Optical and near-infrared observations of the GRB 970616 error box

We report on near-infrared and optical observations of the GRB 970616 error box and of the X-ray sources discovered by ASCA and ROSAT in the region. No optical transient was found either within the IPN band or in the X-ray error boxes, similarly to other bursts, and we suggest that either considerable intrinsic absorption was present (like GRB 970828) or that the optical transient displayed a very fast decline (like GRB 980326 and GRB 980519).Comment: 2 pages with one encapsulated PostScript figure included. Uses Astronomy & Astrophysics LaTeX macros. Accepted for publication in Astronomy & Astrophysics Supplement Serie

Effect of pressure on La-2(WO4)(3) with a modulated scheelite-type structure

We have studied the effect of pressure on the structural and vibrational properties of lanthanum tritungstate La2(WO4)3. This compound crystallizes under ambient conditions in the modulated scheelite-type structure known as the α phase. We have performed x-ray diffraction and Raman scattering measurements up to a pressure of 20 GPa, as well as ab initio calculations within the framework of the density functional theory. Up to 5 GPa, the three methods provide a similar picture of the evolution under pressure of α-La2(WO4)3. At 5 GPa, we begin to observe some structural changes, and above 6 GPa we find that the x-ray patterns cannot be indexed as a single phase. However, we find that a mixture of two phases with C2/c symmetry accounts for all diffraction peaks. Our ab initio study confirms the existence of several C2/c structures, which are very close in energy in this compression range. According to our measurements, a state with medium-range order appears at pressures above 9 and 11 GPa, from x-ray diffraction and Raman experiments, respectively. Based upon our theoretical calculations we propose several high-pressure candidates with high cationic coordinations at these pressures. The compound evolves into a partially amorphous phase at pressures above 20 GPa.We acknowledge the financial support of the Spanish Ministerio de Economia y Competitividad under Grants MAT2010-21270-C04-02/03/04, CTQ2009-14596-C02-01, CSD2007-00045 and the Comunidad de Madrid and European Social Fund S2009/PPQ-1551-4161893. Access to the MALTA Cluster Computer (Universidad de Oviedo), the Atlante Super-computer (Instituto Tecnologico de Canarias, Red Espanola de Supercomputacion), and the MALTA Xcalibur Diffractometer (Universidad Complutense de Madrid) is gratefully acknowledged. C. G. A. wishes to thank the Agencia Canaria de Investigacion, Innovacion y Sociedad de la Informacion, and the European Social Fund of the Gobierno de Canarias for a fellowship. J.A.S. acknowledges financial support through the Juan de la Cierva fellowship program.Sabalisck, N.; Lopez Solano, J.; Guzmán-Afonso, C.; Santamaría Pérez, D.; González-Silgo, C.; Mújica, A.; Muñoz, A.... (2014). Effect of pressure on La-2(WO4)(3) with a modulated scheelite-type structure. Physical Review B (Condensed Matter). 89:1741121-17411211. https://doi.org/10.1103/PhysRevB.89.174112S17411211741121189Maczka, M., Souza Filho, A. G., Paraguassu, W., Freire, P. T. C., Mendes Filho, J., & Hanuza, J. (2012). Pressure-induced structural phase transitions and amorphization in selected molybdates and tungstates. Progress in Materials Science, 57(7), 1335-1381. doi:10.1016/j.pmatsci.2012.01.001Boulahya, K., Parras, M., & González-Calbet, J. M. (2005). A Structural Study of the Solid Solution Eu2(Mo1-xWx)3O12. Zeitschrift für anorganische und allgemeine Chemie, 631(11), 1988-1990. doi:10.1002/zaac.200570039Jeitschko, W. (1973). Crystal structure of La2(MoO4)3, a new ordered defect Scheelite type. Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, 29(10), 2074-2081. doi:10.1107/s0567740873006138Jeitschko, W. (1972). A comprehensive X-ray study of the ferroelectric–ferroelastic and paraelectric–paraelastic phases of Gd2(MoO4)3. Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, 28(1), 60-76. doi:10.1107/s0567740872001876Evans, J. S. O., Mary, T. A., & Sleight, A. W. (1998). Negative Thermal Expansion in Sc2(WO4)3. Journal of Solid State Chemistry, 137(1), 148-160. doi:10.1006/jssc.1998.7744Guzmán-Afonso, C., González-Silgo, C., González-Platas, J., Torres, M. E., Lozano-Gorrín, A. D., Sabalisck, N., … Rodríguez-Carvajal, J. (2011). Structural investigation of the negative thermal expansion in yttrium and rare earth molybdates. Journal of Physics: Condensed Matter, 23(32), 325402. doi:10.1088/0953-8984/23/32/325402Jayaraman, A., Sharma, S. K., Wang, Z., & Wang, S. Y. (1997). Pressure-induced amorphization in the α-phase of Nd2(MoO4)3 and Tb2(MoO4)3. Solid State Communications, 101(4), 237-241. doi:10.1016/s0038-1098(96)00587-xLucazeau, G., Le Bacq, O., Pasturel, A., Bouvier, P., & Pagnier, T. (2011). High-pressure polarized Raman spectra of Gd2(MoO4)3: phase transitions and amorphization. Journal of Raman Spectroscopy, 42(3), 452-460. doi:10.1002/jrs.2731Le Bacq, O., Machon, D., Testemale, D., & Pasturel, A. (2011). Pressure-induced amorphization mechanism in Eu2(MoO4)3. Physical Review B, 83(21). doi:10.1103/physrevb.83.214101Machon, D., Dmitriev, V. P., Sinitsyn, V. V., & Lucazeau, G. (2004). Eu2(MoO4)3single crystal at high pressure: Structural phase transitions and amorphization probed by fluorescence spectroscopy. Physical Review B, 70(9). doi:10.1103/physrevb.70.094117Bandiello, E., Errandonea, D., Martinez-Garcia, D., Santamaria-Perez, D., & Manjón, F. J. (2012). Effects of high-pressure on the structural, vibrational, and electronic properties of monazite-type PbCrO4. Physical Review B, 85(2). doi:10.1103/physrevb.85.024108Kraus, W., & Nolze, G. (1996). POWDER CELL – a program for the representation and manipulation of crystal structures and calculation of the resulting X-ray powder patterns. Journal of Applied Crystallography, 29(3), 301-303. doi:10.1107/s0021889895014920Rodríguez-Carvajal, J. (1993). Recent advances in magnetic structure determination by neutron powder diffraction. Physica B: Condensed Matter, 192(1-2), 55-69. doi:10.1016/0921-4526(93)90108-iKlotz, S., Chervin, J.-C., Munsch, P., & Le Marchand, G. (2009). Hydrostatic limits of 11 pressure transmitting media. Journal of Physics D: Applied Physics, 42(7), 075413. doi:10.1088/0022-3727/42/7/075413Kresse, G., & Hafner, J. (1993). Ab initiomolecular dynamics for liquid metals. Physical Review B, 47(1), 558-561. doi:10.1103/physrevb.47.558Kresse, G., & Furthmüller, J. (1996). Efficient iterative schemes forab initiototal-energy calculations using a plane-wave basis set. Physical Review B, 54(16), 11169-11186. doi:10.1103/physrevb.54.11169Kresse, G., & Joubert, D. (1999). From ultrasoft pseudopotentials to the projector augmented-wave method. Physical Review B, 59(3), 1758-1775. doi:10.1103/physrevb.59.1758Blöchl, P. E. (1994). Projector augmented-wave method. Physical Review B, 50(24), 17953-17979. doi:10.1103/physrevb.50.17953Perdew, J. P., Ruzsinszky, A., Csonka, G. I., Vydrov, O. A., Scuseria, G. E., Constantin, L. A., … Burke, K. (2008). Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces. Physical Review Letters, 100(13). doi:10.1103/physrevlett.100.136406Monkhorst, H. J., & Pack, J. D. (1976). Special points for Brillouin-zone integrations. Physical Review B, 13(12), 5188-5192. doi:10.1103/physrevb.13.5188Pickard, C. J., & Needs, R. J. (2011). Ab initiorandom structure searching. Journal of Physics: Condensed Matter, 23(5), 053201. doi:10.1088/0953-8984/23/5/053201Depero, L. E., & Sangaletti, L. (1997). Cation Sublattice and Coordination Polyhedra inABO4Type of Structures. Journal of Solid State Chemistry, 129(1), 82-91. doi:10.1006/jssc.1996.7234Brown, I. D. (2006). The Chemical Bond in Inorganic Chemistry. doi:10.1093/acprof:oso/9780199298815.001.0001Kresse, G., Furthmüller, J., & Hafner, J. (1995). Ab initioForce Constant Approach to Phonon Dispersion Relations of Diamond and Graphite. Europhysics Letters (EPL), 32(9), 729-734. doi:10.1209/0295-5075/32/9/005Alfè, D. (2009). PHON: A program to calculate phonons using the small displacement method. Computer Physics Communications, 180(12), 2622-2633. doi:10.1016/j.cpc.2009.03.010Sabalisck, N., Mestres, L., Vendrell, X., Cerdeiras, E., Santamaría, D., Lavin, V., … Guzman-Afonso, M. C. (2011). Amorphization in rare earth tungstates with modulated scheelite-type structure under pressure. Acta Crystallographica Section A Foundations of Crystallography, 67(a1), C504-C505. doi:10.1107/s0108767311087228Logvinovich, D., Arakcheeva, A., Pattison, P., Eliseeva, S., Tomeš, P., Marozau, I., & Chapuis, G. (2010). Crystal Structure and Optical and Magnetic Properties of Pr2(MoO4)3. Inorganic Chemistry, 49(4), 1587-1594. doi:10.1021/ic9019876Garg, N., Murli, C., Tyagi, A. K., & Sharma, S. M. (2005). Phase transitions inSc2(WO4)3under high pressure. Physical Review B, 72(6). doi:10.1103/physrevb.72.064106Belsky, A., Hellenbrandt, M., Karen, V. L., & Luksch, P. (2002). New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design. Acta Crystallographica Section B Structural Science, 58(3), 364-369. doi:10.1107/s0108768102006948Shannon, R. D. (1976). Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallographica Section A, 32(5), 751-767. doi:10.1107/s0567739476001551Errandonea, D., & Manjón, F. J. (2008). Pressure effects on the structural and electronic properties of ABX4 scintillating crystals. Progress in Materials Science, 53(4), 711-773. doi:10.1016/j.pmatsci.2008.02.001Kroumova, E., Aroyo, M. I., Perez-Mato, J. M., Kirov, A., Capillas, C., Ivantchev, S., & Wondratschek, H. (2003). Bilbao Crystallographic Server : Useful Databases and Tools for Phase-Transition Studies. Phase Transitions, 76(1-2), 155-170. doi:10.1080/0141159031000076110Manjón, F. J., Errandonea, D., Garro, N., Pellicer-Porres, J., Rodríguez-Hernández, P., Radescu, S., … Muñoz, A. (2006). Lattice dynamics study of scheelite tungstates under high pressure I.BaWO4. Physical Review B, 74(14). doi:10.1103/physrevb.74.144111Manjon, F. J., Errandonea, D., Garro, N., Pellicer-Porres, J., López-Solano, J., Rodríguez-Hernández, P., … Muñoz, A. (2006). Lattice dynamics study of scheelite tungstates under high pressure II.PbWO4. Physical Review B, 74(14). doi:10.1103/physrevb.74.144112Grzechnik, A., Ursaki, V. V., Syassen, K., Loa, I., Tiginyanu, I. M., & Hanfland, M. (2001). Pressure-Induced Phase Transitions in Cadmium Thiogallate CdGa2Se4. Journal of Solid State Chemistry, 160(1), 205-211. doi:10.1006/jssc.2001.9224Gomis, O., Vilaplana, R., Manjón, F. J., Pérez-González, E., López-Solano, J., Rodríguez-Hernández, P., … Ursaki, V. V. (2012). High-pressure optical and vibrational properties of CdGa2Se4: Order-disorder processes in adamantine compounds. Journal of Applied Physics, 111(1), 013518. doi:10.1063/1.3675162Gomis, O., Vilaplana, R., Manjón, F. J., Santamaría-Pérez, D., Errandonea, D., Pérez-González, E., … Ursaki, V. V. (2013). Crystal structure of HgGa2Se4 under compression. Materials Research Bulletin, 48(6), 2128-2133. doi:10.1016/j.materresbull.2013.02.037Errandonea, D., Pellicer-Porres, J., Manjón, F. J., Segura, A., Ferrer-Roca, C., Kumar, R. S., … Aquilanti, G. (2006). Determination of the high-pressure crystal structure ofBaWO4andPbWO4. Physical Review B, 73(22). doi:10.1103/physrevb.73.224103López-Solano, J., Rodríguez-Hernández, P., Radescu, S., Mujica, A., Muñoz, A., Errandonea, D., … Aquilanti, G. (2007). Crystal stability and pressure-induced phase transitions in scheelite AWO4 (A = Ca, Sr, Ba, Pb, Eu) binary oxides. I: A review of recentab initio calculations, ADXRD, XANES, and Raman studies. physica status solidi (b), 244(1), 325-330. doi:10.1002/pssb.200672559Manjón, F. J., Errandonea, D., López-Solano, J., Rodríguez-Hernández, P., Radescu, S., Mujica, A., … Aquilanti, G. (2007). Crystal stability and pressure-induced phase transitions in scheelite AWO4 (A = Ca, Sr, Ba, Pb, Eu) binary oxides. II: Towards a systematic understanding. physica status solidi (b), 244(1), 295-302. doi:10.1002/pssb.200672588López-Solano, J., Rodríguez-Hernández, P., Muñoz, A., & Manjón, F. J. (2006). Theoretical study of theYLiF4phase transitions under pressure. Physical Review B, 73(9). doi:10.1103/physrevb.73.09411

Structure, Velocity Field and Turbulence in NGC 604

The Ha peak intensity, velocity shift and velocity dispersion maps of the giant HII region NGC 604 in M 33, obtained by two dimensional high spatial resolution Fabry-Perot observations with TAURUS II at the 4.2 m William Herschel Telescope in Spain (Sabalisck, 1995), are analyzed via two point correlation functions. The whole system seems to rotate as a rigid body on scales from 50 to 80 pc (the largest studied scale), with a period of $\sim$ 85 Myr. We demonstrate that the cloud seems to be comprised of eddies with varying characteristic scale lengths which range from 10 pc to the largest observed scales. The calculated kinetic energy spectrum may be interpreted as either a manifestation of a double cascading spectrum of forced two-dimensional turbulence, or as a Kolmogorov three-dimensional turbulence (although this last possibility seems unlikely). According to the first interpretation, turbulence is being forced at scales of $\sim$ 10 pc, while an enstrophy (mean-square vorticity) cascade has developed down to the smallest scales resolved and an inverse kinetic energy cascade extends up to scales of $\sim$ 70 pc where a low wave number turn over is observed; if true, this would be the first time that such a phenomenon has been observed outside the Solar System. As for the second interpretation, energy should be injected at the largest scales, $\sim$ 70 pc. In both cases the average intrinsic optical depth consistent with the results is $\sim$ 20 pc.Comment: 27 pages, postscript version including 8 embedded figure

Internal turbulence, virality, and density bounding of the most luminous H II regions in the spiral galaxy M 100

Original article can be found at: http://www.aanda.org/ Copyright The European Southern ObservatoryWe present TAURUS Fabry-Perot velocity data in Ha emission of the disc of the grand design spiral M 100 (NGC 4321). We have studied the emission spectra of the 200 Hii regions most luminous in H , calibrated in luminosity using photometric H imaging from the literature. The emission spectra of individual Hii regions were fitted using one or more Gaussian functions, and analyzed in terms of their velocity dispersion. We concentrate on the question of whether the emission lines show values of their internal velocity dispersions of Hii regions which would be predicted from the virial theorem, and find that in general this is not the case. There is a systematic trend to super-virial line widths, characteristic of the non-equilibrium effects of powerful OB stellar winds, and supernovae. We propose that the lower envelope in velocity dispersion , in the plot of H luminosity v. represents the virialized regions, and give a tentative theoretical explanation for its slope of 2.6 in the log–log plane, in terms of density bounding for the regions of highest luminosity.Peer reviewe

Structures and thermal stability of the [alpha]-LiNH4SO4 polytypes doped with Er3+ and Yb3+

In order to clarify the polymorphism in the lithium sulfate family, LiREx(NH4)1 - xSO4 (0.5 [less-than or equal to] x [less-than or equal to] 4.0 mol%, nominal value; RE = Er3+, Yb3+ and Dy3+) crystals were grown from aqueous solution by slow evaporation between 298 and 313 K. The doping of the samples allowed us to obtain two polymorphic forms, [alpha] and [beta], of LiNH4SO4 (LAS). By means of X-ray diffraction (XRD) in single crystals, we determined the crystal structures of two new [alpha]-polytypes, which we have named [alpha]1- and [alpha]2-LAS. They present the same space group P21/c and the following relation among their lattice parameters: a2 = -c1, b2 = -b1, c2 = -2a1 - c1. In order to evaluate the stability of the new [alpha]-polytypes, we performed thermal analysis, X-ray diffraction and dielectric spectroscopy on single crystals and polycrystalline samples over the cyclic temperature range: 190 [rightwards arrow] 575 [rightwards arrow] 190 K. The results obtained by all the techniques used in this study demonstrate that [alpha]-polytypes are stable across a wide range of temperatures and they show an irreversible phase transition to the paraelectric [beta]-phase above 500 K. In addition, a comparative study of [alpha]- and [beta]-polytypes shows that both polymorphic structures have a common axis, with a possible intergrowth that facilitates their coexistence and promotes the reconstructive [alpha] [rightwards arrow] [beta] transition. This intergrowth was related to small anomalies detected between 240 and 260 K, in crystals with an [alpha]-habit

Structures and thermal stability of the [alpha]-LiNH4SO4 polytypes doped with Er3+ and Yb3+

In order to clarify the polymorphism in the lithium sulfate family, LiREx(NH4)1 - xSO4 (0.5 [less-than or equal to] x [less-than or equal to] 4.0 mol%, nominal value; RE = Er3+, Yb3+ and Dy3+) crystals were grown from aqueous solution by slow evaporation between 298 and 313 K. The doping of the samples allowed us to obtain two polymorphic forms, [alpha] and [beta], of LiNH4SO4 (LAS). By means of X-ray diffraction (XRD) in single crystals, we determined the crystal structures of two new [alpha]-polytypes, which we have named [alpha]1- and [alpha]2-LAS. They present the same space group P21/c and the following relation among their lattice parameters: a2 = -c1, b2 = -b1, c2 = -2a1 - c1. In order to evaluate the stability of the new [alpha]-polytypes, we performed thermal analysis, X-ray diffraction and dielectric spectroscopy on single crystals and polycrystalline samples over the cyclic temperature range: 190 [rightwards arrow] 575 [rightwards arrow] 190 K. The results obtained by all the techniques used in this study demonstrate that [alpha]-polytypes are stable across a wide range of temperatures and they show an irreversible phase transition to the paraelectric [beta]-phase above 500 K. In addition, a comparative study of [alpha]- and [beta]-polytypes shows that both polymorphic structures have a common axis, with a possible intergrowth that facilitates their coexistence and promotes the reconstructive [alpha] [rightwards arrow] [beta] transition. This intergrowth was related to small anomalies detected between 240 and 260 K, in crystals with an [alpha]-habit