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Electronic, bonding and optical properties of the LiGaGe2X6 (X?=?S, Se, and Te) compounds: An ab initio study
摘要： In order to gain an insight into the bonding and to characterize linear and nonlinear optical properties of three infrared LiGaGe2S6, LiGaGe2Se6, and LiGaGe2Te6 compounds, we performed an ab initio calculation based on density functional theory. The calculations determined lattice parameters, band gaps, dipole moments, and second harmonic components that commonly agreed well with the available experimental data. We show in this article, through a large body of calculations, the trend in the row of participating anions, S ? Se ? Te, enhances the polarization effect and the coordination structure distortions in the polyhedra groups forming the bonding pattern of the cell. This one also affects both the electronic and the optical properties, making the compounds more propitious for device-based optical applications.
关键词： Ab initio calculation,Bonding analysis,Optical properties,Electronic population
更新于2025-09-09 09:28:46
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by means of high-pressure–high-temperature Raman experiments
摘要： We report here high-pressure–high-temperature Raman experiments performed on BiVO4. We characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nanopowder). The experimental results are supported by ab initio calculations, which, in addition, provide the vibrational patterns. The temperature and pressure behavior of the fergusonite lattice modes reflects the distortions associated with the ferroelastic instability. The linear coefficients of the zircon phase are in sharp contrast to the behavior observed in the fergusonite phase. The boundary of the fergusonite-to-scheelite second-order phase transition is given by TF ?Sch(K) = ?166(8)P (GPa) + 528(5). The zircon-to-scheelite, irreversible, first-order phase transition takes place at TZ?Sch(K) = ?107(8)P (GPa) + 690(10). We found evidence of additional structural changes around 15.7 GPa, which in the downstroke were found to be not reversible. We analyzed the anharmonic contribution to the wave-number shift in fergusonite using an order parameter. The introduction of a critical temperature depending both on temperature and pressure allows for a description of the results of all the experiments in a unified way.
关键词： zircon,scheelite,BiVO4,fergusonite,high-temperature,phase transition,high-pressure,ab initio calculations,Raman experiments,vibrational patterns
更新于2025-09-09 09:28:46
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UV–vis spectroscopy combined with azastilbene probe as a tool for testing basicity of mesoporous silica modified with nitrogen compounds
摘要： N-bromobenzyl substituted (E)-4′-hydroxy-4-stilbazolium bromide (NBSB), a representative of azastilbene compounds which reveals strong solvatochromism, was used as a probe molecule for testing basicity of nitrogen organic compounds grafted on mesoporous cellular foam silica (MCF). Easy deprotonation of this compound in contact with basic species was the reason for its use for probing the basicity of AP/MCF (AP - (3-aminopropyl) trimethoxysilane), Im/MCF (Im - N-trimethoxysililpropylimidazole) and Az/MCF (Az - (3-azidopropyl)(triethoxy)silane). NBSB and its deprotonated form were analyzed by UV–vis spectroscopy measurements in solutions and UV–vis in solid materials. Ab-initio calculation allowed distinction between the preference of NBSB for interactions with SiOH on the support surface and with nitrogen species of the modi?er. The NBSB protonation - deprotonation equilibrium dependence on pH was indicated. The role of solvent was considered. In water indirect probing of basicity by NBSB was observed via abstraction of proton from solvent by the modi?er and deprotonation of NBSB due to the increased pH of the solution. In acetonitrile direct basicity probing took place in which NBSB was adsorbed in MCF pores. AP/MCF and Im/MCF caused deprotonation of NBSB, as monitored by UV–vis re?ectance spectra. This e?ect was stronger for AP/MCF, which showed higher basicity. Az/MCF could not deprotonate NBSB in either water or acetonitrile.
关键词： Basicity,FTIR,Ab-initio calculations,Hydroxystilbazolium cyanines,UV–vis,Nitrogen compounds on MCF
更新于2025-09-04 15:30:14
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Towards simulation at picometer-scale resolution: Revisiting inversion domain boundaries in GaN
摘要： Motivated by recent high resolution results on the inversion domain boundaries (IDB) in gallium nitride, we re?ne by ab initio DFT calculations the well established atomic model IDB? derived by Northrup et al. This allows us to recover these experimental results obtained by coherent x-ray diffraction and showing small additional shifts of the polarity domains, in particular 8 pm shift along the hexagonal direction. The in?uence of boundary conditions and electrostatic ?elds (IDB-IDB and IDB-surface interactions) on the results and the existence of metastable solutions is carefully discussed to stress the accuracy of the method. These results demonstrate a cross-talk between advanced characterization tools and state-of-the-art ab initio calculations that opens perspectives for the structural analysis of defects in the picometer range.
关键词： picometer resolution,ab initio DFT calculations,coherent x-ray diffraction,gallium nitride,inversion domain boundaries
更新于2025-09-04 15:30:14
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: <i>Ab initio</i> lattice dynamical perspective
摘要： The hydration and ammonization of ZrW2O8 is known to lead to positive and negative thermal expansion behavior, respectively. We report ab initio calculations to understand this anomalous behavior. We identify the crucial low-energy phonon modes involving translations, rotations, and distortions of WO4 and ZrO6 polyhedra, which lead to negative thermal expansion in ZrW2O8 in pure and ammoniated forms; however, the rotation and distortion motions get inhibited on hydration and lead to positive thermal expansion. We demonstrate that the thermal expansion coef?cient could be tailored by engineering the phonon dynamics of a material.
关键词： ZrW2O8,hydration,phonon dynamics,ab initio calculations,ammonization,thermal expansion
更新于2025-09-04 15:30:14
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Unique ion diffusion properties in lead-free halide double perovskites: A first-principles study
摘要： Pb-free halide double perovskites (HDPs) are proposed as potential candidates for various optoelectronic applications to replace the mainstream hybrid organic-inorganic halide perovskites, e.g., CH3NH3PbI3. While it is known that ion di?usion is a critical problem to a?ect the structural and electronic stability of CH3NH3PbI3, the mechanism of ion di?usions in HDPs is still unclear and highly desired to be revealed. In this study, taking Cs2AgInX6 (X = Cl, Br) HDPs as prototypes, for the ?rst time we suggest that the fast ion di?usion of the dominant defects may play an important role in the performance stability of HDPs. Importantly, we ?nd that the Agi+ di?usion in a multi-ion concerted fashion has a much faster di?usion rate, compared to the VAg? and VX? di?usion in a single-ion fashion. It is revealed that HDPs exhibit quite di?erent di?usion properties from CH3NH3PbI3. Furthermore, we demonstrate that the di?usion rate of Agi+ in HDPs can be e?ectively suppressed by applying an epitaxial strain, which opens a promising way to enhance the performance stability of perovskite materials for various device applications.
关键词： Lead-free halide double perovskites,Ion di?usion,Performance stability,ab initio calculations,Solar cells
更新于2025-09-04 15:30:14
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Structural stability and electronic properties of XTO2 (X= Cu, Ag; T=Al, Cr): an ab initio study including X vacancies and Mg doping
摘要： Ab initio density functional theory-based calculations are used to study the structural and electronic properties of CuAlO2, AgAlO2, CuCrO2, and AgCrO2 transparent conducting oxides (TCOs). The hexagonal 2H delafossite polymorph is determined here to be one of the most stable polymorphs, by comparing total energies for different structural phases. The simple antiferromagnetic configuration is chosen to model magnetic effects in CuCrO2 and AgCrO2 due to it having one of the lowest ground state total energies and containing the most semiconductor like behavior of the magnetic configurations considered. Electronic structures of 2H CuAlO2 and AgAlO2 obtained from different approximations for the exchange-correlation functional, GGA (PBE), PBE+U, PBE+mBJ, PBE+mBJ+U, and the Hybrid HSE06 are compared. Supercells are employed to model 6.25% Cu and Ag vacancies, 3.13% O vacancies, and 6.25% Mg doping replacing Al and Cr, from which structural and electronic properties are obtained and used to predict on the effectiveness of these native defects and dopant on increasing the conductivity in all TCOs studied in this work. The obtained partial density of states for the pristine systems supports a model of hole conduction in the a – b plane, perpendicular to the O – X – O dumbbells of the delafossite structure. Additionally, the partial density of states of the defective and doped systems suggest a growth environment deficient in X and saturated in O may increase conductivity in these materials.
关键词： Delafossite,Density functional theory,Transparent conducting oxides,Mg doping,Structural stability,Ab initio,Vacancies,Electronic properties
更新于2025-09-04 15:30:14
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Photon-momentum transfer in diatomic molecules: An <i>ab initio</i> study
摘要： For a molecule, the two-center interference and the molecular scattering phase of the electron are important for almost all the processes that may occur in a laser field. In this study, we investigate their effects in the transfer of linear photon momentum to the ionized electron by absorbing a single photon. The time-dependent Schr?dinger equation of H2+ is numerically solved in the multipolar gauge in which the electric quadrupole term and the magnetic dipole term are explicitly expressed. This allows us to separate the contributions of the two terms in the momentum transfer. For different configurations of the molecular and the laser orientation, the transferred momentum to the electron is evaluated at different internuclear distances with various photon energies and two-center interferences are identified in the whole region. At small electron energies and small internuclear distances, we find significant deviations from the prediction of the classical double-slit model due to the strong mediation of the Coulomb potential. Finally, even for a large internuclear distance, our results show that a varying molecular scattering phase is important at all electron energies, which is beyond the simple prediction of the linear combination of the atomical orbitals.
关键词： diatomic molecules,molecular scattering phase,ab initio study,photon-momentum transfer,two-center interference
更新于2025-09-04 15:30:14
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Effective quenching and excited-state relaxation of a Cu(I) photosensitizer addressed by time-resolved spectroscopy and TDDFT calculations
摘要： Homogenous photocatalytic systems based on copper photosensitizers are promising candidates for noble metal free approaches in solar hydrogen generation. To improve their performance, a detailed understanding of the individual steps is needed. Here, we study the interaction of a heteroleptic copper (I) photosensitizer with an iron catalyst by time-resolved spectroscopy and ab initio calculations. The catalyst leads to rather efficient quenching of the 3MLCT state of the copper complex, with a bimolecular rate being about three times smaller than the collision rate. Using control experiments with methyl viologen, an appearing absorption band is assigned to the oxidized copper complex demonstrating that an electron transfer from the sensitizer to the iron catalyst occurs and the system reacts along an oxidative pathway. However, only about 30% of the quenching events result in an electron transfer while the other 70% experience deactivation indicating that the photocatalytic performance might suffer from geminate recombination.
关键词： Solar hydrogen generation,Time-resolved spectroscopy,Ab initio calculations,Copper photosensitizers,Electron transfer
更新于2025-09-04 15:30:14
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Ab-initio study on electronic and magnetic properties of (Ga,Co) co-doped ZnO
摘要： Using first-principles calculations based on density functional theory within GGA formalism, we have studied the electronic structure and magnetic properties of (Ga,Co) co-doped ZnO system. The effect of impurity distances on ferromagnetic and antiferromagnetic ground state in Co0.056Zn0.944O has been studied. For the closest Co-Co distance, a ferromagnetic ground state with total magnetic moment of ~3.00μμ’ per Co atom has been found. The electronic structure also displays a nearly halfmetallic order. Conversely, for the farthest Co-Co distance an antiferromagnetic ground state was found for Co0.056Zn0.944O. When Zn2+ ions are replaced by Ga ions in Co0.056Zn0.944O, the new (Ga,Co) co-doped ZnO system is more energetically stable. It has also been found that Ga-doping reduces the Co0.056Zn0.944O band gap due to the sp-d exchange interactions, which is in good agreement with the experimental data. Moreover, the Ga-doping changes the nearly halmetallic order of Co0.056Zn0.944O to metallic. Results also show that Ga0.029Co0.056Zn0.915O is still ferromagnetic with a total magnetic moment of ~3.00μμ’ per Co atom. It was also found that the ferromagnetic ground state in (Ga,Co) co-doped ZnO vanishes as Ga concentration increases.
关键词： GGA formalism,(Ga,ab-initio,density functional theory,electronic structure,Co) co-doped ZnO,magnetic properties
更新于2025-09-04 15:30:14