Electrical conductivities of the Ag6PS5X and the Cu6PSe5X (X=Br, I) argyrodites

The Ag₆PS₅Br and Ag₆PS₅I argyrodites crystallize in a face-centered-cubic lattice at room temperature. Both compounds exhibit purely Arrhenius behavior throughout the temperature range 150-400 K with similar activation energies of about 0.23 eV. Cu₆PSe₅Br and Cu₆PSe₅I also crystallize in a face-centered-cubic structure at room temperature. Cu₆PSe₅Br exhibits a distinctive anomaly in electrical conductivity near 286 K while Cu₆PSe₅I undergoes a first-order electrical phase transition near 265 K. Their activation energies above room temperature are 0.13 and 0.30 eV, respectively.     

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.     

Investigations of the optical and EPR spectra for (NiX6) (X=Cl, Br, I) clusters

In this paper, the formulae of optical spectral levels and electron paramagnetic resonance (EPR) spectra in trigonal symmetry of 3d⁸ ions are established on the basis of strong field mechanism and a two spin-coupling (SO) parameters model. Unlike the classical crystal-field approach which has only taken the SO coupling of the central metal ions into account, the contribution of the SO coupling of the ligand ions to the optical and EPR spectra has been included in these formulae. When the optical and EPR spectra of the strong covalent crystals are calculated, the reasonable results can be obtained if the two SO parameters model has been put into action. As an application, the optical and EPR spectra of the (NiX₆)⁴⁻ clusters in CsMgX₃:Ni²⁺ (X=Cl, Br, I) crystals have been studied by the complete diagonalization (of energy matrix) method (CDM). The calculated results agree well with experimental findings. From the investigations, a more valid method to calculate the optical and EPR spectra for 3d⁸ ions clusters is provided.     

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.     

Fine structure in the absorption edge spectrum of NbS2Y2 (Y = Cl,Br, I)

High resolution absorption spectra of single crystals of NbS₂Y₂ (Y = Cl, Br, I), obtained at temperatures between 4.2 and 300 K revealed extensive fine structure in the absorption edge. This structure has been analysed and interpreted in terms of allowed indirect interband transitions, involving phonons corresponding to SS and NbS vibrations, followed by forbidden and by allowed direct transitions. From the shape of the absorption curve associated with the phonon branches of the indirect transitions a binding energy of 23 cm^? for indirect excitons is obtained. A binding energy of 28 cm^? for direct excitons is deduced from the exciton lines observed at the long-wavelength side of the direct transitions. A detailed interpretation of the optical transitions is given in terms of a molecular orbital diagram for the Nb₂S₄ clusters, present in these crystals.     

Isoabsorption and spectrometric studies of optical absorption edge in Cu6AsS5I superionic crystal

Cu₆AsS₅I single crystals were grown using chemical vapour transport method. Two low-temperature phase transitions (PT) are observed from isoabsorption studies: a first-order PT at Т_?=153±1 K and a second-order PT in the temperature interval T_?I=260–280 K. At low temperatures and high absorption levels an excitonic absorption band was revealed in the range of direct optical transitions. At Т>Т_?, the absorption edge has an exponential shape and a characteristic Urbach bundle is observed. The influence of the cationic P→As substitution on the parameters of the Urbach absorption edge, parameters of exciton–phonon interaction, and phase transitions temperatures are studied.     

The origin of disorder in CH 3 HgX (X = Cl, Br and I) crystals investigated by temperature dependent Raman spectroscopy

Methyl-mercury(II) halides CH₃HgX (X = Cl, Br and I) were studied by means of temperature dependent Raman spectroscopy from 10 K to 410 K. In addition to the previously reported soft phonons, new changes in the low frequency spectra were observed at T ≈ 70 K in CH₃HgBr and at T ≈ 100 K in CH₃HgI. The bandwidths of the two internal modes in CH₃HgBr, the CH₃ symmetric stretching band and the C-Hg-Br bending band, rise towards a local maximum at T ≈ 50 K as the temperature is raised from 10 K to 300 K. On the other hand the bandwidths of the two corresponding modes in CH₃HgI crystals monotonously increase with temperature, obeying an Arrhenius law. Besides the three phonon modes present in the Raman spectra of CH₃HgCl at room temperature, the fourth phonon band that has been observed at temperatures below 245 K might correspond to the freezing of methyl librations. The huge bandwidth of the C-Hg-Br bending mode could suggest the presence of additional weak bonding of a mercury atom with bromine atoms from other molecules, thus inducing positional disorder. Received 19 November 1999 and Received in final form 10 November 2000     

Compositional behaviour of Urbach absorption edge and exciton-phonon interaction parameters in Cu6PS5I1−xBrx superionic mixed crystals

Temperature behaviour of optical absorption edge in Cu₆PS₅I_1−xBr_x mixed crystals is studied in the interval 77-325 K. It is shown that the absorption edge has Urbach shape in the 215-325 K temperature interval. The influence of temperature and compositional disorder on the Urbach absorption edge parameters is presented. The mechanism of the Urbach bundle formation and the effect of I→Br anionic substitution on the exciton-phonon interaction parameters is elucidated.     

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.     

First-principles study on the structural,electronic and optical properties of BiOX (X=Cl,Br, I) crystals

In order to investigate systematically the structural, electronic and optical properties of bismuth oxyhalides BiOX (X=Cl, Br, I) semiconductors, the lattice constants, structural characteristics, band structures, densities of states, atomic charge populations and optical properties of BiOX crystals have been calculated using first-principles based on DFT. The calculated indirect band gaps of BiOCl, BiOBr and BiOI crystals are 2.50, 2.10 and 1.59 eV, respectively. The analysis of densities of states and atomic charge populations for BiOX crystals indicates that, (a) the valance band maximum is mainly contributed to O 2p and X np states and the Bi 6p states dominate the conduction band minimum; (b) the contribution of X ns states obviously increases with the increase of X atomic numbers, and the dispersive energy level becomes more and more significant and (c) the sequence of covalent bonding strength between atoms is Bi–O >Bi–I>Bi–Br>Bi–Cl. In addition, the calculated absorption edges of the absorption coefficients I(ω) for BiOCl, BiOBr and BiOI crystals are 355, 448 and 645 nm, respectively, which agree well with our experimental measurements of 376, 442 and 628 nm and the previous reported results of 370, 440 and 670 nm.     

Studies of the optical spectrum band positions and spin-Hamiltonian parameters for Pa ion in ThX4 (X=Cl, Br) crystals

The optical spectrum band positions and spin-Hamiltonian (SH) parameters (g factors g_∥ and g_⊥ and hyperfine structure constants A_∥ and A_⊥) for 5f¹ ion Pa⁴⁺ at the tetragonal Th⁴⁺ site of ThX₄ (X=Cl, Br) crystals are calculated from a complete diagonalization (of energy matrix) method (CDM). In the CDM, the magnetic (or Zeeman) interaction and hyperfine interaction terms are added to the Hamiltonian in the conventional CDM and so the optical and EPR spectra data can be studied in a unified way. The calculated results are in reasonable agreement with experimental values. The possible misprints or small errors in the experimental g factors for Pa⁴⁺ in ThX₄ crystals are pointed out. The results are discussed.     

Theoretical studies of spin-Hamiltonian parameters for Ni ions in CsMgX3 (X=Cl, Br, I) crystals from the two-mechanism model

In this paper, the high-order perturbation formulas of spin-Hamiltonian (SH) parameters (g factors g, g and zero-field splitting D), including both the crystal-field (CF) and for the first time charge-transfer (CT) mechanisms, are established for 3d⁸ ions in trigonal octahedral clusters. By using these formulas, the SH parameters of Ni²⁺ ions in CsMgX₃ (X=Cl, Br, I) crystals are calculated. The results are consistent with the experimental values. The calculations suggest that the sign of Q^CT (Q=Δg, Δg or D, where the g-shift Δg_i=g_i−g_e, g_e≈2.0023 is the value of free-electron) due to CT mechanism is the same as that of the corresponding Q^CF due to CF mechanism, and the relative importance of CT mechanism (characterized by Q^CT/Q^CF) increases with the increasing atomic number of ligand X. So, for the 3dⁿ ML_m clusters with ligand having large atomic number, the reasonable theoretical explanations of all SH parameters should take both CF and CT mechanisms into account. The defect structure of (NiX₆)⁴⁻ impurity centers in CsMgX₃:Ni²⁺ crystals is also considered in our model.     

Halogen bond between hypervalent halogens YF3/YF5 (Y=Cl,Br, I) and H2X (X= O,S, Se)

ABSTRACT

The complexes of H₂X (X?=?O, S, Se) with hypervalent halogens YF₃ and YF₅ (Y?=?Cl, Br, I) have been studied. The σ-hole on the Y atom participates in a halogen bond with the lone pair on the chalcogen atom. In addition, some secondary interactions coexist with the halogen bond in most complexes. The interaction energy correlates with the nature of both X and Y atoms. In most cases, the complex is more stable for the heavier Y atom and the lighter X atom. Of course, there are some exceptions in H₂X···YF₃. YF₃ forms a more stable complex with H₂X than does YF₅. These complexes are dominated by electrostatic interaction and the halogen bond involving H₂S and H₂Se exhibits some covalent character.

Halogen bond plays an important role in chemical reactions and multivalent halogens can regulate chemical reactions by participating in a halogen bond. Thus we compare the effect of the chalcogen electron donor on the strength and nature of halogen bonding involving multivalent halogens.     

First-principles study of the optical properties of PbFX (X = Cl,Br, I) compounds in its matlockite-type structure

We present the results of the ab initio theoretical study of the optical properties for PbFX (X = Cl, Br, I) compounds in its matlockite-type structure using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed 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 Z 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 values of ε₁ ( 0 ) increases with decreasing the energy gap. The reflectivity spectra and absorption coefficient has been calculated and compared with the available experimental data. The optical properties are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure.     

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.     

伽马CuX(X=Cl,Br,I)的电子结构和光学性质的第一性原理计算

基于第一性原理赝势平面波方法对伽马晶体CuCl, CuBr, CuI的体模量、体模量对压强的一阶偏导 数、电子结构、折射率等光学性质进行了计算.计算结果表明,广义梯度近似(GGA)下CuX(X = Cl, Br, I) 晶体的晶格常数与体模量的计算值与实验相差较小.与局域密度近似(LDA)相比, GGA更适合于 CuX(X = Cl, Br, I)晶体 的计算.这三者的价带都来源于Cu的3d态,导带部分主要来自Cu和卤素的s电子贡献,很少部分来自卤素的p电子 贡献.计算得到CuCl, CuBr, CuI的折射率分别为1.887, 2.015, 2.199,与应用Gladstone-Dale半经验关系得到 的结果符合得很好.     

静水压对立方钙钛矿CsSnX3(X=I, Br, Cl)光电性质的第一性原理探究

全无机无铅卤化物钙钛矿已经成为重要的新一代太阳能电池材料.采用密度泛函理论的第一性原理研究了不同静水压下CsSnX₃(X=I, Br, Cl)材料的晶体结构,电子结构和光学性能,并分析了其内在联系.结果表明施加静水压可使材料中Sn-X键长减小,使原子之间的耦合增强,带隙值减小,且随着卤族元素半径的增大,压力效应越明显;随着压力的增加,材料的吸收系数和复折射率增大,吸收光谱出现红移现象,在可见光区和近红外光区吸收增强.相比CsSnBr₃和CsSnCl₃,CsSnI₃在可见光区吸收最佳且受压力作用影响最小,更适用于钙钛矿太阳能电池材料.     

Relativistic density functional investigation of UX6（X=F,Cl,Br and I）

The molecular structures and the vibrational frequencies of uranium hexahalides UX 6(X=F,Cl,Br and I) molecules are investigated by using local density approximation(LDA) and generalised gradient approximation(GGA) functions(BP,BLYP and RPBE) in combination with two different relativistic methods(scalar and scalar+spin-orbit relativistic effects).The calculated results show that the differences are trivial between scalar and scalar+spin-orbit relativistic methods.The vibrational frequencies are also compared with existing experimental values,and overall,the RPBE approach gives the smallest error.The bond dissociation energies(BDEs) of UX 6 are computed by using the RPBE function,thereby obtaining exact vibrational frequencies.In addition,the calculated magnitudes of the spin-orbit effect on the BDE of UX 6(X=F,Cl,Br,and I) are found to be approximately-0.3198,-0.3218,-0.3609 and-0.4415 eV,respectively.