A. N. Kravtsova

Evolution of local electronic structure in alabandite and niningerite solid solutions [(Mn, Fe)S, (Mg, Mn)S, (Mg, Fe)S] using sulfur K- and L-edge XANES spectroscopy

Abstract Synchrotron radiation S K- and L-edge X-ray absorption near-edge structure (XANES) spectra are reported for the cubic, rocksalt (B1) structure sulfides niningerite (MgS), alabandite (MnS), and oldhamite (CaS), and for their solid solutions (Mn,Fe)S and (Mg,Mn)S, and S L-edge XANES spectra are reported also for (Mg,Fe)S solid solutions. Pre-edge features at the S K-edge are attributed to transition of S 1s electrons to the lowest available unoccupied S 3p σ* antibonding states hybridized with metal 3d(eg) states, and at the S L-edge to transition of S 2p electrons to unoccupied S 3s σ*, 4s σ*, and 3d antibonding states hybridized with metal 3d(eg) states, and to a lesser extent 3d(t2g) states. The S K-edge XANES spectra for the solid solutions show a progressive participation of 3d orbitals in metal-S bonding with increase in substitution by Fe in (Mn,Fe)S and (Mg,Fe)S and Mn in (Mg,Mn)S through progressive increase in the area of the pre-edge feature. However, the pre-peak area does not increase linearly in each solid solution series showing that a real change in bulk electronic properties has occurred. Increase in pre-peak area reflects an increase in overall attainability of metal 3d states for hybridization with S 3p σ* antibonding states as proportionally more metal 3d orbitals become available. The S L-edge XANES spectra show progressive evolution of pre-edge features at the L3- and L2-edges (a1 and a2, respectively). Only a2 is present in the S L-edge XANES spectrum of FeS (troilite), and with progressive decrease in Fe content in (Mn,Fe)S and (Mg,Fe)S solid solutions, a1 first appears, then becomes dominant. Since a1 is attributed to transition of S 2p3/2 electrons to S 3s σ* states hybridized with metal 3d(eg) and 3d(t2g) states, this appears to represent an increased contribution from metal-S π-bonding. The results show that the size and position of the pre-edge features to the S K- and L-edges are controlled more by the DOS of hybridized 3d(egβ) and 3d(t2gβ) states and nearest-neighbor coordination of the metal atoms than by the precise coordination of S and the extended structure of the sulfide. The full multiple scattering approach has been applied to the calculation of the S K-edge XANES spectra of MgS, MnS, and CaS. Results are consistent with experimental XANES spectra, especially for the pre-edge features, which are often neglected in such calculations.

X-ray absorption spectroscopic and magneto-chemical analysis of the atomic structure of copper(II) complexes with diacetyl monoxime 1′-phthalazinyl hydrazone

The atomic structure of copper(II) complexes based on diacetyl monoxime 1′-phthalazinyl hydrazone is studied by XANES spectral analysis and magnetochemistry. The XANES spectra at the CuK-edge are measured in C24H24N10Cu2O2 and C24H24N10Cu2O2 complexes. The calculations of the CuK-XANES spectra of the complexes in question are performed for a few structural models based on the full-potential finite difference method. By low-temperature magnetochemistry magnetic exchange parameters are determined, and a quantum chemical simulation of the exchange interaction is carried out within the broken symmetry approximation. Based on a combined analysis of the XANES spectra and magnetic exchange parameters, the most probable structural models of C24H24N10Cu2O2 and C24H24N10Cu2O2 complexes are found.

Atomic and electronic structure of CdS-based quantum dots

The ab initio computer design of the CdS-based quantum dots and the cobalt doped CdS quantum dots is carried out. The characteristics features of the atomic and electronic structures of semiconductor colloidal quantum dots on CdS of different sizes are studied, and the effect of cobalt impurity atoms is estimated. We have proved the sensitivity of the X-ray absorption near-edge structure (XANES) method for the verification of the nanosized atomic structural parameters calculated by the methods of computer modeling for small-scale quantum dots of the CdS family, and for the determination of the local environment parameters of the cobalt atom in the quantum dot.

X-ray spectroscopic identification of garnet from the placer deposits of the Taman peninsula

Garnet from recent placer deposits of the Taman peninsula has been investigated by energy-dispersive X-ray fluorescence microanalysis and X-ray absorption near-edge structure (XANES) spectroscopy. Energy-dispersive X-ray fluorescence microanalysis showed that the chemical composition of the garnet under study corresponds to pyrope-almandine-spessartine series. The Fe K-edge X-ray absorption spectra of garnet have been recorded using a Rigaku R-XAS laboratory spectrometer. Iron K-edge XANES spectra for two iron-containing garnet minals (components), almandine and andradite, have been calculated using the full multiple-scattering and finite-difference methods. Based on a comparison of the experimental and theoretical Fe K-XANES spectra, it is concluded that recent magnetite-garnet placer deposits of the Taman peninsula contain garnet in the form of almandine.

Determination of the local atomic structure of the active center of bromoperoxidase protein via the analysis of x-ray absorption spectra

The existing model of the active center of bromoperoxidase protein, which has been obtained via x-ray diffraction methods, is defined more precisely. The processes occurring in the active center of this protein interacting with hydrogen peroxide are studied. It is shown that the existing model of the active center of bromoperoxidase is not sufficiently accurate. To study the adequacy of certain structural parameters of the active center, the atomic structure has been optimized using electron density functional theory. A new model of the active center of bromoperoxidase protein is proposed.

X-ray spectral diagnostics of synthetic lanthanide silicates

Potassium and rare-earth (Eu, Sm, Yb, Ce) silicate and aluminosilicate crystals are hydrothermally synthesized under isothermal conditions at 500°C and a pressure of 100 MPa. The chemical and structural formulas of the synthesized compounds HK6Eu[Si10O25], K7Sm3[Si12O32], K2Sm[AlSi4O12] · 0.375H2O, K4Yb2[Si8O21], and K4Ce2[Al2Si8O24] are determined. In addition, a synthesis product with Eu, in which the dominant phase is assumed to be K3Eu3+[Si6O15] · 2H2O, is studied. The oxidation state of lanthanides in the silicates under study is determined based on X-ray absorption near-edge structure spectroscopy. The Eu L3-, Sm L3-, Yb L3-, and Ce L3-edge X-ray absorption spectra of the studied silicates and reference samples are recorded using a Rigaku R-XAS laboratory spectrometer. As reference samples, Eu2+S, Eu3+F3, Eu23+O3, Sm23+O3, Yb23+O3, Yb3+F3, Yb3+Cl3, Ce23+O3, and Ce4+O2 are used. Comparison of the absorption edge energies of lanthanide silicates and reference samples shows that Eu, Sm, Yb, and Cе in all the samples studied are in the oxidation state 3+. The synthesized silicates will supplement our knowledge of possible rare-earth minerals existing in hydrothermal systems, which is important for analyzing the distribution spectra of rare elements, which are widely used for diagnostics of geochemical processes and determination of sources of ore materials.

Spin-polarized electronic structure of the core–shell ZnO/ZnO:Mn nanowires probed by X-ray absorption and emission spectroscopy

The combination of X-ray spectroscopy methods complemented with theoretical analysis unravels the coexistence of paramagnetic and antiferromagnetic phases in the Zn0.9Mn0.1O shell deposited onto array of wurtzite ZnO nanowires. The shell is crystalline with orientation toward the ZnO growth axis, as demonstrated by X-ray linear dichroism. EXAFS analysis confirmed that more than 90% of Mn atoms substituted Zn in the shell while a fraction of secondary phases was below 10%. The value of manganese spin magnetic moment was estimated from the Mn Kβ X-ray emission spectroscopy to be 4.3 μB which is close to the theoretical value for substitutional MnZn. However the analysis of L2,3 X-ray magnetic circular dichroism data showed paramagnetic behaviour with saturated spin magnetic moment value of 1.95 μB as determined directly from the spin sum rule. After quantitative analysis employing atomic multiplet simulations such difference was explained by a coexistence of paramagnetic phase and local antiferromagnetic coupling of Mn magnetic moments. Finally, spin-polarized electron density of states was probed by the spin-resolved Mn K-edge XANES spectroscopy and consequently analyzed by band structure calculations.

Synthesis and structure modeling of ZnS based quantum dots

Quantum dots (QDs) based on zinc sulfide are synthesized by a microwave method in an aqueous medium using dioctyl sodium sulfosuccinate (DS) or 4,4′-bipyridine (BP). Based on the analysis of X-ray diffraction profiles the conclusion is drawn that QDs obtained have a structure of cubic zinc blende with an average particle size of 5.6 nm for the ZnSDS sample and 4.8 nm for ZnSBP. Transmission electron microscopy images show the presence of spherical aggregates of particles only for ZnSDS. FTIR data indicate the presence of sulfate ions in both samples; DS remains in the sample, facilitating the QD agglomeration, while BP is effectively washed out. From the optical diffuse reflectance spectra the band gap is estimated, which turns out to be larger than the expected one due to the presence of elemental sulfur in the samples and partial oxidation of the QD surface. The QD structure based on ZnS particles is also modeled in the work. The possibility to employ X-ray absorption near-edge spectroscopy for the verification of atomic structural parameters around zinc sites in QDs based on zinc sulfide is demonstrated.

In silico study of the atomic and electronic structure of quantum dots of the CdTe family doped with atoms of rare earth elements

An in silico study of semiconductor quantum dots of the CdTe family doped with atoms of rare earth elements is performed based of density functional theory. An ab initio computer design of quantum dots based on CdTe nanoparticles doped with Eu и Gd atoms is carried out. Partial densities of states of CdTe:Eu and CdTe:Gd quantum dots are calculated and analyzed. X-ray absorption near edge (XANES) spectra near the Eu K-, L1-, and L3- and Gd K-, L1-, and L3-edges of CdTe:Eu and CdTe:Gd quantum dots are calculated. The sensitivity of XANES spectroscopy for the verification of parameters of a nanosized atomic structure of quantum dots based on CdTe particles doped with atoms of rare earth elements and the determination of the local atomic structure around the atoms of rare earth elements in quantum dots is demonstrated.

Doped CdTe-based quantum dots

Colloidal semiconductor CdTe-based quantum dots are investigated. An ab initio computer design of quantum dots based on nanoparticles of CdTe and CdTe doped with atoms of transition elements (Co, Mn) is executed. Partial densities of the electron states of the investigated quantum dots are calculated. The sensitivity of X-ray absorption near edge structure (XANES) spectroscopy for verifying the parameters of the nanoscale atomic structure of small quantum dots based on CdTe, and for determining the parameters of the local environment around cadmium atoms and doping atoms in quantum dots was proved.