First-principles investigations on the mechanical,thermal,electronic,and optical properties of the defect perovskites Cs_2SnX_6(X= Cl,Br, I)

The mechanical properties, thermal properties, electronic structures, and optical properties of the defect perovskites Cs_2SnX_6(X = Cl, Br, I) were investigated by first-principles calculation using PBE and HSE06 hybrid functional. The optic band gaps based on HSE06 are 3.83 eV for Cs_2SnCl_6, 2.36 eV for Cs_2SnBr_6, and 0.92 eV for Cs_2SnI_6, which agree with the experimental results. The Cs_2SnCl_6, Cs_2SnBr_6, and Cs_2SnI_6 are mechanically stable and they are all anisotropic and ductile in nature. Electronic structures calculations show that the conduction band consists mainly of hybridization between the halogen p orbitals and Sn 5s orbitals, whereas the valence band is composed of the halogen p orbitals. Optic properties indicate that these three compounds exhibit good optical absorption in the ultraviolet region, and the absorption spectra red shift with the increase in the number of halogen atoms. The defect perovskites are good candidates for probing the lead-free and high power conversion efficiency of solar cells.     

Ab initio study of structural,elastic, and high-pressure properties of rubidium halides RbX (X = F,Cl, Br and I)

We present first-principle calculations on the structural, elastic, and high-pressure properties of rubidium halides compounds, using the pseudo-potential plane-waves approach based on density functional theory, within the generalized gradient approximation. Results are given for lattice constant, bulk modulus and its pressure derivative. The pressure transition at which these compounds undergo structural phase transition from NaCl-type to CsCl-type structure are calculated and compared with previous calculations and available experimental data. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's modulus and Poisson's ratio for ideal polycrystalline RbF, RbCl, RbBr, and RbI aggregates. We estimated the Debye temperature of these compounds from the average sound velocity.     

单层金属卤化物CoX_2(X=Cl、Br、I)可调能隙和磁性研究

本文采用密度泛函理论系统的研究了二维单层金属卤化物CoX_2(X=Cl,Br,I)的结构稳定性、电子性质和磁性质.三种卤化物的束缚能分别是9.01、8.04和6.95 eV,表明Co原子和卤素原子间存在强相互作用.三种材料的能带结构都显示了间接带隙半导体特性.三种材料的总磁矩都是3 μ_B,主要来源于Co原子的磁矩.为了实现对材料物性的调控,我们考虑了双轴应变.发现压缩应变不仅可以显著增强铁磁态的稳定性,还可以实现体系从间接带隙半导体向直接带隙半导体的转变.     

Spectroscopy and dissociation of sulfuryl halides SO2X2 (X=F,Cl)

The spectroscopy and dissociation of the sulfuryl halides SO₂F₂ and SO₂Cl₂ have been studied in detail using ab initio methods. The possibility of various dissociation channels has been explored taking into account that the fragmented atoms and molecules can stay in their ground state only. An interesting pattern was observed in their dissociation energy spectra for the dissociation channels. The singlet potential energy surfaces for the exit channels of these molecules have been analysed. The release of halogen molecules after dissociation is discussed from an industrial point of view. Finally, the enthalpy of formation of these molecules was computed using the ab initio results. Our results agree very well with the experimental values available.     

The structural properties of RbMnX<Subscript>3</Subscript> (X = F,Cl, Br) halides

The results of nonempirical calculation of energies of three polytypes (cubic, two-layer hexagonal, and six-layer hexagonal) are given for RbMnX₃ (X = F, Cl, Br) crystals. The calculation is performed using an ionic crystal model with regard for the deformability and the dipole and quadrupole polarizabilities of ions. The behavior of these crystals under the action of hydrostatic pressure is studied. It is demonstrated that, at normal pressure, the RbMnCl₃ and RbMnBr₃ crystals have a six-layer hexagonal structure. At pressures above 11 kbar, RbMnCl₃ passes to a phase with a cubic structure; RbMnBr₃ at pressures above 90 kbar passes to a phase with a two-layer hexagonal structure. The RbMnF₃ crystal under normal conditions has a cubic structure and experiences no phase transformations under the effect of pressure. The obtained results are in satisfactory agreement with the known experimental data.     

Cation mobility in the Cu6PS5X (X=Cl,Br, I) argyrodites

The electrical conductivity of Cu₆PS₅Cl shows purely Arrhenius behavior throughout the temperature range 170–450 K with no evidence of the 241 K thermodynamic phase transition previously reported. Cu₆PS₅Br exhibits two changes in activation energy. The first, at 251 K, apparently coincides with a previously determined thermodynamic transition. The second, at 321 K, is likely associated with a conduction transformation from ionic to electronic. The conductivity of Cu₆PS₅I is characterized by a cusp at a temperature of 194 K, far removed from a previously reported thermodynamic transition at 221 K. In addition, a broad and continuous change in activation energy appears to coincide with another previously reported phase transition at 270 K.     

The photoelectron spectra of the oxalyl halides (COX)2, [X = F,Cl and Br]

We have examined the HeI photoelectron spectra of the oxalyl halides (COX)₂, (X = F, Cl, and Br). We assume the presence of the trans-isomer onWe have used semi-empirical calculations (SCF MO), and Koopmans' Theorem, as aids in assigning the spectral bands.     

High-pressure magnetic and structural properties of TiOX (X=Cl,Br)

We report high-pressure diffraction and magnetization measurements to demonstrate that the partial collapse of electronic gap at high-pressure insulator to metal transition reported in TiOCl (C. Kuntscher et al. Phys. Rev. B 74 184402 (2006).) corresponds to a Ti³⁺–Ti³⁺ dimerization at room temperature within the space group P2₁/m. The shortest Ti–Ti distance is comparable to that of the Ti metal, but a Peierls-like distortion prevents a metallic behaviour.     

Polarized Raman scattering from oriented single crystals of A2BX4 halides (A = Cs,B = Zn,X = Cl,Br, I)

Raman spectra of isostructural tetrahalozincates viz. Cs₂ZnI₄, Cs₂ZnBr₄ and Cs₂ZnCl₄ single crystals have been measured in various scattering orientations using polarized laser excitation. The lattice dynamics of the crystals is probed down to 70 K in order to cover the successive phase sequences. Except Cs₂ZnI₄, none of the other salts shows the characteristic commensurate-incommensurate features of lattice instabilities. The unit cell dynamics is understood in terms of 84 phonon branches originating from various internal modes of tetrahedral ZnX²⁻₄ group and external lattice modes. The enormous splittings, responsible for a complete loss of degeneracy in the internal modes for all the three salts, suggest a considerable distortion particularly at low temperatures in the tetrahedral geometry of the ZnX²⁻₄ group.     

Room-temperature scintillation properties of cerium-doped REOX (RE=Y, La, Gd, and Lu; X=F, Cl, Br, and I)

The scintillation properties of cerium-doped oxyhalides following the general formula REOX (RE=Y, La, Gd, and Lu; X=F, Cl, Br, and I) are reported. These materials were synthesized under dry conditions as microcrystalline powders from conventional solid state reactions. The room temperature X-ray excited emission and scintillation decay curves were measured and analyzed for each material. Additionally, the hygroscopic nature of the oxychlorides and oxybromides was compared to that of their corresponding rare earth halides. The yttrium, lanthanum, and gadolinium oxychlorides, and all of the oxybromides and oxyiodides are found to be activated by Ce³⁺. GdOBr doped with 0.5% Ce³⁺ has the highest light output with a relative luminosity of about one-half that of LaBr₃: Ce³⁺. It displays a single exponential decay of 30 ns.     

Optical properties of the alkaline-earth fluorohalides matlockite-type structure SrFX (X=Cl, Br, I) compounds

The optical properties of the SrFX (X=Cl, Br, I) compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0). We find that the value of ε₁(0) increases on decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data.     

晶体NiX2(X=Cl,Br,I)的光谱和电子顺磁共振谱研究

在强场耦合图像中,采用双自旋-轨道耦合(SO)参量模型建立了过渡族3d2(3d8)离子的三角对称下全组态光谱能级和电子顺磁共振(EPR)公式.与经典的晶体场理论(仅考虑中心金属离子的自旋-轨道耦合作用)相比较,该公式还包括了配体离子的自旋-轨道耦合作用的贡献,这一模型在应用于计算共价性较强的晶体光谱和电子顺磁共振谱可得到合理的结果.作为验证,用完全对角化方法研究了品体NiX2(X=Cl,Br,I)的光谱和电子顺磁共振谱,结果表明,理论与实验很好地符合.建立的全组态谱能级和电子顺磁共振公式为更精确地计算光谱和电子顺磁共振谱提供了一条可行方法.     

Electrical properties of the Ag6PSe5X (X=Cl, Br, I) argyrodites

The total electrical conductivities at room temperature of Ag₆PSe₅Cl, Ag₆PSe₅Br, and Ag₆PSe₅I were found to be 2.0×10⁻⁴, 5.6×10⁻⁴, and 6.8×10⁻⁴ S/cm, respectively. In the chloride and iodide compounds, the electronic contribution comprises approximately 1% of the total conductivity, although it exceeds 10% of the total conductivity in the bromide compound. Ag₆PSe₅Cl and Ag₆PSe₅Br exhibit purely Arrhenius behavior throughout the temperature range 150-300 K. Ag₆PSe₅I exhibits a second-order anomaly in electrical conductivity at 324 K.     

Rydberg states and ionization potentials of GeX,SnX, and PbX (X = F,Cl, Br,I)

Thirty-nine unattributed electronic states of group IV monohalides have been interpreted in terms of Rydberg configurations. Ionization potential and quantum defects have been derived.     

Calculation of vibrational spectra and force and vibrational-rotational constants of germanium halides of the type GeHnX4−n (X=F,Cl, Br,I; n=0 to 2)

Calculations are made of the frequencies, the form of the normal vibrations, and the constants of the potential energy of displacements of atoms during normal vibrations of germanium halides of the type GeH_nX_4–n (X=F, Cl, Br, I; n=0 to 2) and their deuterium replacements. For displacements of atoms during normal vibrations a calculation is made of the constants of vibrational-rotational interaction in the molecules GeHX₃ and GeX₄. The validity of the calculation is confirmed by the way in which the sum rule for the - and -constants is fulfilled. The mean-square amplitudes are calculated for tetrahedral molecules GeX₄ (X=F, Cl, Br, I). An analysis is given of the strong field of germanium halides GeH_nX_4–n in connection with properties of the structure of the Ge-halide bond.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 9, pp. 54–58, September, 1973.     

CX2+CH2O (X=F, Cl, Br)环加成反应的密度泛函理论计算

用密度泛函理论,在B3LYP/6-311 G(d)水平上研究了CX2 CH2O(X=F,Cl,Br)环加成反应一条三过渡态三中间体路径的反应机理,全参数优化了反应势能面各驻点的几何构型,用内禀反应坐标(IRC)和频率分析方法,对过渡态进行了验证.用高级电子相关校正的耦合簇[CCSD(T)/6-311 G(d)]方法对优化构型进行了单点能计算.采用经Wigner校正的Eyring过渡态理论和热力学方法,研究了该反应通道的热力学及动力学性质.从热力学和动力角度综合分析,该途径CF2与GH2O的环加成反应难以发生,而CCl2及CBr2与CH2O反应的适宜温度范围均为400～1000K,如此,反应既具有较大的自发趋势和平衡常数,又具有较快的反应速率.     

X-ray excited optical luminescence studies on the system BaXY (X,Y=F, Cl, Br, I)

The present paper reports the experimental observations on the x-ray excited optical luminescence (XEOL) along with the afterglow and colour center features found for the barium salts, represented by the formula,BaXY, whereX andY are the halides. The system thus consists of four dihalides (BaF₂,. . . ,BaI₂) and six mixed halides (BaFCl,. . . ,BaBrI). To start with, it was found that on two of the binary halides of barium, BaClI and BaBrI, no literature exists, and so these were prepared for the first time and their crystal structures were determined. An x-ray generator of 3-kW rating was next coupled to a spectrometer via a high throughput fiberoptic sensor for recording the luminescence spectra under x-irradiation. Also presented in this paper are the observations on the BaXY compounds in which about 0.1 mole% of Eu²⁺ was doped, in order to study the efficiency between the prompt luminescence and the photostimulated luminescence in these compounds. The crystal structure varies from fluorite (BaF₂), to matlockite (BaFX) and finally to orthorhombic (BaCl₂,. . .,BaBrI) for these compounds. Hence searching for systematics and empirical relations in the observed XEOL behaviour of these compounds is still a challenging problem.     

Impeding anion exchange to improve composition stability of CsPbX_3(X= Cl,Br) nanocrystals through facilely fabricated Cs_4Pb_6 shell

Inorganic lead halide perovskite nanocrystals(NCs) with superior photoelectric properties are expected to have excellent performance in many fields. However, the anion exchange changes their features and is unfavorable for their applications in many fields. Hence, impeding anion exchange is important for improving the composition stability of inorganic lead halide perovskite NCs. Herein, CsPb X_3(X = Cl, Br) NCs are coated with Cs_4PbX_6 shell to impede anion exchange and reduce anion mobility. The Cs_4PbX_6 shell is facily fabricated on CsPbX_3 NCs through high temperature injection method.Anion exchange experiments demonstrate that the Cs_4 PbX_6 shell completely encapsulates CsPbX_3 NCs and greatly improves the composition stability of CsPbX_3 NCs. Moreover, our work also sheds light on the potential design approaches of various heterostructures to expand the application of CsPbM_3(M = Cl, Br, I) NCs.     

Stability,electronic structure,and optical properties of lead-free perovskite monolayer Cs3B2X9(B=Sb,Bi;X=Cl,Br,I)and bilayer vertical heterostructure Cs3B2X9/Cs3B’2X9(B,B’=Sb,Bi;X=Cl,Br,I)

The lead-free perovskites Cs₃B₂X₉(B=Sb,Bi;X=Cl,Br,I)as the popular photoelectric materials have excellent optical properties with lower toxicity.In this study,we systematically investigate the stable monolayer Cs₃B₂X₉and bilayer vertical heterostructure Cs₃B₂X₉/Cs3B02X9(B,B0=Sb,Bi;X=Cl,Br,I)via first-principles simulations.By exploring the electrical structures and band edge positions,we find the band gap reduction and the band type transition in the heterostructure Cs₃B₂X₉/Cs3B02X9 due to the charge transfer between layers.Furthermore,the results of optical properties reveal light absorption from the visible light to UV region,especially monolayer Cs3Sb2I9 and heterostructure Cs3Sb2I9/Cs3Bi2I9,which have absorption peaks in the visible light region,leading to the possibility of photocatalytic water splitting.These results provide insights for more two-dimensional semiconductors applied in the optoelectronic and photocatalytic fields.