| 1. |
- Akahama, Yuichi, et al.
(författare)
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Structural stability and equation of state of simple-hexagonal phosphorus to 280 GPa : Phase transition at 262 GPa
- 2000
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Ingår i: Physical Review B (Condensed Matter). - 0163-1829. ; 61:5, s. 3139-3142
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Tidskriftsartikel (refereegranskat)abstract
- A structural phase transition and the equation of state of the simple-hexagonal (sh) phase, P-V, of phosphorus were investigated at pressures up to 280 GPa using monochromatic synchrotron x-ray diffraction. The P-V phase, which was stable above 137 GPa, has shown a structural transition to a higher-pressure phase, P-VI, above 262 GPa, while the c/a ratio of the sh phase slightly increased with increasing pressure from 0.948 to 0.956. The structure of the P-VI phase has been proposed to be bcc. The bulk modulus (B0) and the atomic volume at atmospheric pressure (V0) of the sh phase were estimated to be 84.1 GPa and 14.2 Å3, with a fixed value of the pressure derivative of the bulk modulus (B′0).
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| 2. |
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| 3. |
- Carlson, Johan, et al.
(författare)
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Echo-cancellation in a single-transducer ultrasonic imaging system
- 2002
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Ingår i: NORSIG 2002. - Trondheim : The Institute of Electrical and Electronics Engineers. - 8299315840
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Konferensbidrag (refereegranskat)abstract
- During the last ten years, time-reversal of acoustic fields have been shown to be a very useful technique in ultrasonic imaging and testing. With the use of transducer arrays, it is possible to steer the sound beam to an arbitrary position within the medium, even if the medium is inhomogeneous or contains scatterers. The major drawback with traditional beamforming and time-reversal techniques is that they require the use of transducer arrays. Recently a new technique was presented that, with the use of a waveguide, makes it possible to focus sound arbitrarily, with only one transducer elements. A problem with the setup is that the signal-to-noise ratio (SNR) is degraded because of interfering echoes from the waveguide. In this paper, we present an echo-cancellation scheme that results in an SNR gain of approximately 33 dB. This enables the new technique to be used in pulse-echo mode, where this was not previously possible.
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| 4. |
- Carlson, Stefan, et al.
(författare)
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Beamline I811 - status report
- 2000
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Ingår i: MAX-Lab Activity Report. ; , s. 302-303
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Forskningsöversikt (refereegranskat)
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| 5. |
- Carlson, Stefan
(författare)
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High-pressure studies of the cubic to rhombohedral transformation in NbO2F
- 2000
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Ingår i: Journal of Applied Crystallography. - 0021-8898. ; 33:4, s. 1175-1176
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Tidskriftsartikel (refereegranskat)abstract
- The Pm3̄m to R3̄c high-pressure transition of NbO2F has been studied in detail using diamond anvil cells and synchrotron X-ray radiation. The transition starts at 0.28 GPa and is complete at 0.65 GPa. The bulk modulus for the cubic phase became 24.8 (11) GPa, which is roughly two times higher than for the rhombohedral phase.
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| 6. |
- Carlson, Stefan, et al.
(författare)
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High-pressure transformations of NbO2F
- 2000
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Ingår i: Acta Crystallographica. Section B: Structural Science. - 0108-7681. ; 56:2, s. 189-196
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Tidskriftsartikel (refereegranskat)abstract
- The ReO3-type structure NbO2F, niobium dioxyfluoride, has been studied at high pressures using diamond anvil cells and synchrotron X-ray radiation. High-pressure powder diffraction measurements have been performed up to 40.1 GPa. A phase transition from the cubic (Pm3m) ambient pressure structure to a rhombohedral (R3c) structure at 0.47 GPa has been observed. Rietveld refinements at 1.38, 1.96, 3.20, 6.23, 9.00 and 10.5 GPa showed that the transition involves an a-a-a- tilting of the cation-anion coordination octahedra and a change of the anion-anion arrangement to approach hexagonal close packing. Compression and distortion of the Nb(O/F)6 octahedra is also revealed by the Rietveld refinements. At 17-18 GPa, the diffraction pattern disappears and the structure becomes X-ray amorphous.
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| 7. |
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| 8. |
- Cerenius, Yngve, et al.
(författare)
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Crystallography at MAX-lab
- 2004
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Ingår i: Synchrotron Radiation in Natural Science. - 1644-7190. ; 3:1-2, s. 13-16
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Tidskriftsartikel (refereegranskat)
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| 9. |
- Dinnebier, RE, et al.
(författare)
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Bulk moduli and high-pressure crystal structures of minium, Pb3O4, determined by X-ray powder diffraction
- 2003
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Ingår i: American Mineralogist. - 0003-004X. ; 88:7, s. 996-1002
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Tidskriftsartikel (refereegranskat)abstract
- We report the pressure dependence of the crystal structure of lead tetroxide (P less than or equal to 41.05 GPa, T 298 K) using high-resolution angle-dispersive X-ray powder diffraction. Pb3O4 shows two reversible phase transitions in the measured pressure range. The crystal structures of the modifications identified have in common frameworks of Pb+4O6 octahedra and irregular Pb+2O4+1 respectively Pb2+O6+1 polyhedra. At ambient conditions, Pb3O4 crystallizes in space group P4(2)/mbc (phase 1). Between 0.11 and 0.3 GPa it exhibits a displacive second order phase transition to a structure with space group Pbam (phase II). A second displacive phase transition occurs between 5.54 and 6.6 GPa to another structure with space group Pbam (phase 111) but halved c dimension. A non-linear compression behavior over the entire pressure range is observed, which can be described by two Vinet relations in the ranges from 0.28 to 5.54 GPa and from 6.6 to 41.05 GPa. The extrapolated bulk moduli of the high-pressure phases were determined to be K-0 = 21(2) GPa for phase 11 and K-0 = 91(3) GPa for phase III. The crystal structures of all phases were refined from X-ray diffraction powder data collected at several pressures between 0.06 and 41.05 GPa. Except for their cell dimensions, phases I and 11 were found to be isostructural to the corresponding phases at low temperatures, whereas phase III can be derived from the Sr2PbO4 aristotype. With increasing pressure, the lone pair which is localized at Pb2+ adopts increasingly pure s-character, which is reflected by the similar coordination polyhedra of Ph2+ in Pb3O4 (phase 111) and of Sr2+ in Sr2PbO4.
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| 10. |
- Eriksson, Stefan, et al.
(författare)
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Lobe cell convection and field-aligned currents poleward of the region 1 current system
- 2002
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 107:A8
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Tidskriftsartikel (refereegranskat)abstract
- [1] We present a case and statistical study of plasma convection in the Northern Hemisphere during summer conditions using electric field, magnetic field, and particle data taken during dawn-dusk directed orbits of the FAST satellite. To our knowledge, this set provides the most comprehensive combination of data as yet presented in support of lobe cell convection from an ionospheric perspective this far from the noon sector. In particular, we study the current systems and convection patterns for all passes in July 1997 that show evidence for six large-scale field-aligned currents (FACs) rather than the usual system of four FACs associated with the region 1/region 2 current systems. A total of 71 passes out of 232 in the study had the extra pair of FACs. The extra pair of FACs in 30 of the 71 cases lies either on the dawnside or on the duskside of the noon-midnight meridian, and their position is strongly correlated with the polarity of the IMF By (negative and positive, respectively). This is consistent with the IMF dependence of a three-cell convection pattern of coexisting merging, viscous, and lobe-type convection cells. The occurrence of the asymmetric FAC pair was also strongly linked to conditions of IMF |B-y/B-z | > 1. The extra pair of FACs in these cases was clearly associated with the lobe cell of the three-cell convection system. The remaining 41 cases had the pair of FACs straddling the noon-midnight meridian. The extra pair of FACs was often (20 cases out of 30) observed at magnetic local times more than three hours away from noon, rather than being confined to regions near noon and the typical location of the cusp. Such a current system consisting of a pair of FACs poleward of the nearest region 1 current is consistent with the IMF B-y-dependent global MHD model developed by Ogino et al. [1986] for southward IMF conditions, as well as with other magnetospheric and ionospheric convection models that include the effects of merging occuring simultaneously at both low-latitude dayside and high-latitude lobe and flank magnetopause reconnection sites. Finally, the presence of the additional FACs and three-cell convection well away from noon show that the entire dayside ionosphere is affected by IMF-dependent processes, rather than only a limited region around noon.
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