| 1. |
- Carini, Giovanni, et al.
(författare)
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Hypersonic attenuation in cesium borate glasses: Relaxation and anharmonicity
- 2009
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 521-22, s. 247-250
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Konferensbidrag (refereegranskat)abstract
- The internal friction and the sound velocity in a cesium borate glass (Cs(2)O)(0.14)(B(2)O(3))(0.86) were measured at ultrasonic and hypersonic frequencies by Brillouin scattering measurements over the temperature range between 15 and 300 K. The analysis of the experimental results show that, in addition to thermally activated local motions of structural defects, the vibrational anharmonicity contributes substantially to the temperatures dependences of both the hypersonic loss and velocity. It has been also proved that above 150 K the vibrational anharmonicity appears to be governed by the Akhiezer mechanism of phonon viscosity.
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| 2. |
- Ahmed, Istaq, 1972, et al.
(författare)
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Crystal structure and proton conductivity of BaZr0.9Sc0
- 2008
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Ingår i: Journal of the American Ceramic Society. - : Wiley. - 0002-7820 .- 1551-2916. ; 91:9, s. 3039-3044
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Tidskriftsartikel (refereegranskat)abstract
- Solid-state sintering has been used to prepare the perovskite BaZr0.9Sc0.1O3-delta. Analysis of X-ray powder diffraction data shows that an increase of the unit cell parameter, a, was observed after deuteration. Rietveld analysis of room-temperature neutron powder diffraction data confirmed cubic symmetry (space group Pm-3m). Dynamic thermogravimetric analysis indicates that the hydration process occurs below 335 degrees C and approximately 58% of the theoretical number of protonic defects can be filled. The presence of protons/deutrons is seen from the strong O-H/O-D stretch band in the infrared spectrum of the hydrated/deuterated samples. The proton conductivity of a prehydrated sample was investigated under dry and wet Ar atmospheres
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| 3. |
- Ahmed, Istaq, 1972, et al.
(författare)
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Location of deuteron sites in the proton conducting perovskite BaZr0.50In0.50O3-y
- 2008
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388. ; 450:1-2, s. 103-110
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution neutron powder diffraction data have been collected on deuterated and dried samples of the perovskite BaZr0.5In0.5O2.75 at 5 K and room temperature, respectively. Inspection of Fourier nuclear density maps for the deuterated phase have allowed the deuteron position to be refined on a 12h (1/2, y, 0) crystallographic site, with y = 0.217(4) yielding a chemically reasonable O-D distance of 0.92(2) angstrom. Evidence for anisotropy of the deuteron position is also found consistent with a 24k crystallographic site (0.56, 0.21, 0) indicative of displacements of the ion towards neighbouring oxygen ions. The presence of static oxygen disorder in both the dried and deuterated samples is apparent from the structural analyses. Raman spectra confirm short range deviations from cubic symmetry for both dried and hydrated samples.
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| 4. |
- Granath, Mats, 1972, et al.
(författare)
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Anharmonic softening of Raman active phonons in iron-pnictides: Estimating the Fe isotope effect due to anharmonic expansion
- 2009
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Ingår i: Physical Review B. - 1098-0121. ; 79:23, s. 235103-
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Tidskriftsartikel (refereegranskat)abstract
- We present Raman measurements on the iron-pnictide superconductors CeFeAsO1-xFx and NdFeAsO1-xFx. Modeling the Fe-As plane in terms of harmonic and a cubic anharmonic Fe-As interactions, we calculate the temperature dependence of the energy and lifetime of the Raman active Fe B-1g mode and fit to the observed energy shift. The shifts and lifetimes are in good agreement with those measured also in other Raman studies which demonstrate that the phonon spectrum, at least at small wave numbers, is well represented by phonon-phonon interactions without any significant electronic contribution. Even at zero temperature there is a non-negligent effect of interactions on the phonon energy, which for the Fe B-1g mode corresponds to 6 cm(-1) or 3% of the total energy of the mode. We also estimate the anharmonic expansion from Fe (56 -> 54) isotope substitution to Delta a approximate to 5.1 x 10(-4) angstrom and Delta d(Fe-As) approximate to 2.5 x 10(-4) angstrom and the shift of harmonic zero-point fluctuations of bond lengths less than or similar to 3 x 10(-5) angstrom(2), giving a total relative average decrease in electronic hopping integrals of vertical bar delta vertical bar/t less than or similar to 2.0 x 10(-4). For a nonphonon-mediated weak-coupling superconductor this gives an isotope exponent alpha similar to 10(-2). The results pose a serious challenge for any theory of superconductivity in the pnictides that does not include electron-phonon interactions to produce a sizable Fe isotope effect.
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| 5. |
- Holmlund, Joakim, 1968, et al.
(författare)
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Two-magnon Raman scattering from the Cu3O4 layers in (Sr-2,Ba-2)Cu3O4Cl
- 2009
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969 .- 1098-0121 .- 1550-235X. ; 79:8
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Tidskriftsartikel (refereegranskat)abstract
- (Sr-2,Ba-2)Cu3O4Cl2 are antiferromagnetic insulators which are akin to the parent compounds of the cuprate superconductors but with two distinct magnetic ordering temperatures related to two magnetic Cu-I and Cu-II spin sublattices. Here we present a study of these materials by means of Raman spectroscopy. Following the temperature and polarization dependence of the data, we readily identify two distinct features at around 3000 and 300 cm(-1) that are related to two-magnon scattering from the two sublattices. The estimated spin-exchange coupling constants for the Cu-I and Cu-II sublattices are found to be J(I)similar to 139-143(132-136) meV and J(II)similar to 14 (11) meV for Sr(Ba) compounds. Moreover, we observe modes at around 480 and 445 cm(-1) for the Sr and Ba containing samples, respectively, which disappears at the ordering temperature of the Cu-II. We argue that these modes may also be of magnetic origin and possibly related to interband transitions between the Cu-I-Cu-II sublattices.
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| 6. |
- Karlsson, Maths, et al.
(författare)
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Short-range structure of proton-conducting perovskite BaInxZr1-xO3-x/2 (x=0-0
- 2009
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 20:10, s. 3480-3486
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Tidskriftsartikel (refereegranskat)abstract
- In a systematic study, we investigate the effect of dopant level and hydration on the short-range structure of the proton-conducting perovskite-type oxide BaInxZr1-xO3-x/2 (x = 0 - 0.75), using infrared and Raman spectroscopy. The results show that the doping leads to significant local distortions of the average cubic structure of these materials. By increasing the In concentration from x = 0 to x = 0.75, new bands appear and grow in intensity in both the IR and the Raman spectra, showing that the local distortions become successively more and more pronounced. The structural distortions are largely uncorrelated to the presence of oxygen vacancies, but instead are mainly driven by the size and charge difference between the In3+ and Zr4+ ions, which leads to displacements of the cations and to tilting of the (In/Zr)O-6 octahedra. On the basis of our results, we suggest that there is a threshold between x = 0.10 and x = 0.25 where the structural distortions propagate throughout the whole perovskite structure. Comparison of our spectroscopic data with the proton conductivity for the same materials indicates that the presence of extended structural distortions is favorable for fast proton transport.
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| 7. |
- Mattsson, Johan, 1969, et al.
(författare)
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Effects of hydrogen bonding on supercooled liquid dynamics and the implications for supercooled water
- 2009
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 79:17, s. 174205-
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Tidskriftsartikel (refereegranskat)abstract
- The supercooled state of bulk water is largely hidden by unavoidable crystallization, which creates an experimentally inaccessible temperature regime - a "no man's land." We address this, and circumvent the crystallization problem by systematically studying the supercooled dynamics of hydrogen bonded oligomeric liquids (glycols), where water corresponds to the chain ends alone. This approach permits a "dilution of water" by altering the hydrogen bond concentration via variations in chain length. We observe a dynamic crossover in the temperature dependence of the structural relaxation time for all glycols, consistent with the common behavior of most supercooled liquids. We find that the crossover becomes more pronounced for increasing hydrogen bond concentrations, which leads to the prediction of a marked dynamic transition for water within "no man's land" at T similar to 220 K. Interestingly, the predicted transition thus takes place at a temperature where a so called "strong-fragile" transition has previously been suggested. Our results, however, imply that the dynamic transition of supercooled water is analogous to that commonly observed in supercooled liquids. Moreover, we find support also for the existence of a secondary relaxation of water with behavior analogous to that of the secondary relaxation observed for the glycols.
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