EXAFS Studies of Zn1-xMnxS Ternary Compounds

n/

Ab initio and nuclear inelastic scattering studies of Fe3_3Si/GaAs heterostructures

The structure and dynamical properties of the Fe$_3$Si/GaAs(001) interface are investigated by density functional theory and nuclear inelastic scattering measurements. The stability of four different atomic configurations of the Fe$_3$Si/GaAs multilayers is analyzed by calculating the formation energies and phonon dispersion curves. The differences in charge density, magnetization, and electronic density of states between the configurations are examined. Our calculations unveil that magnetic moments of the Fe atoms tend to align in a plane parallel to the interface, along the [110] direction of the Fe$_3$Si crystallographic unit cell. In some configurations, the spin polarization of interface layers is larger than that of bulk Fe$_3$Si. The effect of the interface on element-specific and layer-resolved phonon density of states is discussed. The Fe-partial phonon density of states measured for the Fe$_3$Si layer thickness of three monolayers is compared with theoretical results obtained for each interface atomic configuration. The best agreement is found for one of the configurations with a mixed Fe-Si interface layer, which reproduces the anomalous enhancement of the phonon density of states below 10 meVComment: 14 pages, 9 figures, 4 table

Structure and elastic properties of Mg(OH)2_2 from density functional theory

The structure, lattice dynamics and mechanical properties of the magnesium hydroxide have been investigated with static density functional theory calculations as well as \it {ab initio} molecular dynamics. The hypothesis of a superstructure existing in the lattice formed by the hydrogen atoms has been tested. The elastic constants of the material have been calculated with static deformations approach and are in fair agreement with the experimental data. The hydrogen subsystem structure exhibits signs of disordered behaviour while maintaining correlations between angular positions of neighbouring atoms. We establish that the essential angular correlations between hydrogen positions are maintained to the temperature of at least 150 K and show that they are well described by a physically motivated probabilistic model. The rotational degree of freedom appears to be decoupled from the lattice directions above 30K

Revisiting properties of CaCoSinO2n+2. Crystal and electronic structure

In a public space there are several reports of materials with general stoichiometry CaCoSinO2n+2. Pyroxene CaCoSi2O6 is probably the best-known representative for n = 2 but not much is known about materials with n = 3 and n = 4. In this study, attempts were carried out to synthesize those phantom materials and it was found that they do not exist as a single phase. A quantitative XRD analysis revealed that their chemical composition is correct but the formula should be written as CaCoSi2O6 + (n-2)SiO2. Similar qualitative conclusions were drawn from investigation of magnetic (DC magnetometry) and electronic properties using X-ray Photoelectron Spectroscopy (XPS) and Si K edge X-ray Absorption Spectroscopy (XAS). Additionally, the DFT ab initio calculations were carried out to obtain electronic signature from band structure of CaCoSi2O6. The apparent influence of the excess of SiO2 on magnetic properties of this “series” can be understood in terms of presence and suppression of secondary phases like Ca2CoSi2O7, which form when the starting materials are not homogenized properly. Addition of surplus SiO2 suppresses their formation leaving clear signature from CaCoSi2O6, which also can be synthesized from stoichiometric mixture using proper techniques

Random Microscopic Model of Quaternary Alloys

A random model of microscopic structure of zinc-blende type A$\text{}_{1-x}$B$\text{}_{x}$C$\text{}_{1-y}$D$\text{}_{y}$ quaternary alloys, based entirely on the elastic bond-stretching and bond-bending forces of two-element components AC, AD, BC, and BD, was constructed. The model was applied to Cd$\text{}_{1-x}$Mn$\text{}_{x}$Te$\text{}_{1-y}$Se$\text{}_{y}$ quaternary alloys and the calculated nearest-neighbor distances were compared with the experimental data. Additionally, a possibility of particle exchange (within each sublattice) was included into the model and a possibility of studying the preferential coordination was discussed

ΑCΤΑ PHYSICΑ POLONICA A MANGANESE DISTRIBUTION IN CdMnTeSe CRYSTALS. EXAFS DATA ANALYSIS*

The extended X-ray absorption fine structure (EXAFS) technique has been used to investigate the first shell coordination in zinc-blende quaternary In tetrahedrally coordinated III-V and II-VI semiconducting compounds there exist two sublattices occupied respectively by anions and cations. In Α1-x B x C and ΑC1-yDy ternary or A1-xBxC1-yDy quaternary compounds a basic question concerns a random or non-random substitution of the Α by B and C by D ions in respective sublattices. The first answer to this question has been obtained from infrared phonon mode analysis for GaΡl- and CdTe1-y Sey [4] ternary alloys. In all these experiments an additional feature in the infrared reflectivity spectrum has been observed and interpreted as a result of next nearest neighbours clusterin

Heating-Induced Conversion of Sr-Contaminated Brushite - EXAFS Data Analysis

The extended X-ray absorption fine structure method was applied to determine changes of the coordination parameters of Sr atoms during the thermal conversion of Sr-contaminated brushite into calcium pyrophosphate. The results revealed that the changes of the crystallographic structure induced by heating are correlated with the rearrangement of the Sr environment. It was also confirmed that Sr may be used as a marker of Ca in cases of thermal conversions of orthophosphate groups into pyrophosphate groups

EXAFS Studies of Zn1−x\text{}_{1-x}Mnx\text{}_{x}S Ternary Compounds

We performed extended X-ray absorption fine structure (EXAFS) measurements of Zn$\text{}_{1-x}$Mn$\text{}_{x}$S solid solution for various concentrations x in the range of 0 ≤ x ≤ 0.4. Data were collected on Zn and Mn K-edges with the use of synchrotron radiation from the ADONE storage ring in Frascati utilizing the Si(111) channel-cut monochromator. Applying the usual procedure of data reduction described elsewhere, we found well-defined different nearest-neighbor Zn-S and Mn-S distances, according to the previous results, almost independent on x. For Mn-S distances in the range of 0.1 ≤ x ≤ 1.0 we found, within the limit of experimental error, a constant value equal to 2.430±0.008 Å. For Zn-S distances for concentration changing from x = 0.0 to x = 0.4 we observed a weak, linear increase from 2.343±0.008 Å to 2.354 ± 0.008 Å, respectively

Influence of magnetic interaction on lattice dynamics of FeBO

The lattice dynamics of the antiferromagnetic FeBO3 crystal has been calculated by ab initio density-functional theory and measured by nuclear inelastic absorption spectroscopy. The calculations for the antiferromagnetic phase reproduce the experimental lattice parameters of the unit cell and provide phonon dispersion relations which agree well with the measured partial density of phonon states for the Fe atoms. Calculations for the nonmagnetic configuration lead to a smaller crystal volume and drastically higher phonon frequencies for the Fe atoms