Physics Commons^™

Response Of The Isothermal Mode Grüneisen Tensor Across Phase Boundaries, Jasmine K. Hinton

UNLV Theses, Dissertations, Professional Papers, and Capstones

The assumptions for the 1912 Grüneisen parameter are reviewed, particularly in the cases of anisotropy, high temperatures, and across phase boundaries. Two main case studies are shown: β-Sn, and Cd. The main techniques of this work involve resistively heated diamond anvil cells with both optical Raman spectroscopy and x-ray diffraction. It is found in Sn that the isothermal mode Grüneisen tensor along increasing isotherms diverges from the single-valued temperature aggregate at the onset of melt, and this is proposed to use as a method of exploring melt phase boundaries in other systems. This method is examined once again on another …

Stoichiometric Determination Of Hydride Materials At Extreme Conditions, Gregory Alexander Smith

UNLV Theses, Dissertations, Professional Papers, and Capstones

Hydrogen was predicted to be a high-temperature superconductor at near-megabar conditions in 1968,[1] but only recently was been experimentally observed.[2] This is due to the extraneous metrological constraint of requiring 5 megabars of pressure to stabilize. A more practical approach for synthesis of high-temperature superconductors has been pro-posed through the use of hydride compounds. Recently, a surge of rare earth hydrides have achieved critical superconducting transition temperatures (T_C ) close to room temperature.[3, 4, 5, 6] However, due to limitations of the necessary instrumentation to achieve megabar pressures, many techniques traditionally used to measure stoichiometry are unavailable.Three works presented in …

Towards The Electronic Response Of Carbon-Based Van Der Waals Heterostructures In A Diamond Anvil Cell, George Thomas Foskaris

UNLV Theses, Dissertations, Professional Papers, and Capstones

The nanoscale regime of materials has been at the forefront of research and interest in condensed matter physics for many years. In a merger of the fields of two-dimensional (2D) materials and high pressure physics, we present an investigation of the electronic response of carbon-based, van der Waals (vdW) heterostructures in a diamond anvil cell (DAC). Combining these fields presents us with the ability to study the characteristics of such systems both optically, and through electrical transport. Properties such as conductance, band structure, and layer number are considered. The samples in this study are assembled using exfoliation and stacking techniques …

Towards Highly Sensitive Capacitance Measurements Of A Quantum Anomalous Hall Phase In Van Der Waal Heterostructures, Kayla Cerminara

UNLV Theses, Dissertations, Professional Papers, and Capstones

One of the pioneering achievements in condensed matter physics of the 20th century is the observation of the quantum Hall e↵ect (QHE) in which the Hall resistance in a two-dimensional (2D) sample takes on quantized values in the presence of a strong perpendicular magnetic field. The precise quantization of the hall resistance to one part in a billion has provided a practical, worldwide resistance standard. A long-standing goal has been to realize a similar state of matter but without the need of a strong quantizing magnetic field. The quantum anomalous Hall e↵ect (QAHE) is such a state that is predicted …

Ab-Initio And Empirical Simulations Of Aluminum And Copper Metal, William Wolfs

UNLV Theses, Dissertations, Professional Papers, and Capstones

In this work, I perform detailed calculations on the bulk and electronic properties of aluminum and copper metal. Originally, I was motivated by experimental work on the solidsolid phase changes in pure aluminum. These phase changes were well predicted by density functional theory(DFT) but difficult or impossible to predict using embedded atom method potentials(EAM). EAM potentials are in wide use to describe many properties of bulk materials, and it seemed worrying that something so basic as a phase change could not be predicted. I began running high precision calculations with DFT and compared the results to EAM potentials which had …

High Pressure Synchrotron Xrd, Nfs, And Nrixs Studies Of Fesb2, Melanie Alden White

UNLV Theses, Dissertations, Professional Papers, and Capstones

This work combines high pressure synchrotron X-ray diffraction (XRD), nuclear forward scattering (NFS), and nuclear resonant inelastic X-ray scattering (NRIXS) experiments on iron diantimonide (FeSb2), a promising thermoelectric material. NRIXS uses the interaction between excited nuclear and lattice states to directly measure the phonon density of states (PDOS) for samples containing certain Mössbauer isotopes. For our experiments, we synthesized FeSb2 enriched with the isotope 57Fe and measured the partial PDOS for 57Fe sites in the FeSb2 lattice. NFS, also known as synchrotron Mössbauer spectroscopy, is a sister technique to NRIXS that studies the hyperfine structure of the excited Mössbauer nuclei. …

Investigations Of Technetium Metal And The Synthesis Of Binary Technetium Nitrides Under Extreme Conditions, Emily Siska

UNLV Theses, Dissertations, Professional Papers, and Capstones

Technetium (Tc) is the lightest of the radioactive elements and has no stable isotopes. Significant quantities of Tc are not naturally occurring on earth. However, technetium is found in high fission yield in nuclear reactors and produced for medical imaging. With its long half life, and high mobility in the environment make it of particular interest. To that end, the fundamental chemistry of Tc and Tc compounds is not as well understood compared to neighboring elements on the periodic table. Therefore, fundamental studies designed to better understand this transition metal, coupled with more targeted investigation at high temperature and pressure …

Development Of Co2 Laser-Heating For The Study Of Wide Band Gap Oxide Materials, Christian Matthew Childs

UNLV Theses, Dissertations, Professional Papers, and Capstones

The ability to access a vast region of the pressure-temperature landscape using energy density tuning enables exotic states of matter to be probed. A well documented method for such exploration, under static conditions, is the use of the laser-heated diamond anvil cell (LH-DAC), accessing a combination of high pressure (>300 GPa) and high temperature (>5000 K). This thesis presents our development of direct CO2 laser heating techniques to study a series of wide band gap insulators, La2Sn2O7, ZrO2, and CeO2, under high pressure conditions. The lasing frequency of CO2 lasers is such that these wide band gap materials …

Applied Symmetry For Crystal Structure Prediction, Scott William Fredericks

UNLV Theses, Dissertations, Professional Papers, and Capstones

This thesis presents an original open-source Python package called PyXtal (pronounced "pi-crystal") that generates random symmetric crystal structures for use in crystal structure prediction (CSP). The primary advantage of PyXtal over existing structure generation tools is its unique symmetrization method. For molecular structures, PyXtal uses an original algorithm to determine the compatibility of molecular point group symmetry with Wyckoff site symmetry. This allows the molecules in generated structures to occupy special Wyckoff positions without breaking the structure's symmetry. This is a new feature which increases the space of search-able structures and in turn improves CSP performance.

It is shown that …

Equation Of State Of H2o Ice Using Melt-Recrystallization, Zachary Michael Grande

UNLV Theses, Dissertations, Professional Papers, and Capstones

The recent surge in exoplanet discoveries due to advancements in astrophysical technology and analysis has brought the reliability of early equation of state measurements into question as they are the limiting factor when modeling composition of these planets. H2O content is among the most important for the search of habitable planets as well as in understanding planetary dynamics and atmosphere formation. Over the last three decades the equation of state of H2O has been investigated with various techniques but, has suffered from anisotropic strain and poor powder statistics resulting in a large discrepancy in equation of state fits. At pressures …

High-Pressure High-Temperature Exploration Of Phase Boundaries Using Raman Spectroscopy, Jasmine Kashmir Hinton

UNLV Theses, Dissertations, Professional Papers, and Capstones

Metastability of states can provide interesting properties that may not be readily accessible in a material’s ground state. Many materials show high levels of polymorphism, indicating a rich energy landscape and a potential for metastable states. Melt crystallization techniques provide a potential route to these states. We use a resistively heated diamond anvil cell (DAC) with fine control of a system’s pressure and temperature to explore these systems. Raman spectroscopy is used to track subtle structural changes across phase boundaries. Organic systems, such as glycine and aspirin, were our initial interest due to their high polymorphism and reported low melting …

Specific Heat, Magnetic Susceptibility, And The Effect Of Pressure On Structural Properties And Valence Of Eumn2si2, Euco2si2, And Eu5in2sb6, Brian Edward Light

UNLV Theses, Dissertations, Professional Papers, and Capstones

Many intermetallic solids containing elements from the rare earth series show interesting and unusual behavior associated with 4f electrons. This behavior includes unusual magnetic order, strongly correlated electrons, intermediate valence, heavy fermions, the Kondo effect, superconductivity, and non-Fermi liquid (NFL) to name a few. When long range magnetic order is suppressed to T = 0 K by the application of an external tuning parameter such as pressure, magnetic field, or chemical doping, a quantum critical point (QCP) appears in which strong quantum fluctuations give rise to many of the mentioned unusual properties.

Most of the past studies on unusual 4f …

Super Rapid Crystal Growth And Quench Of Monoclinic Bi-Ii* During Dynamic Compression, Zachary Allen Fussell

UNLV Theses, Dissertations, Professional Papers, and Capstones

We show that monoclinic Bi-II* forms during a dynamic compression regime with crystal growth rates from melt of ≈ 70 m/s. This extreme quench rate implies crystallization by non-diffusive processes and indicates that the liquid had a high degree of pre-ordering. Using ambient condition single crystal structure analysis we show for the first time that the monoclinic distorted phase of Bi (Bi-II) exists at ambient pressure, albeit bound to formation under dynamic compression. We review the pressure, temperature, and time conditions for formation and growth of this structure.

Crystallographic Exploration Of Fundamental Technetium Species At Nonambient Conditions, Daniel S. Mast

UNLV Theses, Dissertations, Professional Papers, and Capstones

The study of mater under extreme conditions has yielded fascinating insight into the physical and chemical nature of materials that under more modest conditions are considered mundane but have quite unusual properties when under pressure. Examples of this are the lightest of elements found in the first two rows and in the heaviest elements of the lanthanide and actinide series. Pure elements, though the simplest possible chemical systems, have proven to be among the most complex and interesting. Technetium metal is a simple HCP transition metal that, due to its half-filled d-shell, has a rich chemistry and an important place …

Predicting The Ground-State Structure Of Sodium Boride, Xin-Ling He, Xiao Dong, Quansheng Wu, Zhisheng Zhao, Qiang Zhu, Artem R. Oganov, Yongjun Tian, Dongli Yu, Xiang-Feng Zhou, Hui-Tian Wang

Physics & Astronomy Faculty Research

Binary borides has been a subject of extensive research. However, the exact compositions and crystal structures of sodium borides remained controversial. Here, using the ab initio variable-composition evolutionary algorithm, a new stable Na2B30 with I212121 symmetry (I212121-Na2B30) is found, which is -7.38meV/atom lower in energy than the Imma-Na2B30 structure reported by experimentalists. Interestingly, the Imma-Na2B30 structure is predicted to be a topological nodal line semimetal, which may result in superior electronic transport. In contrast, I212121-Na2B30 is an ultrahard semiconductor with an unprecedented open-framework structure, whose interstitial helical boron sublattice enhances its hardness and energetic stability.

Instrumentation And Measurement Of Thermoelectric And Structural Properties Of Binary Chalcogenides And Half-Heusler Alloys At Extreme Conditions Using A Paris-Edinburgh Press, Jason Lee Baker

UNLV Theses, Dissertations, Professional Papers, and Capstones

Understanding the high-pressure behavior of transport properties has been a driving force in the study of materials under extreme conditions for well over a century being pioneered by P.W. Bridgman in the early 20th century. Research dedicated to the study of these properties leads to a variety of important applications: exploration of insulator to semi-conductor to metal, structural, and electronic phase transitions, correlation of pressure-induced structural phase transitions and the electronic properties along phase boundaries, identifying electronic topological transitions, testing the validity of theoretical models and providing input parameters for calculations at high-pressure and high-temperature conditions, exploration of sample synthesis …

High-Pressure Properties Of Several Narrow Bandgap Semiconductors From First-Principles Calculations, Andrew Michael Alvarado

UNLV Theses, Dissertations, Professional Papers, and Capstones

The electronic, thermodynamic, and structural properties of three semiconducting materials, ZnO, InN, and PbS, at high pressure are investigated utilizing first-principles calculations based on density function theory. The first two systems, ZnO and InN, crystalize as hexagonal structures at ambient conditions and transition to a cubic structure at higher pressures. The last system, PbS, is cubic at ambient conditions, but transitions to an orthorhombic structure at higher pressure. At ambient conditions, these materials are well known semiconductors with vast amount of research and a variety of wide ranging applications in electrical devices. However, there is a lack of understanding of …

Impedance Spectroscopy Studies Of Yttria Stabilized Zirconia Under Extreme Conditions, Quinlan Blaine Smith

UNLV Theses, Dissertations, Professional Papers, and Capstones

Yttria Stabilized Zirconia (YSZ) is of interest for many industries. Varying amounts of Yttria (Y2O3) can be doped into Zirconia (ZrO2) to create materials with specific characteristics. For instance, 3mol% YSZ (3YSZ) is known to be a super hard material and is used as a coating on drill tips and as an abrasive. Eight mol% YSZ (8YSZ) is commonly used as a solid electrolyte in Solid Oxide Fuel Cells because of its good ionic conducting abilities and stability at high temperatures. In this thesis project, a novel experimental setup was created and used to study the ionic conductivity of (3 …

High Pressure Behavior Of Mullite-Type Oxides: Phase Transitions, Amorphization, Negative Linear Compressibility And Microstructural Implications, Patricia Kalita

UNLV Theses, Dissertations, Professional Papers, and Capstones

Even though mullite occurs rarely in nature, it is perhaps one of the most important phases in both traditional and advanced ceramics. Existing and emerging applications of mullite and mullite-type materials include: high-temperature composites, aerospace materials, ballistic shielding for military applications and even non-linear optical materials. There are many uncertainties regarding the basic physical properties of mullite-type materials, particularly in terms of their high-pressure structural stability and mechanical behavior that are important to address for emerging applications of mullites as engineering materials. This work is the first reported comprehensive investigation of the high –pressure structural behavior of several different mullites …

Synthesis And High-Pressure Structural Studies Of Aux2 (X= Al, Ga, In) Compounds, Jason Lee Baker

UNLV Theses, Dissertations, Professional Papers, and Capstones

Three intermetallic compounds, AuX₂ (X = Al, Ga, In), were synthesized by arc-melting proper stoichiometric ratios of high purity Au, Al, Ga, and In. They were found to be single-phase in the CaF₂ type Fm3m crystalline structure. Interest in these particular intermetallic materials is due to intriguing pressure-induced behavior such as electronic topological transitions (ETTs), and structural phase transitions as well as their use in a variety of practical applications such as a solar spectral absorber and in electronic circuitry. In this study, the high-pressure structure of these materials was studied using high resolution synchrotron x-ray diffraction (XRD) …

Spectroscopic Studies Of Melamine At High Pressure, Martin Donald Galley

UNLV Theses, Dissertations, Professional Papers, and Capstones

We have performed mid- and far- Infra Red (IR) absorption, Raman spectroscopy, and angular dispersive x-ray diffraction (XRD) studies on melamine under high pressure and room temperature. We have verified the presence of two prior reported phase transitions, the first between 1-2 GPa, and the second between 7-9 GPa. We have also found evidence of a third unreported phase transition between 14-16 GPa, during which, there was a sudden disappearance of all low energy peaks (^-1 ) in both the Raman and IR spectra. The far-IR peak movement experiences a discontinuity as the rate of peak movement suddenly changes. …

Raman Spectroscopic Study Of Solid Solution Spinel Oxides, Brian D. Hosterman

UNLV Theses, Dissertations, Professional Papers, and Capstones

Solid solution spinel oxides of composition MgxNi1−xCr2O4, NiFexCr2−xO4, and FexCr3−xO4 were synthesized and characterized using x-ray diffraction and Raman spectroscopy. Frequencies of the Raman-active modes are tracked as the metal cations within the spinel lattice are exchanged. This gives information about the dependence of the lattice vibrations on the tetrahedral and octahedral cations. The highest-frequency Raman-active mode, A1g, is unaffected by substitution of the divalent tetrahedral cation, whereas the lower frequency vibrations are more strongly affected by substitution of the tetrahedral cation. The change in wavenumber of many phonons is nonlinear upon cation exchange. All detected modes of MgxNi1−xCr2O4 and …

Graphene: Material That Will Change The Future, Jigar Desai, Darryl Reese

Festival of Communities: UG Symposium (Posters)

Graphene is the most recent material discovered by scientists and is a star on the horizon of materials science and condensed matter physics. The one atom thick, two dimensional materials is an amazing conductor of electricity. Although graphene was not discovered completely until 2004, it has already revealed potential applications and scientists have begun researching ways of developing graphene products for the market. Only two products have been successfully produced so far, but scientists have encountered amazing results. This material has many potential applications in the real world and is about to change the future in a positive way.

Measurement System For High Pressure Characterizations Of Materials, Matthew K. Jacobsen

UNLV Theses, Dissertations, Professional Papers, and Capstones

Thermoelectric materials have long been investigated for possible use as power sources. This application was recently put to use in the Voyager space program, powering the deep space probes. Despite the usefulness of these materials, the use of pressure to investigate the material properties has only recently become interesting. As such, the work in this document was to developing a system for concurrently measuring the necessary properties. This system is capable of measuring the electrical resistivity, thermal conductivity, and Seebeck coefficient in the pressure range from 0 - 10 GPa. The results for zinc, almandine garnet, and nickel are presented …

Chemical And Electronic Structure Of Surfaces And Interfaces In Compound Semiconductors, Sujitra Pookpanratana

UNLV Theses, Dissertations, Professional Papers, and Capstones

The interface formation between two different materials is important in applications for optoelectronic devices. Often, the success or performance of these devices is dependent on the formation of these heterojunctions. In this work, the surface and interfaces in such materials for optoelectronic devices are investigated by a suite of X-ray analytical techniques including X-ray photoelectron (XPS), X-ray excited Auger electron (XAES), and X-ray emission (XES) spectroscopies to provide novel insight.

For the group III-nitrides (e.g., AlxGa1-xN) used in many light emitting devices, a significant challenge exists to form an Ohmic contact. The electron affinities and band gaps of GaN and …

Partial Phonon Density Of States Of 57-Iron And 161-Dysprosium In Dyfe3 By Nuclear Resonant Inelastic X-Ray Scattering Under High Pressure, Elizabeth Anne Tanis

UNLV Theses, Dissertations, Professional Papers, and Capstones

The dual partial phonon density of states (DOS) from two different Mossbauer isotopes (161Dy and 57Fe) in the same material (DyFe3) was successfully measured using the nuclear resonant inelastic x-ray scattering (NRIXS) technique at high pressure. Nuclear inelastic scattering measurements yield an in-depth understanding of the element-specific dynamic properties. The Debye temperatures , the Lamb-Mossbauer factor, and the vibrational contributions to the Helmholtz free energy, specific heat , entropy and internal energy are calculated directly from the phonon density of states.

Photon Density Of States Of 47-Iron And 161-Dysprosium In Dyfe3 By Nuclear Resonant Inelastic X-Ray Scattering Under High Pressure, Elizabeth Anne Tanis

UNLV Theses, Dissertations, Professional Papers, and Capstones

The dual partial phonon density of states (DOS) from two different Mossbauer isotopes (161Dy and 57Fe) in the same material (DyFe3) was successfully measured using the nuclear resonant inelastic x-ray scattering (NRIXS) technique at high pressure. Nuclear inelastic scattering measurements yield an in-depth understanding of the element-specific dynamic properties. The Debye temperatures, the Lamb-Mossbauer factor (fLM), and the vibrational contributions to the Helmholtz free energy (Fvib), specific heat (cV ), entropy (Svib) and internal energy (Uvib) are calculated directly from the phonon density of states.

Finite Strain Studies Of Single Crystal Fe3p Under High Pressures, John William Howard

UNLV Theses, Dissertations, Professional Papers, and Capstones

Fe3P (synthetic schreibersite) is a phosphide occurring in iron alloys. Phosphorousis often considered an undesired impurity causing brittleness. Conversely, in some cases the addition of iron phosphides to certain materials is beneficial (e.g.properties of certain frictional materials are enhanced). In terrestrial rock, we do not find Fe3P, although (Fe;Ni)3P (natural schreibersite) is found in nearly all iron-containing meteorites. In this project, we examine the unit cell parameters of Fe3P as function of pressure and derive the respective axial and bulk compressibilities. Both Vinet and Birch-Murnaghan formulations were used to relate pressure and unit cell volume, and a comparison of each …

X-Ray Diffraction Study Of Dysprosium Single Crystal Samples In A Diamond Anvil Cell (Dac), Ognjen Grubor-Urosevic

UNLV Theses, Dissertations, Professional Papers, and Capstones

X-ray diffraction is the basis of crystallography, the study of the structure of crystals. It uses X-rays of a wavelength on the order of the size of atoms, so it can resolve the positions of individual atoms in a crystal. Illuminating the crystal with a well-collimated X-ray beam produces X-rays diffracted in a certain direction for a specific crystal orientation. By analyzing the relative phase of the incoming and outgoing scattered X-rays, the unique arrangements of atoms can be determined and the structure of the crystal can be solved.

There is a long standing controversy regarding the crystal structures and …

Ac Susceptibility And Heat Capacity Studies Of The Geometrically Frustrated Pyrochlores Terbium Titanium Tin Oxides And Holmium Titanates, Daniel Antonio

UNLV Theses, Dissertations, Professional Papers, and Capstones

Materials with a geometrically frustrated magnetic pyrochlore lattice have been of interest due to their unusual ground states. Tb 2 Ti 2 O 7 is known for having a spin liquid ground state which does not transition to a long-range ordered state down to at least 50 mK. Ho 2 Ti 2 O 7 has a macroscopically degenerate spin ice ground state which resembles that of the proton ordering in water ice. Heat Capacity measurements of Tb 2 Ti 2- x Sn x O 7 were done down to 0.36 K and AC magnetic susceptibility measurements of Ho 2- x …