Michael Pravica | University of Nevada, Las Vegas (original) (raw)

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Papers by Michael Pravica

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), Feb 2, 2021

$Research paper thumbnail of High-pressure x-ray diffraction and Raman spectroscopy of phase transitions in Sm2O3$

Journal of Applied Physics, 2013

The structural phase transitions in Sm2O3 with mixed phases of cubic and monoclinic as starting m... more The structural phase transitions in Sm2O3 with mixed phases of cubic and monoclinic as starting material were studied by in situ high-pressure synchrotron angle dispersive x-ray diffraction and Raman scattering measurements up to 40.1 GPa and 41.0 GPa, respectively. The x-ray diffraction data indicate that the monoclinic and cubic phases begin to transform to a hexagonal phase at 2.5 and 4.2 GPa, respectively. The hexagonal phase is stable up to at least 40.1 GPa and could not be quenched to ambient conditions. These phase transitions have also been confirmed by Raman spectroscopy. A third-order Birch-Murnaghan fit based on the observed pressure-volume data yields zero pressure bulk moduli B0 = 149(2), 153(7), and 155(5) GPa for cubic, monoclinic, and hexagonal phases, respectively, when their first pressure derivatives (B0′) were fixed as 4. The pressure coefficients of Raman peaks and the mode Grüneisen parameters of different Raman modes were also obtained. Coupled with previous ...

Laboratory-X-ray Raman scattering (XRS) is a sensitive bulk probe that yields information about t... more Laboratory-X-ray Raman scattering (XRS) is a sensitive bulk probe that yields information about the local structure of systems and compounds in the soft x-ray regime. The availability of high energy synchrotron radiation has allowed combining this method with different sample environmental conditions such as extreme pressures and temperatures. Here we present the C k-edge measurements under pressure to investigate the bonding changes of multi walled carbon nanotubes (MWCNT) up to 25 GPa. The results show that the MWCNT transform to a superhard phase similar to diamond around 15 GPa. Further compression results in a structural collapse leading to an amorphous state.

Advances in Magnetic Resonance, 1990

Physical Chemistry Chemical Physics

The critical roles of high pressure and the presence of water molecules in an initial solid-state... more The critical roles of high pressure and the presence of water molecules in an initial solid-state compound (Cs2C2H2O5) in the monochromatic X-ray induced synthesis of cesium superoxide (CsO2) are experimentally demonstrated.

APS March Meeting Abstracts, 2018

Bulletin of the American Physical Society, 2019

Chimica Oggi-chemistry Today, 2018

High Pressure Research, 2021

We report theoretical and experimental investigations on the structures of strontium and magnesiu... more We report theoretical and experimental investigations on the structures of strontium and magnesium oxalates, and corresponding Raman spectra at high pressure. These systems have shown progress in the generation of CO2 and in the synthesis of energetic doped polymeric carbon monoxide after X-ray irradiation and simultaneous application of high pressure. Density functional perturbation theory (DFT) was used to calculate the zone center optical phonons in monoclinic and triclinic strontium oxalate, and the ambient triclinic phase of magnesium oxalate. Vibration modes were also determined in terms of atomic displacements for both compounds. The simulations were compared to experimental Raman spectra in an effort to elucidate the details of the phase transition between monoclinic and triclinic phases. Additional phonon dispersion calculations of the compounds were performed to gain better insight into the dynamic phase stability in strontium and magnesium oxalates under high pressure.

Bulletin of the American Physical Society, 2018

Bulletin of the American Physical Society, 2017

Journal of Applied Physics, 2016

The structure stability under high pressure and thermal expansion behavior of Na3OBr and Na4OI2, ... more The structure stability under high pressure and thermal expansion behavior of Na3OBr and Na4OI2, two prototypes of alkali-metal-rich antiperovskites, were investigated by in situ synchrotron X-ray diffraction techniques under high pressure and low temperature. Both are soft materials with bulk modulus of 58.6 GPa and 52.0 GPa for Na3OBr and Na4OI2, respectively. The cubic Na3OBr structure and tetragonal Na4OI2 with intergrowth K2NiF4 structure are stable under high pressure up to 23 GPa. Although being a characteristic layered structure, Na4OI2 exhibits nearly isotropic compressibility. Negative thermal expansion was observed at low temperature range (20–80 K) in both transition-metal-free antiperovskites for the first time. The robust high pressure structure stability was examined and confirmed by first-principles calculations among various possible polymorphisms qualitatively. The results provide in-depth understanding of the negative thermal expansion and robust crystal structure...

Submitted for the SHOCK13 Meeting of The American Physical Society Studies in useful hard x-ray i... more Submitted for the SHOCK13 Meeting of The American Physical Society Studies in useful hard x-ray induced chemistry MICHAEL PRAV-ICA, LIGANG BAI, DANIEL SNEED, University of Nevada, Las Vegas, CHANGY-ONG PARK, HP-CAT, Carnegie Geophysical Laboratory-The observed rapid decomposition of potassium chlorate (via 2KClO 3 + hν → 2KCl +3O 2) via synchrotron hard x-ray irradiation (>10 keV) has enabled experiments that are developing novel and useful hard x-ray chemistry. We have observed a number of radiation-induced in situ decomposition reactions in various substances which release O 2 , H 2 , N 2 , NH 3 , and H 2 O in a diamond anvil cell (DAC) at ambient and high pressures. These novel acatalytic and isothermal reactions represent a highly controllable, penetrating, and focused method to initiate chemistry (including x-ray induced combustion) in sealed and/or isolated chambers which maintain matter under extreme conditions. During our studies, we have typically observed a slowing of decomposition with pressure including phase dependent decomposition of KClO 3. Energy dependent studies have observed an apparent resonance near 15 keV at which the decomposition rate is maximized. This may enable use of much lower flux and portable x-ray sources (e.g. x-ray tubes) in larger scale experiments. These developments support novel means to load DACs and control chemical reactions providing novel routes of synthesis of novel materials under extreme conditions.

Journal of Physics: Conference Series, Oct 1, 2017

This paper discusses our attempt to synthesize higher oxidation forms of cesium fluoride by press... more This paper discusses our attempt to synthesize higher oxidation forms of cesium fluoride by pressurizing cesium fluoride in a fluorine-rich environment created via the x-ray decomposition of potassium tetrafluoroborate. This was done in order to confirm recent theoretical predictions of higher oxidation forms of CsF n. We discuss the development of a technique to produce molecular fluorine in situ via useful hard x-ray photochemistry, and the attempt to utilize this technique to form higher oxidation states of cesium fluoride. In order to verify the formation of the novel stoichiometric species of CsF n. X-ray Absorption Near Edge Spectroscopy (XANES) centered on the cesium K-edge was performed to probe the oxidation state of cesium as well as the local molecular coordination around Cs.

A sample of magnesium oxalate, confined within a 100 μm diameter beryllium gasket, pressurized in... more A sample of magnesium oxalate, confined within a 100 μm diameter beryllium gasket, pressurized inside of a Diamond Anvil Cell to about 100,000 times atmospheric pressure. The picture was taken with an optical microscope looking through a diamond. The sample was irradiated with 10 keV x-rays produced at the Advanced Photon Source at the Argon National Lab in Chicago. The yellow area shows material that has been decomposed to a doped form of polymeric carbon monoxide by exposure to hard x-rays, while white area is unadulterated Magnesium Oxalate.https://digitalscholarship.unlv.edu/image_of_research/1021/thumbnail.jp

Physical chemistry chemical physics : PCCP, 2021

We examined the high pressure behavior of stannous oxalate via Raman and X-ray absorption spectro... more We examined the high pressure behavior of stannous oxalate via Raman and X-ray absorption spectroscopy (XAS) inside a diamond anvil cell. Phase transitions were observed to occur near 2.6 and 15 GPa which were reversible upon decompression to ambient conditions. When further pressurized above 15 GPa, the colorless material sustains irreversible chemical alterations and becomes bright red colored - darkening at higher pressures. Another irreversible phase transition occurred above 20 GPa. Concomitant with color change of the sample, we observed a softening of the ν(C-C) modes of the C2O42- anion via Raman spectroscopy. We performed a separate XAS experiment which indicates that the Sn2+ cation undergoes a partial reduction of the 2+ oxidation state with pressure which persists when the sample was depressurized to ambient conditions. Thus, electron density within the C-C bond in the oxalate anion appears to migrate toward the tin cation with pressure. This observation suggests that pr...

Physical chemistry chemical physics : PCCP, Jan 18, 2018

X-ray induced damage has been known for decades and has largely been viewed as a tremendous nuisa... more X-ray induced damage has been known for decades and has largely been viewed as a tremendous nuisance. We, on the other hand, harness the highly ionizing and penetrating properties of hard X-rays to initiate novel decomposition and synthetic chemistry. Here, we show that powdered cesium oxalate monohydrate pressurized to ≤0.5 GPa and irradiated with X-rays of energies near the cesium K-edge undergoes molecular and structural transformations with one of the final products exhibiting a new type of bcc crystal structure that has previously not been observed. Additionally, based on cascades of ultrafast electronic relaxation steps triggered by the absorption of one X-ray photon, we propose a model explaining the X-ray induced damage of multitype bounded matter. As X-rays are ubiquitous, these results show promise in the preparation of novel compounds and novel structures that are inaccessible via conventional methods. They may offer insight into the formation of complex organic compounds...

Metaphilosophy, 2017

Its not uncommon for scientists to give different explanations of the same phenomenon, but we cur... more Its not uncommon for scientists to give different explanations of the same phenomenon, but we currently lack clear guidelines for deciding whether to treat such accounts as competitors. This article discusses how science studies can help create tools and guidelines for thinking about whether explanations compete. It also specifies how one family of discourse rules enables there to be differing accounts that appear to compete but dont. One hopes that being more aware of the linguistic mechanisms making compatible accounts appear to compete will prevent people from wasting resources trying to show which account is right.