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

Forcing Cesium into Higher Oxidation States via Useful Hard X-ray Induced Chemistry at Extreme Conditions

Recent theoretical work published in Nature Chemistry postulates the existence of cesium in high oxidation states when bonding with fluorine. It is thus predicted to behave as a p-block element (such as xenon) at pressures above 5 GPa. At these pressures, fluorine atoms may bond with the inner p-shell electrons forming CsFn, where n may vary from 2 up to 6; thus the oxidation state of Cs may change up to 6+. My research focused on physically synthesizing these compounds and to verify that, given the right conditions, bonding doesn\u27t only occur with valence electrons, but with the inner p-shell electrons as well, much like what occurs in xenon chemistry. The difficulty of proving this experimentally is that working with fluorine is extremely difficult and dangerous, and has only been studied at high pressure in one earlier study. For the past two years our group has been working on developing a new field of science we call: Useful Hard X-ray Photochemistry. By utilizing the highly penetrating, highly focused, and highly ionizing characteristics of hard x-rays, we can decompose relatively safe and inert solid and liquid samples into simple molecules. We have successfully used our technique to produce O2, H2, N2, Cl2, and most recently F2 in situ within a diamond anvil cell. We have also successfully and safely combusted O2 and H2 into water by creating a segregated mixture of potassium perchlorate and ammonia borate within a diamond anvil cell. We have developed a new method to produce molecular fluorine in situ, giving us a safe mechanism to supply excess fluorine that is available to react with cesium in order to experimentally verify the theoretical prediction of the unexpected stoichiometries of cesium compounds. By using techniques such as x-ray absorption fine structure spectroscopy, x-ray diffraction, and Raman spectroscopy, coupled with our new techniques of in situ hard x-ray photochemistry, we sought to experimentally demonstrate this theoretical behavior of inner shell bonding and open the door to a better understanding of chemical bonding under extreme conditions. This thesis discusses the results of our attempt to synthesize these novel CsFn (n\u3e1) compounds

An emerging technique: multi-ice-core multi-parameter correlations with Antarctic sea-ice extent

ABSTRACT. Using results stemming from the International Trans-Antarctic Scientific Expedition (ITASE) ice-core array plus data from ice cores from the South Pole and Siple Dome we investigate the use of sodium (Na+), non-sea-salt sulfate (nssSO4 2–) and methylsulfonate (MS–) as proxies for Antarctic sea-ice extent (SIE). Maximum and mean annual chemistry concentrations for these three species correlate significantly with maximum, mean and minimum annual SIE, offering more information and clarification than single ice-core and single species approaches. Significant correlations greater than 90% exist between Na+ and maximum SIE; nssSO4 2– with minimum and mean SIE; and MS– with mean SIE. Correlations with SIE within large geographic regions are in the same direction for all ice-core sites for Na+ and nssSO4 2– but not MS–. All ice cores display an SIE correlation with nssSO4 2– and MS–, but not all correlate with Na+. This multi-core multi-parameter study provides the initial step in determining which chemical species can be used reliably and in which regions as a building block for embedding other ice-core records. Once established, the resulting temporal and spatial matrix can be used to relate ice extents, atmospheric patterns, biological productivity and site conditions

A new room-temperature equation of state of Bi up to 260 GPa

At room temperature, bismuth undergoes several structural transitions with increasing pressure before taking on a body-centered cubic (bcc) phase at approximately 8 GPa. The bcc structure is stable to the highest measured pressure and its simplicity, along with its high compressibility and atomic number, make it an enticing choice as a pressure calibrant. We present three data sets on the compression of bismuth in a diamond anvil cell in a neon pressure medium, up to a maximum pressure of about 260 GPa. The use of a soft pressure medium reduces deviatoric stress when compared to previous work. With an expanded pressure range, higher point density, and a decreased uniaxial stress component, we are able to provide more reliable equation of state (EOS) parameters. We also conduct density functional theory (DFT) electronic-structure calculations that confirm the stability of the bcc phase at high pressure.Comment: 7 pages, 5 figures in main text; 7 pages, 2 figures in supplemen

Effort estimation of FLOSS projects: A study of the Linux kernel

This is the post-print version of the Article. The official published version can be accessed from the link below - Copyright @ 2011 SpringerEmpirical research on Free/Libre/Open Source Software (FLOSS) has shown that developers tend to cluster around two main roles: “core” contributors differ from “peripheral” developers in terms of a larger number of responsibilities and a higher productivity pattern. A further, cross-cutting characterization of developers could be achieved by associating developers with “time slots”, and different patterns of activity and effort could be associated to such slots. Such analysis, if replicated, could be used not only to compare different FLOSS communities, and to evaluate their stability and maturity, but also to determine within projects, how the effort is distributed in a given period, and to estimate future needs with respect to key points in the software life-cycle (e.g., major releases). This study analyses the activity patterns within the Linux kernel project, at first focusing on the overall distribution of effort and activity within weeks and days; then, dividing each day into three 8-hour time slots, and focusing on effort and activity around major releases. Such analyses have the objective of evaluating effort, productivity and types of activity globally and around major releases. They enable a comparison of these releases and patterns of effort and activities with traditional software products and processes, and in turn, the identification of company-driven projects (i.e., working mainly during office hours) among FLOSS endeavors. The results of this research show that, overall, the effort within the Linux kernel community is constant (albeit at different levels) throughout the week, signalling the need of updated estimation models, different from those used in traditional 9am–5pm, Monday to Friday commercial companies. It also becomes evident that the activity before a release is vastly different from after a release, and that the changes show an increase in code complexity in specific time slots (notably in the late night hours), which will later require additional maintenance efforts

Prevalence of Pretransition Disordering in the Rutile-to-Cacl2 Phase Transition of Geo2

The ability to tailor a material\u27s electronic properties using density driven disordering has emerged as a powerful route to materials design. The observation of anomalous structural and electronic behavior in the rutile to CaCl2 phase transition in SnO2 led to the prediction that such behavior is inherent to all oxides experiencing such a phase transition sequence [Smith et al., J. Phys. Chem. Lett. 10, 5351 (2019)1948-718510.1021/acs.jpclett.9b01633]. Here, the ultrawide band gap semiconductor GeO2 is confirmed to exhibit anomalous behavior during the rutile to CaCl2 phase transition. A phase pure rutile GeO2 sample synthesized under high-pressure, high-temperature conditions is probed using synchrotron diffraction and x-ray and optical spectroscopy under high pressure conditions. Density functional theory calculations show that the enthalpic barrier to displacing an oxygen along the B1g librational mode decreases with pressure leading up to the rutile to CaCl2 phase transition. The band structure of the distorted state shows that such oxygen displacements form small polarons

High-Pressure Equation of State of Cesium Fluoride to 120 GPa

We have performed a high pressure synchrotron x-ray diffraction study of the ionic salt, cesium fluoride (CsF), up to 120 GPa. We observed the B1 → B2 phase transition near 5 GPa as previously reported. Beyond this pressure, no phase transitions were determined to have occurred up to the highest pressure studied. Unit cell data were calculated from the known B2 (CsCl) structure for all of the pressures studied above 5 GPa, and an equation of state (EOS) was fit to the data using a 3rd order Birch-Murnaghan equation in this phase. Density Functional Theory (DFT) was also employed to compute an EOS for comparison purposes. Our experimental results agreed very well with both sets of the predicted EOS

An ice-core proxy for northerly air mass incursions into West Antarctica

A 200-year proxy for northerly air mass incursions (NAMI) into central and western West Antarctica is developed from the examination of 19 shallow (21–150 m deep) Antarctic ice-core non-sea-salt (nss) Ca2+ concentration records. The NAMI proxy reveals a significant rise in recent decades. This rise is unprecedented for at least the past 200 years and is coincident with anthropogenically driven changes in other large-scale Southern Hemisphere (SH) environmental phenomena such as greenhouse gas (GHG) induced warming, ozone depletion, and the associated intensification of the SH westerlies. The Hysplit trajectory model is used to examine air mass transport pathways into West Antarctica. Empirical orthogonal function analysis, in combination with trajectory results, suggests that atmospheric circulation is the dominant factor affecting nssCa2+ concentrations throughout central and western West Antarctica. Ozone recovery will likely weaken the spring-summer SH westerlies in the future. Consequently, Antarctica could lose one of its best defences against SH GHG warming. Copyright 2011 Royal Meteorological Societ

Climate Variability in West Antarctica Derived from Annual Accumulation-Rate Records from ITASE Firn/Ice Cores

Thirteen annually resolved accumulation-rate records covering the last similar to 200 years from the Pine Island-Thwaites and Ross drainage systems and the South Pole are used to examine climate variability over West Antarctica. Accumulation is controlled spatially by the topography of the ice sheet, and temporally by changes in moisture transport and cyclonic activity. A comparison of mean accumulation since 1970 at each site to the long-term mean indicates an increase in accumulation for sites located in the western sector of the Pine Island-Thwaites drainage system. Accumulation is negatively associated with the Southern Oscillation Index (Sol) for sites near the ice divide, and periods of sustained negative Sol (1940-42, 1991-95) correspond to above-mean accumulation at most sites. Correlations of the accumulation-rate records with sea-level pressure (SLP) and the SOI suggest that accumulation near the ice divide and in the Ross drainage system may be associated with the midlatitudes. The post-1970 increase in accumulation coupled with strong SLP-accumulation-rate correlations near the coast suggests recent intensification of cyclonic activity in the Pine Island-Thwaites drainage system