A.A. Lavrentyev

Manifestation of Anomalous Weak Space-Charge-Density Acentricity for a Tl4HgBr6 Single Crystal

Density functional theory (DFT) calculations within the concept of the MBJ+U+SO (modified Becke-Johnson potential + U + spin orbit) approach were performed for a Tl4HgBr6 single crystal for the first time assuming weak noncentrosymmetry (space group P4nc). Excellent agreement was achieved between the calculated and experimental band-gap-energy magnitudes as well as the density of electronic states measured by the X-ray photoelectron spectroscopy method. It is a very principal result because usually the DFT calculations underestimate the energy-gap values. In the present study, we carry out calculations of the optical properties (absorption coefficient, real and imaginary parts of the dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient, and optical reflectivity dispersions). It has been established that the principal origin of the observed weak acentricity is determined by delocalized band states at the top of the valence band originating from the p states of the Br atoms.

Ab initio calculations of the electron structure of the compounds AgGaS2, CdGa2S4, InPS4

Abstract Crystalline structures of the compounds in the row AgGaS2–CdGa2S4–InPS4 originate from the sphalerite structure with the gradual growth of the deficiency of metallic atoms from defect-free Ag4IGa4IIIS8 to □2IICd2IIGa4IIIS8 and further to □2IIIIn2III□2VP2VS8, where the symbol □ denotes a pseudovacancy. With the FEFF8 full multiple-scattering code, the local partial electron densities states of all atoms of all compounds investigated have been calculated. A comparison of the calculated electron densities with the experimental X-ray K- and L2,3-spectra of emission and absorption of sulfur and phosphorus shows good correspondence between the form and energy positions of the fine structure elements. For the compounds investigated there is no significant influence of the defectiveness of the crystalline lattice on the electron density of states.

Electron energy structure of defect chalcopyrite CdGa2Se4

The CdGa2Se4 compound, with a crystalline lattice of defect chalcopyrite, shows birefringence. However, in contrast to the similar CdGa2S4 compound, CdGa2Se4 has no experimentally observed isotropic points in the optical spectral range. In this study, the electron energy structure and optical spectra of CdGa2Se4 are calculated for three different experimentally obtained sets of crystalline parameters. Only one of these sets shows the absence of isotropic points.

X-ray spectra of some ternary crystal and glass semiconductors

Abstract X-ray absorption K-edges, fluorescent K-bands, and primary emission L2,3-bands of sulphur in crystalline and amorphous phases of semiconductor compound AsSI, AgAsS2, GeAsS, GeS2, and As2S3 have been investigated. The superposition of X-ray spectra of different kinds using the common energy scale reveals the electron structure of the compounds investigated and the character of chemical bonding in them. For AsSI, AgAsS2, GeAsS and As2S3 it is found that by transition from crystal to glass the electron density of states distribution in the valence and conduction bands does not change because of the too little changes of the short-range ordering by amorphization.

Electron energy structure of the semiconductors TlAsS2, Tl3AsS3, Tl3AsS4, Tl3PS4 and Tl3TaS4

The X-ray emission K and L2,3 bands of sulphur and phosphorus as well as their X-ray K absorption spectra are obtained for the semiconductors TlAsS2, Tl3AsS3, Tl3AsS4, Tl3PS4 and Tl3TaS4. The upper part of the valence band in the compounds investigated is formed of the S p states mainly, but the S s states are localized near the bottom of the valence band and hardly take part in the bonding. Comparison of the X-ray spectra of S and P in the isoelectronic and isostructural compounds Tl3AsS4 and Tl3PS4 has made it possible to determine the energy position of the p states of arsenic in Tl3AsS4. The energy distributions of the valence electron states of sulphur and its neighbours in all the compounds investigated are analysed and the covalent-ionic mechanism of chemical bonding is proposed.

XANES and unoccupied DOS of sulfides and selenides of Zn and Cd

The compounds of type A II B VI (A = Zn, Cd; B = S, Se) with zincblende structure have been investigated both experimentally by X-ray spectroscopy and theoretically by two different approaches : the high order multiple scattering (FEFF7 code) and the full multiple scattering approximation using the method of the local coherent potential. The XANES of K-spectra of sulfur in cubic modification of ZnS and CdS have been obtained with X-ray Johann spectrograph, but the L 2,3 -spectra of sulfur and K-spectra of Zn and Se in ZnSe and ZnS were taken from the literature. The experimental spectra have been compared with the calculated curves of the density of unoccupied electron states. Calculations were performed with the crystal potentials of the FEFF code. The forms of calculated curves agree well with the experimental spectra. A satisfactory correspondence of the theory to the experiment has been obtained.

The electronic structure of defect chalcopyrite CdGa2Se4 as determined from first principles calculations and X-ray spectroscopy studies

The total and partial densities of states of all components of CdGa2Se4 are calculated using the modified ab initio augmented plane wave (APW) method implemented in the WIEN2k software package. The results of the APW calculations indicate that in the CdGa2Se4 compound, the Sep states are the main contributions to the valence band, and their contribution is maximum at the top of the valence band, meanwhile at the bottom of the conduction band the contributions of the Gas* states dominate. According to the theoretical APW-calculations, the Cdd and Gap electronic states also contribute sufficiently to the valence band (mostly at the bottom and at the top of the band respectively) in CdGa2Se4. The joint alignment of CdLβ2,15, GaKβ2 and SeKβ2 X-ray emission bands and of the X-ray photoelectron spectrum of the valence electrons obtained for the CdGa2Se4 single crystal, in a common energy scale, shows good agreement of the obtained theoretical and experimental data referring to the characteristic properties of the electronic structure of the CdGa2Se4 compound.

X-ray K-absorption spectra of phosphorus and sulfur in InPS4: experiment and theory

Abstract The K-edge X-ray absorption near edge spectra (XANES) of P and S in InPS 4 semiconductors is investigated both experimentally and theoretically. Theoretical calculations carried out using high order multiple scattering theory with the ab initio FEFF7 code are found to be in good agreement with experiment. The notable correspondence between the XANES features for P and S absorbers allows one to interpret the spectra in terms of strongly mixed free S and P p-states.

Quantum-mechanical calculations for the electron structure of phosphorus-containing sulfides Sn2P2S6 and Tl3PS4

The electron structure of phosphorus-containing sulfides Sn2P2S6 and Tl3PS4 is studied experimentally (by means of X-ray spectroscopy) and theoretically (via ab initio band calculations). Partial electronic state densities calculated using the WIEN2k software package correspond to their experimental analogs (the X-ray K- and {vnL}2,3-spectra of P and S). As a result of the greater electronegativity of sulfur, the electron densities of the p- and {pms}-states of P are shifted by approximately 3.5 eV toward lower energies, relative to the similar electronic states of S.

Aspects of simulating metal-insulator transitions in CrS and CoS

The electron-energy structure of CrS and CoS monosulfides at a temperature of 0 K is investigated theoretically using the augmented plane wave approach in combination with the WIEN2k program code. Computations of the total and partial densities of electron states are performed in the LDA + U approximation with allowance for the antiferromagnetic ordering of magnetic moments of 3d-metal atoms. In our models, the optical forbidden band gap Eg in the CrS compound emerges in the monoclinic crystal structure but is missing from the NiAs structure. For the CoS compound in the NiAs structure, computations of the electron-energy structure are performed for both the highand low-spin configurations of the Co atom’s 3d7 shell. The optically forbidden Eg band gap appears in the high-spin configuration with strong correlations of d-electrons, while CoS remains a conductor in the low-spin configuration.