Atomic level structure of Ge-Sb-S glasses: chemical short range order and long Sb-S bonds

The structure of Ge$_{20}$Sb$_{10}$S$_{70}$, Ge$_{23}$Sb$_{12}$S$_{65}$ and Ge$_{26}$Sb$_{13}$S$_{61}$ glasses was investigated by neutron diffraction (ND), X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) measurements at the Ge and Sb K-edges as well as Raman scattering. For each composition, large scale structural models were obtained by fitting simultaneously diffraction and EXAFS data sets in the framework of the reverse Monte Carlo (RMC) simulation technique. Ge and S atoms have 4 and 2 nearest neighbors, respectively. The structure of these glasses can be described by the chemically ordered network model: Ge-S and Sb-S bonds are always preferred. These two bond types adequately describe the structure of the stoichiometric glass while S-S bonds can also be found in the S-rich composition. Raman scattering data show the presence of Ge-Ge, Ge-Sb and Sb-Sb bonds in the S-deficient glass but only Ge-Sb bonds are needed to fit diffraction and EXAFS datasets. A significant part of the Sb-S pairs has 0.3-0.4 {\AA} longer bond distance than the usually accepted covalent bond length (~2.45 {\AA}). From this observation it was inferred that a part of Sb atoms have more than 3 S neighbors.Comment: 23 pages, 6 figures, submitted to Journal of Alloys and Compound

Inner relaxations in equiatomic single phase high entropy cantor alloy

The superior properties of high entropy multi functional materials are strongly connected with their atomic heterogeneity through many different local atomic interactions. The detailed element specific studies on a local scale can provide insight into the primary arrangements of atoms in multicomponent systems and benefit to unravel the role of individual components in certain macroscopic properties of complex compounds. Herein, multi edge X ray absorption spectroscopy combined with reverse Monte Carlo simulations was used to explore a homogeneity of the local crystallographic ordering and specific structure relaxations of each constituent in the equiatomic single phase face centered cubic CrMnFeCoNi high entropy alloy at room temperature. Within the considered fitting approach, all five elements of the alloy were found to be distributed at the nodes of the fcc lattice without any signatures of the additional phases at the atomic scale and exhibit very close statistically averaged interatomic distances 2.54 2.55 with their nearest neighbors. Enlarged structural displacements were found solely for Cr atoms. The macroscopic magnetic properties probed by conventional magnetometry demonstrate no opening of the hysteresis loops at 5 K and illustrate a complex character of the long range magnetic order after field assisted cooling in 5 T. The observed magnetic behavior is assigned to effects related to structural relaxations of Cr. Besides, the advantages and limitations of the reverse Monte Carlo approach to studies of multicomponent systems like high entropy alloys are highlighte

Pressure Driven Electronic Band Gap Engineering in Tin(IV)-O,N Compounds

The intrinsic link between long-range order, coordination geometry, and the electronic properties of a system must be understood in order to tailor function-specific materials. Although material properties are typically tailored using chemical dopants, such methods can cause irreversible changes to the structure, limiting the range of functionality. The application of high pressure may provide an alternative “clean” method to tune the electronic properties of semiconducting materials by tailoring their defect density and structure. We have explored a number of optoelectronic relevant materials with promising characteristics, specifically Sn-(O,N) compounds which have been predicted to undergo pressure-mediated opening of their optical band gaps. Tin (IV) oxide (SnO$_2$) is a wide band gap semiconductor that belongs to a class of materials known as transparent conducting oxides (TCO). In SnO$_2$ we have discovered pressure-driven disorder in its rutile structure that may explain the origins of its conductivity. We predict this property to be a universal phenomenon across all rutile-structured materials, and could present a new route for strain engineering meta-stable states in rutile structures that are recoverable to ambient conditions. We have also developed a better understanding of the mechanisms that drive the pressure mediated band gap opening in tin (IV) nitride (Sn$_3$N$_4$). X-ray absorption spectroscopy (XAS) is a multifaceted technique that can help elucidate how the behavior of lighter anion species affects the electronic band structure, structural properties, and vibrational dynamics of a material. In this thesis I will discuss how XAS can be combined together with x-ray diffraction (XRD) and Raman spectroscopy to construct a detailed picture regarding the different atomic species in Sn-(O,N) compounds. One difficulty with building a quantitative description based off of experimental data is that many of these materials are known to have highly kinetically hindered phase transitions. Because of this, they exhibit mixed phasing across a wide range of extreme conditions, leading to severe non-hydrostatic stresses within the system. By utilizing \textit{in situ} CO$_2$ laser annealing, I demonstrate that ground state structures can be accessed, overcoming many of the challenges that have thus far prevented a complete understanding of anion disordering and the role that it plays in a materials electronic properties