Structural Studies and Valence Band Splitting Parameters in Ordered Vacancy Compound AgGa7Se12 (original) (raw)
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The Journal of Physical Chemistry B, 2013
Theoretical and experimental studies of the Ag x Ga x Ge 1−x Se 2 (x = 0.333, 0.250, 0.200, 0.167) single crystals are performed. These crystals possess a lot of intrinsic defects which are responsible for their optoelectronic features. The theoretical investigations were performed by means of DFT calculations using different exchangecorrelation potentials. The experimental studies were carried out using the modulated VUV ellipsometry for dielectric constants and birefringence studies. The comparison of the structure obtained from X-ray with the theoretically optimized structure is presented. The crucial role of the intrinsic defect states is manifested in the choice of the exchange correlation potential used. The data may be applicable for a large number of the ternary chalcogenides which are sensitive to the presence of the local disordered states near the band edges.
We report an ab initio calculation and study of the structural and electronic properties of AgAlM 2 (M = S, Se, Te) chalcopyrite semiconductors using the density functional theory (DFT)-based self-consistent tight-binding linear muffin tin orbital (TB-LMTO) method. The calculated equilibrium values of the lattice constants, anion displacement parameter (u), tetragonal distortion (η = c/2a) and bond lengths are in good agreement with experimental values. Our study suggests these semiconductors to be direct band gap semiconductors with band gaps 1.98 eV, 1.59 eV and 1.36 eV, respectively. These values are in good agreement with experimental values, within the limitation of the local density approximation (LDA). Our explicit study of the effects of anion displacement and p-d hybridization show that the band gap increases by 9.8%, 8.2% and 5.1%, respectively, for AgAlM 2 (M = S, Se, Te) due to former effect and decreases by 51%, 47% and 42%, respectively, due to latter effect.
Science of Advanced Materials
Ag 2 In 2 GeS 6 and Ag 2 InSiS 6 are two interesting quaternary-sulfide single crystals. Starting from our previous investigation on Ag 2 In 2 GeS 6 single crystals, the Ag 2 In 2 SiS 6 is investigated here. We demonstrate the effect of replacing Ge by Si on the electronic structure and the bonding properties. We have used X-ray diffraction (XRD) data for Ag 2 In 2 Ge(Si)S 6 single crystals as input to our theoretical calculations using the all-electron full potential linearized augmented plane wave method to solve the Kohn Sham Density Functional Theory (DFT) equations. As remarkable finding, our calculations show that, on replacing Ge by Si atom, the environment of the S atoms is changed significantly. The energy gap depends on the exchange correlation function. For the local density approximation (LDA) the energy gap is 0.76 eV, while, based on the modified Becke-Johnson approximation (mBJ), the energy gap increases to 1.98 eV. We should emphasize that this energy gap in Ag 2 In 2 SiS 6 is almost the same as that obtained for Ag 2 In 2 GeS 6 (1.96 eV). Another significant finding is that when we replace Ge by Si the conductions bands move away from the Fermi energy while the valence bands are almost unchanged. In order to support the theoretical calculation the calculated total density of states below E F (TDOS-VB) of Ag 2 In 2 SiS 6 single crystals is compared with our experimentally measured valence-band X-ray photoelectron spectroscopy (XPS-VB). The theoretical spectrum reproduces the general features structure of the measured XPS-VB faithfully.
Ag 2 In 2 GeS 6 and Ag 2 InSiS 6 are two interesting quaternary-sulfide single crystals. Starting from our previous investigation on Ag 2 In 2 GeS 6 single crystals, the Ag 2 In 2 SiS 6 is investigated here. We demonstrate the effect of replacing Ge by Si on the electronic structure and the bonding properties. We have used X-ray diffraction (XRD) data for Ag 2 In 2 Ge(Si)S 6 single crystals as input to our theoretical calculations using the all-electron full potential linearized augmented plane wave method to solve the Kohn Sham Density Functional Theory (DFT) equations. As remarkable finding, our calculations show that, on replacing Ge by Si atom, the environment of the S atoms is changed significantly. The energy gap depends on the exchange correlation function. For the local density approximation (LDA) the energy gap is 0.76 eV, while, based on the modified Becke-Johnson approximation (mBJ), the energy gap increases to 1.98 eV. We should emphasize that this energy gap in Ag 2 In 2 SiS 6 is almost the same as that obtained for Ag 2 In 2 GeS 6 (1.96 eV). Another significant finding is that when we replace Ge by Si the conductions bands move away from the Fermi energy while the valence bands are almost unchanged. In order to support the theoretical calculation the calculated total density of states below E F (TDOS-VB) of Ag 2 In 2 SiS 6 single crystals is compared with our experimentally measured valence-band X-ray photoelectron spectroscopy (XPS-VB). The theoretical spectrum reproduces the general features structure of the measured XPS-VB faithfully.
Revista Mexicana de Física
The primary goal of this study is to investigate the effect of different exchange-correlation functionals on the optoelectronic and elastic properties of the Ag2O chalcogenide compound. For the electronic structures and optical spectra, the Tran-Blaha modified Becke-Johnson approach combined with GGA and with GGA+U (mBJ-GGA-PBEsol and mBJ-GGA-PBEsol+U, respectively) was used. The available theoretical and experimental data for the bandgap energy were reported to determine whether there is a correlation with our results. The electronic structure revealed that our compound is a direct semiconductor at the R-symmetry point with a bandgap of 1.22 eV, which this value agrees well with the experimental values for the first time. The elastic constants were also evaluated using the IRelast package, which revealed that the compound was mechanically stable. Finally, the optical response was systematically studied, and it was found that Ag2O exhibited excellent optical efficiency.
Crystallography and optical energy gap values for AgGa(Se1−zTez)2alloys
Journal of Applied Physics, 1985
Polycrystalline samples of Cd~x(AgIn),nln~,Te, (z + y + z = 1) alloys are prepared by a melt and anneal technique. Debye-Scherrer X-ray powder photographs are used to determine equilibrium conditions and lattice parameter values. It is found that in addition to the zincblende and chalcopyrite structures, a partially ordered structure is obtained, plus a two-phase field at higher zvalues. Room temperature measurements of optical absorption are made t o give values of the optical energy gap E , for all single phase samples. It is found that the variation of a is practically linear with composition. However, while E, varied linearly with composition inside a phase field, the resulting lines have different aiming points a t z = I, the values being 2.83 eV for zinc blmde, 1.90 eV for the ordered phase, and 1.36 eV for the chalcopyrite phase. Thus the values of E, give a very good indication of the phase boundaries. Des kchantillons polycristallins d'alliages de Cdi,(AgInj,Mnn,Te, (x + y + z = 1) ont 6th prbparks par une technique de fonte e t recuit. Les conditions d'kquilibre et les valeurs des parametres de maille ont ktb determinbes B partir des films de diffraction des rayons-X par la mkthode Debye-Scherrer. On trouve qu'en plus des structures blende et chalcopyrite, une structure partiellement ordonnee est obtenue, ainis qu'un champ oh deux phases coexistent B plus hautes valeurs de z. Des mesures d'absorption optique ont btk faites 8. l'ambiante pour obtenir les valeurs de bandes interdites optiques E , sur tous les kchantillons presentant une seule phase. On trouve que la variation de a est pratiquement linbaire avec la composition. Toutefois, alors qu'Eg varie lineairement avec la composition B l'intbrieur d'un champ de phase donn8, les droites ainsi obtenues s'extrapolent sur des valeurs diffbrentes pour z = 1, ces valeurs 6tant de 2,83 eV pour la phase blende, 1,90 eV pour la phase or donnee et 1,36 eV pour la phase chalcopyrite. Done les valeurs d'E, fournissent de tres bonnes indications des limites des phases.
Electronic and structural properties of A Al 2 Se 4 (A ¼Ag, Cu, Cd, Zn) chalcopyrite semiconductors
We have studied the structural and electronic properties of defect chalcopyrite semiconductors A Al 2 Se 4 (A ¼Ag, Cu, Cd, Zn) using density functional theory (DFT) based first principle technique within tight binding linear muffin-tin orbital (TB-LMTO) method. Our calculated structural parameters such as lattice constants a and c, tetragonal distortion (Z ¼ c=2a) are in good agreement with experimental work. Anion displacement parameters, bond lengths and bulk modulus are also calculated. Our band structure calculation suggests that these compounds are direct band gap semiconductors having band gaps 2.40, 2.50, 2.46 and 2.82 eV for A Al 2 Se 4 (A¼ Ag, Cu, Cd, Zn) respectively. Calculated band gaps are in good agreement with other experimental and theoretical works within LDA limitation. We have made a quantitative estimation of the effect of p-d hybridization and structural distortion on the electronic properties. The reduction in band gap due to p-d hybridization is 19.47%, 21.29%, 0% and 0.7% for A Al 2 Se 4 (A¼ Ag, Cu, Cd, Zn) respectively. Increment of the band gap due to structural distortion is 11.62%, 2.45%, 2.92% and 9.30% in case of AgAl 2 Se 4 , CuAl 2 Se 4 , CdAl 2 Se 4 and ZnAl 2 Se 4 respectively. We have also discussed the bond nature of all four compounds.
Journal of Physics: Condensed Matter, 2003
The structural, electronic and optical properties of tetragonal nonlinear optical (NLO) crystals, AgGa(S x Se 1−x) 2 (x = 0.0, 0.25, 0.5, 0.75, and 1.0), were investigated theoretically and experimentally. The results obtained indicated that the electronic bandgaps, optical properties and bulk moduli of these compounds were linearly dependent on the substitution concentration of cations. From partial density of state analysis, it was found that the electronic states near the band edges of AgGa(S x Se 1−x) 2 were a simple proportional mixture of the atomic orbitals of sulfur and selenium. A cell-volume effect was proposed as the major cause of the linear dependence of material properties on the substitution concentration. It was calculated that the second-order NLO susceptibilities were scaled with the cubic power of bandgap, although a minor deviation existed. This deviation arose from the optical transition moment products.
Electronic structure of photoresponsive Ag6M2O7 (M = Si, Ge)
Journal of the Ceramic Society of Japan, 2016
The electronic structures of Ag 6 M 2 O 7 (M = Si, Ge) are investigated by the scalar-relativistic full potential linearized augmented plane wave plus local orbital (FLAPW+lo) method using the modified Becke-Johnson (MBJ) potential combined with the local density approximation (LDA) correlation. For Ag 6 M 2 O 7 (M = Si, Ge), the valence band maximums (VBM) are approximately located at the X (Si) or A (Ge) and the conduction band minimums (CBM) at the ¥ both, indicating that Ag 6 M 2 O 7 are an indirect energy gap material. The fundamental band gaps of Ag 6 Si 2 O 7 and Ag 6 Ge 2 O 7 are calculated to be 1.69 and 1.42 eV, respectively, in the MBJ-LDA calculation. The results are a remarkably contrast to the underestimation based on the generalized gradient approximation (GGA) calculation. On the other hand, there is no distinct difference in the effective masses of photogenerated holes and electrons near the VBM and CBM between MBJ-LDA and GGA approaches. The optical properties of Ag 6 M 2 O 7 (M = Si, Ge) are contemplated from spectral dependence of the complex dielectric function, ¾ (½) = ¾ 1 (½) + i¾ 2 (½).