, benzo[c][1,2,5]thiadiazole, benzo[c][1,2,5]oxadiazole, and benzo[c][1,2,5]selenadiazole. The computed optical absorption spectra of the designed compounds lie in the visible and near-infrared regions. Of all the studied copolymers, -CN substituted and Se-based compound displays the lowest HOMO-LUMO (E _{H - L}) gap and optical band gap (E _opt). The exciton binding energy (E _b) is found to be smaller for O-incorporated compounds and -CN substituted copolymer as well, inferring more ICT. The electron-hole coherence concentrated over the D-A units is nearly the same for -CN and -NO₂ substituted compounds, but larger in -F derivatives, indicating weak electron-hole coupling in the formers. Comparatively larger dipole moment (6.421 Debye-9.829 Debye) and charge transfer length (D _CT) (1.976 Å-3.122 Å) for -CN derivatives lead to enhanced ICT properties. The designed donors yield good hole mobilities (0.127-6.61 cm² V⁻¹ s⁻¹) and the predicted power conversion efficiencies are calculated to be as high as ~6%-7% for –CN and –NO₂ substituted compounds.     

DFT study of the half-metallic variant perovskites A2OsX6 (A = Rb/Cs; X = Cl/Br) for spintronic and thermoelectric applications

We present our results of the spin-polarized calculations on the structural, magneto-electronic, thermodynamic, and thermoelectric properties of vacancy-ordered double perovskites A₂OsX₆ (A = Rb/Cs; X = Cl/Br). We utilized the Wu-Cohen generalized gradient approximation (Wu-GGA) and the mBJ scheme to determine a more reliable electronic structure. The compounds exhibit negative formation energy, suitable tolerance factor, and a stable phonon spectrum, indicating their stability. The compounds show half-metallicity, acting as semiconductors with direct band gaps between 2 and 3 eV in the spin-up orientation while metallic in the spin-down. Each compound shows a total spin magnetic moment of 2.00 μB per formula unit, with Os-t²g states contributing the most (~1.5 μB). The computed thermoelectric coefficient indicates the usability of these compounds across a wide temperature range (200–800 K) with high electrical conductivity and low electronic thermal conductivity. The compounds exhibit high Seebeck coefficient and figure of merit (ZT), making them suitable for thermoelectric applications. With ferromagnetic and half-metallic characteristics, these compounds could be promising candidates for spintronics, thermoelectronic, and data storage applications.     

Bandgap Engineering of Lead‐Free Double Perovskite Cs2AgBiBr6 through Trivalent Metal Alloying

The double perovskite family, A₂M^IM^IIIX₆, is a promising route to overcome the lead toxicity issue confronting the current photovoltaic (PV) standout, CH₃NH₃PbI₃. Given the generally large indirect band gap within most known double perovskites, band‐gap engineering provides an important approach for targeting outstanding PV performance within this family. Using Cs₂AgBiBr₆ as host, band‐gap engineering through alloying of In^III/Sb^III has been demonstrated in the current work. Cs₂Ag(Bi_1−x M_x )Br₆ (M=In, Sb) accommodates up to 75 % In^III with increased band gap, and up to 37.5 % Sb^III with reduced band gap; that is, enabling ca. 0.41 eV band gap modulation through introduction of the two metals, with smallest value of 1.86 eV for Cs₂Ag(Bi_0.625Sb_0.375)Br₆. Band structure calculations indicate that opposite band gap shift directions associated with Sb/In substitution arise from different atomic configurations for these atoms. Associated photoluminescence and environmental stability of the three‐metal systems are also assessed.     

Structural Variety of Defect Perovskite Variants M3E2X9 (M = Rb,Tl, E = Bi,Sb, X = Br,I)

The compounds Rb₃Sb₂Br₉, Rb₃Sb₂I₉, Rb₃Bi₂Br₉, Rb₃Bi₂I₉, and Tl₃Bi₂Br₉ were synthesized and their crystal structures determined from single crystal X‐ray diffraction data. The compounds Rb₃Sb₂Br₉, Rb₃Sb₂I₉, and Rb₃Bi₂I₉ crystallize in the Tl₃Bi₂I₉ type of structure (space group P2₁/n, no. 14). Rb₃Bi₂Br₉ and Tl₃Bi₂Br₉ crystallize in a new but closely related type of structure (space group P2₁/a, no. 14). Both structure types feature characteristic double layers comprising corner‐sharing EX₆ octahedra. The space groups are set in a way that the stacking direction of the layers is the [001] direction. The group‐subgroup relations to cubic perovskite ABO₃ are discussed. Differences between M₃E₂X₉ types are attributed to distortions of the underlying MX₃ close packing. Depending on the atomic size ratio, the distortions are quantified by an order parameter.     

A first‐principles investigation of the hydrogen bond interaction and the sensitive characters in cis‐1,3,4,6‐tetranitrooctahydroimidazo‐[4,5‐d]imidazole

A theoretical study of structural and electronic properties of cis‐1,3,4,6‐tetranitrooctahydroimidazo‐[4,5‐d]imidazole (BCHMX) crystal is performed using density functional theory. The band structure, the total density of states, the atomic orbit projected density of states (PDOS) of C, N, O, and H, and Mulliken population analysis are discussed. The study by analyzing the PDOS shows that the structure of BCHMX crystal possesses C? H···O intra‐ and intermolecular hydrogen bonding. There are hydrogen bonds between H₃‐1s and O₅‐2p orbits, H₂‐1s and O₆‐2p orbits of intramolecules and between H₂‐1s and O₁‐2p orbits of intermolecules. The reasons for the smaller impact sensitivity compared with β‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane and 1,3,5‐trinitro‐1,3,5‐triazinane are also explored from the band gap in the crystal and the weakest bond dissociation energy in single molecule. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Electronic structure and bonding of the hydrides Mg3TH7 (T = Mn,Re) from first principles

From pseudo-potentials and all-electrons computations within density functional theory, desorption energies within range of MgH₂ and covalent like hydrogenated intermetallic compounds are identified for hydrogen rich Mg₃TH₇, (T = Mn, Re). The rhenium based compound is found with a lower desorption energy which has been quantified from the analysis of the Bader charges within the {TH₆}^5- complex anion as related with a decreasingly ionic charge on hydrogen from Mn to Re. The electronic densities of states show insulating compounds in agreement with literature relevant to this class of salt-like hydrides with a larger band gap for the Re compound. From chemical bonding analyses stronger Mn–H bonding versus Re–H is identified in agreement with desorption energies magnitudes favoring Mg₃ReH₇.     

Unexpected Synergy between Magnetic Iron Chains and Stacked B6 Rings in Nb6Fe1−xIr6+xB8

The synergistic combination of experiment and density functional theory has led to the discovery of the first ferromagnetic material, Nb₆Fe_1?xIr_6+xB₈, containing in its crystal structure iron chains embedded in stacked B₆ rings. The strong ferromagnetic Fe–Fe interactions found in the iron chains induce an unexpected strengthening of the B–B interactions in the B₆ rings. Beside these strong B–B interactions, strong interlayer metal–boron bonds (Ir–B and Nb–B) ensure the overall structural stability of this phase, while the magnetic Fe–Fe interactions are mainly responsible for the observed ferromagnetic ordering below T_C=350 K.     

The Special Case of the Spectral Emission of a Tb3+ Mono Metal Complex

The fine structure in the spectral lines of the visible fluorescence of Tb³⁺ complexes are replaced by a single peak in the case of a singular molecular complex Tb(H₃PTC)₃, where H₄PTC represents perylene-3,4,9,10-tetracarboxylic acid, and its emission wavelength depends on the film thickness. This single peak challenges the old creed that the f-orbital electrons of Tb³⁺ are always protected from the influence of the surrounding atoms. We perform density functional theory calculations to show that the wavefunction of the ground state is localized and in addition, spin-polarized, and this facilitates fluorescent transitions under UV to the first excited state instead of the fundamental state. We discuss the possibility of making a spintronic device with the molecule, Tb(H₃PTC)₃.     

Stabilizing RbPbBr3 Perovskite Nanocrystals through Cs+ Substitution

ABX₃-type halide perovskite nanocrystals (NCs) have been a hot topic recently due to their fascinating optoelectronic properties. It has been demonstrated that A-site ions have an impact on their photophysical and chemical properties, such as the optical band gap and chemical stability. The pursuit of halide perovskite materials with diverse A-site species would deepen the understanding of the structure–property relationship of the perovskite family. In this work we have attempted to synthesize rubidium-based perovskite NCs. We have discovered that the partial substitution of Rb⁺ by Cs⁺ help to stabilize the orthorhombic RbPbBr₃ NCs at low temperature, which otherwise can only be obtained at high temperature. The inclusion of Cs⁺ into the RbPbBr₃ lattice results in highly photoluminescent Rb_1−xCs_xPbBr₃ NCs. With increasing amounts of Cs⁺, the band gaps of the Rb_1−xCs_xPbBr₃ NCs decrease, leading to a redshift of the photoluminescence peak. Also, the Rb_1−xCs_xPbBr₃ NCs (x=0.4) show good stability under ambient conditions. This work demonstrates the high structural flexibility and tunability of halide perovskite materials through an A-site cation substitution strategy and sheds light on the optimization of perovskite materials for application in high-performance optoelectronic devices.