High-pressure preparation and thermoelectric properties of Bi0.85Sb0.15

Nanocrystalline Bi_0.85Sb_0.15 powders were prepared by a novel mechanical alloying method. The bulk samples were formed by applying a pressure of 6 GPa at different pressing temperatures and times. Electrical conductivity, Seebeck coefficients, and thermal conductivity were measured in the temperature range 80–300 K. The Seebeck coefficient reaches a maximum value of −173 μV/K at 150 K. The largest figure of merit, 3.46 × 10⁻³ K⁻¹, achieved in this experiment is 50% higher than that of its single-crystal counterpart at 200 K.     

Electronic,optical and bonding properties of MgYZ2 (Y=Si,Ge; Z=N,P) chalcopyrites from first principles

The electronic and optical properties of MgYZ₂ (Y=Si, Ge; Z=N, P) compounds are carried out using first-principle calculations within the density functional theory. The calculations show close correspondence to the available experimental data compared to the previous theoretical calculations. Band gap decreases by changing the cations Y from Si to Ge as well as Z from N to P in MgYZ₂. The N/P p-states contribute majorly in the density of states. Bonding nature of the herein studied compounds is predicted from the electron density plots. Optical response of these compounds is noted from the complex refractive index, reflectivity and optical conductivity. The direct band gap and the high reflectivity of these compounds in the visible and ultraviolet regions of electromagnetic energy spectrum ensure their applications in optoelectronic and photonic domains.     

First principles calculations of the electronic structure and optical properties of (001), (011) and (111) Ga0.5Al0.5As surfaces

Using the first-principles plane-wave pseudo-potential method based on density function theory (DFT), the electronic structure and optical properties of Ga_0.5Al_0.5As (001), (011) and (111) surfaces are calculated. Result shows that (001) surface is reconstructed, (011) surface is not reconstructed but wrinkled, (111) surface is only relaxed. (111) is the most stable surface. (001) surface owns the lowest work function. Absorption coefficient and reflectivity of these surfaces are smaller than bulk, the transmittance of the surfaces are larger than the bulk, which is helpful for the incident light to excite photoelectrons. The decrease of the absorption coefficient and reflectivity at (001) surface are the largest. Calculation of electronic structure and optical properties predict that the (001) surface should have the strongest photoemission.     

Electronic structure,chemical bonding and optical properties of Di-2-pyrymidonium dichloride diiodide (C4H5ClIN2O) from first-principles

The electronic structure and electronic charge density of the monoclinic phase Di-2-pyrymidonium dichloride-di-iodide compound is studied by using the local density approximation (LDA) and Engel Vosko generalized gradient approximation (EVGGA). Using LDA for exchange correlation potential, we have optimized the atomic positions taken from the X-ray crystallographic data by minimization of the forces acting on the atoms. From the relaxed geometry the electronic structure, electronic charge density and the optical properties were determined. Band structures disclose that this compound has indirect energy band gap. The obtained energy band gap value using EVGGA (2.010 eV) is larger than that obtained within LDA (1.781 eV). To envision the chemical bonding nature between the composition of the investigated compound, the distribution of charge density was discussed in the (−1 0 1) crystallographic plane. The contour plot shows partial ionic and strong covalent bonding between C–O, N–C and C–H atoms. The optical properties of Di-2-pyrymidonium dichloride-di-iodide are obtained by the calculation of the dielectric function.     

Structural analysis and valence variation in ytterbium pnictides YbX (X=P,As) under high pressure

We have studied the pressure induced structural phase transition, pressure volume relationship and valence variation in ytterbium pnictide compounds (YbX; X=P, As). An improved interaction potential model (IIPM) (including the polarizability effect) has been applied. We have also studied the trivalent and intermediate valence in ytterbium compounds and predicted the phase transition pressures, volume collapses for two different valences. The variations of the second-order elastic constants with pressure and stability condition have shown systematic trends in YbX compounds similar to those observed in other compounds of NaCl structure. The pressure volume relationship curves show that ytterbium pnictide compounds undergo NaCl to CsCl type structural transition. The present study also confirms that these compounds behave similar to the rare earth cerium pnictide compounds and they are trivalent in nature.     

The Shubnikov-de Haas effect and thermoelectric properties of Tl-doped Sb2Te3 and Bi2Se3

Semiconductors - The influence of doping with Tl on the Shubnikov-de Haas effect at T = 4.2 K in magnetic fields up to 38 T in p-Sb2−x Tl x Te3 (x = 0, 0.005, 0.015, and 0.05) and...     

Highly efficient n-(Bi, Sb)2Te3 thermoelectric materials for temperatures below 200 K

The possibility of using an n-type Bi_2?x Sb_xTe₃ solid solution in thermoelectric refrigerators at T<200 K is considered. It is shown that, if the material under consideration is optimized for the above temperature region, the temperature dependence of the Seebeck coefficient α becomes less pronounced, and the crystal-lattice thermal conductivity κ_L decreases as compared to what is observed in a conventional n-Bi₂Te_3?y Se_y solid solution. These factors and a high mobility of charge carriers μ₀ bring about an increase in the parameter β ～ ZT, where Z is the thermoelectric efficiency.     

Ab initio studies of structural,electronic, optical,elastic and thermal properties of Ag-chalcopyrites (AgAlX2: X=S,Se)

The ab-initio calculations for the structural, electronic, optical, elastic and thermal properties of Ag-chalcopyrites (AgAlX₂: X=S and Se) have been reported using the full potential linearized augmented plane wave (FP-LAPW) method. In this paper, the recently developed Tran–Blaha modified Becke–Johnson potential is used along with the Wu-Cohen generalized gradient approximation (WC-GGA) for the exchange-correlation potential. Results are presented for lattice constants, bulk modulus and its pressure derivative, band structures, dielectric constants and refractive indices. We have also computed the six elastic constants (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, C₆₆). The thermodynamical properties such as thermal expansion, heat capacity, Debye temperature, entropy, bulk modulus are calculated employing the quasi-harmonic Debye model at different temperatures (0–900 K) and pressures (0–8 GPa) and the silent results are interpreted. Hardness of the materials is calculated for the first time at different temperatures and pressures.     

Ni掺杂CoSb3化合物的热电性能及机理研究

采用机械合金化结合冷等静压的方法制备了CoSb3和Co3.5Ni0.5Sb12化合物,并测量了其热电性能。利用基于密度泛函理论赝势平面波的方法对Ni掺杂前后的CoSb3的能带结构和态密度进行了计算,实验和理论计算结果表明:CoSb3的费密面位于导带和价带之间,其电阻率随温度的升高而降低,为非简并半导体;Co3.5Ni0.5Sb12的费密面进入导带,其电阻率随温度的升高而增大,为n型简并半导体;本实验条件下,Co3.5Ni0.5Sb12化合物的功率因子在550 K时出现最大值2 292.92μW/(m.K2),是未掺杂CoSb3化合物最大值的12倍。     

First principles study of electronic and optical properties of the ternary SnSb4S7 using modified Becke-Johnson potential

The electronic and optical properties of SnSb₄S₇ compound are calculated by the full-potential linearized augmented plane-wave (FP-LAPW) method. The density of states (DOS) is carried out by the modified Becke-Johnson (mBJ) exchange potential approximation based on density functional theory (DFT). The compound SnSb₄S₇ has a monoclinic structure with the space group P2₁/m with lattice parameters of a=11.331 Å, b=3.865 Å and c=13.940 Å. The band gap is calculated to be 0.8 eV. The optical parameters, like dielectric constant, refractive index, reflectivity and energy loss function were also calculated and analyzed. The present work provides information about variation of the electronic and optical properties which reveals that SnSb₄S₇ is suitable for optoelectronic devices.     

(Bi2Te3)0.90(Sb2Te3)0.05(Sb2Se3)0.05热压合金微观结构和电学性能相关性研究

通过熔炼,研磨制备N型(Bi2Te3)0．90(Sb2Te3)0．05(Sb2Se3)0．05热电材料的粉末,热压制备混合粉末热压合金。通过SEM和XRD研究热压合金的微观结构,在室温测量热压合金样品的电学性能。结果表明热压合金在微观结构和电学性能上存在各向异性,从而预示能够在增强材料机械强度的同时提高其热电性能。     

First principles study of structural,electronic, elastic and magnetic properties of gadolinium hydride system GdHx (x=1, 2, 3)

The structural, electronic, elastic and magnetic properties of gadolinium and its hydrides GdH_x (x=1, 2, 3) are investigated by using Vienna ab-initio simulation package with the generalized gradient approximation parameterized by Perdew, Burke and Ernzerhof (GGA-PBE) plus a Hubbard parameter (GGA-PBE+U) in order to include the strong Coulomb correlation between localized Gd 4f electrons. At ambient pressure all the hydrides are stable in the ferromagnetic state. The calculated lattice parameters are in good agreement with the experimental results. The bulk modulus is found to decrease with the increase in the hydrogen content for the gadolinium hydrides. A pressure-induced structural phase transition is predicted to occur from cubic to hexagonal phase in GdH and GdH₂ and from hexagonal to cubic phase in GdH₃. The electronic structure reveals that mono and di-hydrides are metallic, whereas trihydride is half-metallic at normal pressure. On further increasing the pressure, a half-metallic to metallic transition is also observed in GdH₃. The calculated magnetic moment values of GdH_x (x=1, 2, 3) are in accord with the experimental values.     

First-principles calculations of structural,elastic, electronic and optical properties of XO (X=Ca,Sr and Ba) compounds under pressure effect

The structural, elastic, electronic and optical properties of XO (X= Ca, Sr and Ba) compounds were investigated by the density functional theory. A good agreement was found between our calculated results and the available theoretical and experimental data of the lattice constants. Young's modulus, Poisson ratio, bulk modulus, elastic constants and their pressure derivatives are also calculated. SrO and BaO compounds present a transition phase at 39.72 and 27.28 GPa. The SrO compound shows a change from direct band gap (Γ–Γ) to indirect band gap (Γ–X) at about 15 GPa. The top of the valence bands reflects the s electronic character for all structures. We investigate the effective mass of electrons as function of pressure at the Γ point for CaO, SrO and BaO compounds. Calculations of the optical spectra have been performed for the energy range 0–60 eV. The origin of the spectral peaks was interpreted based on the electronic structures. The enhancement of pressure increases the static dielectric function and refractive index of CaO, SrO and BaO.     

Synthesis,structure and optical properties of Sb2Se3

Antimony selenide (Sb₂Se₃) films were deposited by chemical bath deposition on glass substrates. Deposited films were characterized by X-ray diffraction, scanning electron microscopy and UV–vis–NIR spectroscopy. X-ray analysis of these films confirms an orthorhombic structure. Evaluation of band gap from optical spectra shows absorption due to indirect transition occurring at 1.16 eV. For the first time, optical properties including refractive index, extinction coefficient, optical conductivity, reflectivity, absorption coefficient and energy-loss spectrum were calculated by a self-consistent approach using the Wien2k code.     

Thermoelectric properties of the (Bi,Sb)2Te3-based material obtained by spark plasma sintering

The dependence of the thermoelectric properties of the nanostructured bulk (Bi,Sb)₂Te₃ material on the composition and the spark plasma-sintering (SPS) temperature T _SPS has been studied. It has been revealed that the Bi_0.4Sb_1.6Te₃ solid solution sintered at a temperature of 450–500°C has a thermoelectric figure of merit ZT = 1.25–1.28. The dependence of thermoelectric properties on the sintering temperature T _SPS above 400°C is correlated to the transformation of the fine structure of the material due to the rearrangement of point vacancy-donor defects in the process of repeated recrystallization. It has been established that point structural defects make a considerable contribution to the formation of the thermoelectric properties of nanostructured material.     

Selenium alloying of indium sulfide: Ab-initio study of structural,electronic and optical features

In the framework of density functional calculations and using the Linear Augmented Plane Waves with local orbital method (LAPW+lo), we have investigated the structural, electronic and optical properties of indium sulfoselenide (InS_1−xSe_x). The present study confirms that InS_1−xSe_x are indirect band gap materials. The non-linear dependence concentration x of the theoretical forbidden energy band is clearly visible and the microscopic origins of gap bowing are identified and calculated. In order to identify the angular momentum characteristics of the bands structure, the total and partial densities of states (DOS and PDOS) are analyzed. The feature bonding is also investigated from charge density study. Using the projected total densities of states (DOS) and the bands structure, we have calculated the linear optical properties, namely, the complex dielectric function ε(ω), refractive index n(ω), absorption coefficient α(ω) and the electron energy loss L(ω). In particular, we take into account the effect of Se composition on anisotropy, real part of the dielectric function and refractive index. The present alloy is found to be a challenging material in optoelectronic, medical and photovoltaic devices. Good agreements are found with the available experimental and theoretical results.