| 1. |
- Iota, Valentin, et al.
(author)
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Six-fold coordinated carbon dioxide VI
- 2007
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In: Nature Materials. - : Springer Science and Business Media LLC. - 1476-1122 .- 1476-4660. ; 6, s. 34-38
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Journal article (peer-reviewed)abstract
- Under standard conditions, carbon dioxide (CO2) is a simple molecular gas and an important atmospheric constituent, whereas silicon dioxide (SiO2) is a covalent solid, and one of the fundamental minerals of the planet. The remarkable dissimilarity between these two group IV oxides is diminished at higher pressures and temperatures as CO2 transforms to a series of solid phases, from simple molecular to a fully covalent extended-solid V, structurally analogous to SiO2 tridymite. Here, we present the discovery of an extended-solid phase of CO2: a six-fold coordinated stishovite-like phase VI, obtained by isothermal compression of associated CO2-II (refs 1,2) above 50 GPa at 530–650 K. Together with the previously reported CO2-V (refs 3–5) and a-carbonia6, this extended phase indicates a fundamental similarity between CO2 (a prototypical molecular solid) and SiO2 (one of Earth's fundamental building blocks). We present a phase diagram with a limited stability domain for molecular CO2-I, and suggest that the conversion to extended-network solids above 40–50 GPa occurs via intermediate phases II (refs 1,2), III (refs 7,8) and IV (refs 9,10). The crystal structure of phase VI suggests strong disorder along the c axis in stishovite-like P42/mnm, with carbon atoms manifesting an average six-fold coordination within the framework of sp3 hybridization.
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| 2. |
- Jacobsohn, L. G., et al.
(author)
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Positron annihilation spectroscopy of sputtered boron carbide films
- 2005
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In: Diamond and related materials. - : Elsevier BV. - 0925-9635 .- 1879-0062. ; 14:2, s. 201-205
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Journal article (peer-reviewed)abstract
- Positron annihilation spectroscopy (PAS) was carried out on boron carbide films deposited by sputtering and the results correlated to the bombardment conditions during film growth. Films were deposited with substrate bias voltages in the range of 0 to −200 V with a working pressure of 5 mTorr of Ar. Films deposited with bias voltages from −100 to −200 V present the same type of defect and the defect concentration increased linearly with the bias voltage. This defect was ascribed to vacancies in agreement with Monte Carlo simulations of Ar+ bombardment of boron carbide. On the other hand, films deposited with 0 V bias presented a higher S parameter values, whose origin was tentatively attributed to a relatively more open nanosized columnar structure, as suggested by the structure zone model. Annealing up to 800 °C for 30 min did not change the defect content.
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| 3. |
- Jacobsohn, L. G, et al.
(author)
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Role of intericosahedral chains on hardness of sputtered boron carbide films
- 2004
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:21, s. 4173-
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Journal article (peer-reviewed)abstract
- The relationship between the structure and mechanical properties of sputter-deposited boron carbide films was investigated. Changes in the structure induced by annealing were characterized in terms of chemical composition, chemical bonding, and concentrations of defects and trapped impurities. The creation of intericosahedral chains for higher annealing temperatures was revealed by infrared and Raman measurements, and the intensity of the infrared band at 1500 cm–1 was found to be related to the hardness. The presence of residual trapped Ar atoms and of open-volume defects is insensitive to relatively high annealing temperatures and does not influence the recovery of the hardness. Our results suggest postdeposition annealing as a pathway to enhance the mechanical properties of boron carbide films.
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| 4. |
- Jenei, Zsolt, 1978-
(author)
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Experimental investigation of molecular solids and vanadium at high pressure and temperature
- 2009
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Doctoral thesis (other academic/artistic)abstract
- Understanding high pressure effects on simple molecular system is of great interest for condensed matter science and geophysics. Accessing the static pressure and temperature regions found in planetary interiors is made possible by the development of the Diamond Anvil Cell technique. We developed a double sided resistive heating method for the membrane DAC operating in low pressure (<0.5 mTorr) pressure environment requiring only 175 W input power to reach sample temperatures up to 1300 K. We applied this technique successfully to study molecular solids at high temperatures, such as H2, N2 and CO2. We made an attempt to determine the melting line of hydrogen and present data up to 26 GPa in agreement with literature. Raman spectroscopy of Nitrogen indicates a high stability of the ε molecular phase, while θ phase is only accessible via certain P, T paths. Studies of solid CO2 at high pressure and temperature lead to the discovery of a six-fold coordinated stishovite-like phase VI, obtained by isothermal compression of associated CO2-II above 50 GPa at 530-650 K, or by isobaric heating of CO2-III above 55 GPa. From our X-ray diffraction experiment on isothermally compressed H2O we report a coexistence of ice VII and symmetric ice X from the start of the transition pressure 40GPa to just below 100 GPa and a volume change of 4% across the transition. Vanadium, a transition metal undergoes a phase transition upon compression unlike other elements (Nb, Ta) from its group. We confirm the bcc phase transition to rhombohedral structure at 62 GPa under quasi hydrostatic compression in Ne pressure medium. Compression without pressure medium results in a much lower 30 GPa transition pressure at room temperature and 37 GPa at 425 K, pointing to a positive phase line between the bcc and rhombohedral crystalline phases.
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| 5. |
- Jenei, Zsolt, 1978-, et al.
(author)
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High pressure/temperature structural phase transition in vanadium: Influence of non-hydrostatic conditions
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Other publication (other academic/artistic)abstract
- Vanadium undergoes a phase transition upon compression from bcc to rhombohedral structure at 62 GPa. In this paper we confirm the bcc to rhombohedral phase transition at 61.5 GPa under quasi hydrostatic compression in Ne pressure medium. Under non-hydrostatic condition we find the phase transition occurring at 30 GPa at ambient temperature, and 37 GPa at 425 K. We find the hydrostatic transition is hindered and it can occur at much lower pressure under non-hydrostatic condition.
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| 6. |
- Jenei, Zsolt, 1978-, et al.
(author)
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High temperature experiments using a resistively heated membrane driven diamond anvil cell
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Other publication (other academic/artistic)abstract
- A reliable high-performance heating method using resistive heaters and a membrane driven diamond anvil cell (mDAC) is presented. Two micro-heaters are mounted in a mDAC and use electrical power of less than 150 W to achieve sample temperatures up to 1200 K. For temperature measurement we use two K-type thermocouples mounted near the sample. The approach can be used for in-situ Raman spectroscopy and x-ray diffraction at high pressures and temperatures. A W-Re alloy gasket material permits stable operation of mDAC at high temperature. Using this method, we made an isothermal compression at 900 K to pressures in excess of 100 GPa and isobaric heating at 95 GPa to temperatures in excess of 1000 K. As an example, we present high temperature Raman spectroscopy measurements of nitrogen at high pressures
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| 7. |
- Jenei, Zsolt, 1978-, et al.
(author)
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Phase diagram of nitrogen: in-situ Raman spectroscopy investigation
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Other publication (other academic/artistic)abstract
- In this paper we confirm the existence of previously reported θ molecular phase of nitrogen and present in-situ Raman spectra of different molecular phases of nitrogen at temperatures over 900 K at 100 GPa pressure. Our investigation indicate very high transition barrier between ε and θ phases, θ being accessible only from previously quenched samples. Our experimental results indicate a “steeper" slope of δ/ε phase boundary than the extrapolation of previously determined phase line.
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| 8. |
- Simpson, P. J., et al.
(author)
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Observation of fluorine-vacancy complexes in silicon
- 2004
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 85:9, s. 1538-
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Journal article (peer-reviewed)abstract
- We show direct evidence, obtained by positron annihilation spectroscopy, for the complexing of fluorine with vacancies in silicon. Both float zone and Czochralski silicon wafers were implanted with 30 keV fluorine ions to a fluence of 2×1014 ions/cm2, and studied in the as-implanted condition, and after annealing to 650 °C for 10 and for 30 min. The "2-detector" background reduction technique for positron annihilation was applied. The spectra reveal a significant concentration of fluorine-vacancy complexes after annealing, for both Czochralski and float zone material, supporting the results of computer simulations of the implantation and annealing process.
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