| 1. |
- Jacobsen, Henrik, et al.
(författare)
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Spin dynamics of the director state in frustrated hyperkagome systems
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9969 .- 2469-9950. ; 104:5
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Tidskriftsartikel (refereegranskat)abstract
- We present an experimental study of the magnetic structure and dynamics of two frustrated hyperkagome compounds, Gd3Ga5O12 and Gd3Al5O12. It has previously been shown that Gd3Ga5O12 exhibits long-range correlations of multipolar directors that are formed from antiferromagnetic spins on loops of ten ions. Using neutron diffraction and reverse Monte Carlo simulations we prove the existence of similar magnetic correlations in Gd3Al5O12, showing the ubiquity of these complex structures in frustrated hyperkagome materials. Using inelastic neutron scattering we shed further light on the director state and the associated low-lying magnetic excitations. In addition, we have measured quasielastic dynamics that show evidence of spin diffusion. Finally, we present AC susceptibility measurements on both Gd3Ga5O12 and Gd3Al5O12, revealing a large difference in the low-frequency dynamics between the two otherwise similar compounds.
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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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Effect of co-doping on proton conductivity in perovskite oxides BaZr0.9In0.05M0.05O3−δ (M = Yb3+ or Ga3+)
- 2010
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 35:12, s. 6381-6391
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Tidskriftsartikel (refereegranskat)abstract
- BaZr0.9In0.05M0.05O3-d (M ¼ Ga3+ or Yb3+) has been prepared by solid-statesynthesis route.Rietveld analysis of neutron powder diffraction data on as-prepared samples showed thatboth samples crystallise in the cubic space group Pm-3m. Scanning electron microscopyanalysis showed that the grains intheBaZr0.9In0.05Ga0.05O3-d sample were larger than the BaZr0.9In0.05Yb0.05O3-d sample. Dynamic thermogravimetric analysis indicates that the proton concentration was higher inpre-hydrated BaZr0.9In0.05Ga0.05O3-d sample (40%oftheoretical)than that of BaZr0.9In0.05Yb0.05O3-d sample (28%oftheoretical).Proton conductivity was studied on pre-hydrated samples(under both dry and wetAr atmospheres).Thebulk proton conductivities of the heating cycle of pre-hydrated BaZr0.9In0.05M0.05O3-d (M ¼ Ga3+ or Yb3+) samples were 2.1 * 10-5 S cm-1 and 1.9 * 10-4 Scm-1 at 350C, respectively.The effect of co-doping onproton conductivity was investigatedand the results are compared with single doped systems e.g. BaZr0.9M0.1O3-d (M=Ga3+, In3+ orYb3+) samples.
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| 4. |
- Ahmed, Istaq, 1972, et al.
(författare)
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Influence of microstructure on electrical properties in BaZr0.5In0.5O3-delta proton conductor
- 2008
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 179:21-26, s. 1155-1160
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Tidskriftsartikel (refereegranskat)abstract
- Two different preparative routes, one a wet chemical route (WCR) and the other a traditional solid state sintering route (SSR), have been used to synthesise the oxygen deficient perovskite BaZr0.5In0.5O3-delta. Analysis of X-ray powder diffraction data showed that both phases adopt cubic crystal structures. Environmental scanning electron microscope (ESEM) images showed that a smaller grain size was obtained for the WCR sample. Dynamic then-no-gravimetric analysis indicated significant mass losses for hydrated samples compared to their respective dried samples. Proton conductivity was investigated for hydrated and as-prepared phases under dry and wet atmospheres, respectively. The hydrated sample prepared via the solid state sintering route displayed a higher proton conductivity under dry conditions, reflecting the presence of fewer grain boundaries in this sample. (c) 2008 Elsevier B.V. All rights reserved
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| 5. |
- 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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| 6. |
- Ahmed, Istaq, 1972, et al.
(författare)
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Proton conduction in Perovskite Oxide BaZr0.5Yb0.5O3-δ Prepared by Wet Chemical Route
- 2008
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 1945-7111 .- 0013-4651. ; 155:11, s. 97-102
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Tidskriftsartikel (refereegranskat)abstract
- A wet chemical route has been used to synthesize the oxygen deficient perovskite Ba Zr0.5 Yb0.5 O3-δ. Analysis of X-ray powder diffraction data showed that both dried and hydrated samples adopt cubic crystal structures of space group Pm 3- m. Dynamic thermogravimetric analysis showed a significant mass loss for the hydrated sample compared to the dried sample, indicating that ∼28% of the oxygen vacancies are filled by protonic defects. The strong O-H stretch band, 2500-3500 cm-1, in the IR absorbance spectrum also clearly manifests the presence of significant levels of protons in the hydrated material. Proton conductivity was investigated on prehydrated (under dry Ar) and as-prepared (under wet Ar) samples. The heating cycle of the prehydrated sample showed higher proton conductivity compared to the cooling cycle, especially in the intermediate temperature range (150-550°C). Finally, comparison with data for Ba Zr0.9 Yb0.1 O3-δ revealed that the more heavily doped sample showed higher proton conductivity compared to the more lightly doped sample. © 2008 The Electrochemical Society.
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| 7. |
- Ahmed, Istaq, 1972, et al.
(författare)
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Structural study and proton conductivity in Yb-doped BaZrO3
- 2007
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Ingår i: Solid State Ionics. - 0167-2738. ; 178:(7-10), s. 515-520
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Tidskriftsartikel (refereegranskat)abstract
- Traditional solid state sintering has been used to prepare the perovskite BaZr0.9Yb0.1O3-delta. Analysis of X-ray powder diffraction data shows that an increase of the unit cell parameter, a, was observed compared to undoped BaZrO3. Rietveld analysis of room temperature neutron powder diffraction data confirmed cubic symmetry (space group Pm-3m). Dynamic thermogravimetric analysis (TGA) indicates that the hydration process occurs below 200 degrees C. The strong O-H stretch band, 2500-3500 cm(-1), in the infrared absorbance spectrum clearly manifests the presence of protons in the hydrated material. Proton conductivity was investigated on hydrated and as-prepared samples under dry and wet atmospheres, respectively. Keywords: X-ray diffraction; neutron diffraction; Rietveld refinement; proton conductor; perovskite
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| 8. |
- Ahmed, Istaq, 1972, et al.
(författare)
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Structural study and proton conductivity of BaZr0.90Ga0.10O2.95
- 2006
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Ingår i: Advances of Solid Oxide Fuel Cells II, Ceramic Engineering and Science Proceedings. - Hoboken, NJ, USA : John Wiley & Sons, Inc.. ; 27:4, s. 105-117
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Tidskriftsartikel (refereegranskat)abstract
- Traditional solid state sintering has been used to prepare the perovskite BaZr0.9Ga0.1O2.95. Analysis of X-ray powder diffraction data shows that a decrease of the unit cell parameter a was observed compared to the undoped BaZrO3, which confirms successful substitution of Ga3+ for Zr4+ at the B site. Rietveld analysis of room temperature neutron powder diffraction data confirmed cubic symmetry (space group Pm-3m) for both as- prepared and deuterated BaZr0.90Ga0.10O2.95 samples. The strong O-H stretch band (2500-3500 cm−1) in the infrared absorbance spectrum clearly manifests the presence of protons in the pre-hydrated material. The bulk and total conductivities of prehydrated BaZr0.9Ga0.1O2.95 are 1.17 × 10−5 and 3.55 × 10−6 Scm−1, respectively at 400°C, which are more than one order of magnitude higher than for dried a sample at the same temperature. In contrast, the total conductivity of pre-hydrated and dried samples is similar at higher temperature, e. g. T > 800 °C. The higher activation energy (e. g. 0.7 eV) for pre-hydrated sample compared to typical value (0.4-0.5 eV) of proton conduction may suggest that the protons are trapped in the material.
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| 9. |
- Ahmed, Istaq, 1972, et al.
(författare)
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Synthesis and structural characterization of perovskite type proton conducting BaZr1-xInxO3-delta (0.0 <= x <= 0.75)
- 2006
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738 .- 1872-7689. ; 177:17-18, s. 1395-1403
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Tidskriftsartikel (refereegranskat)abstract
- Solid state sintering has been used to prepare the cubic perovskite structured compounds BaZr1-xInxO3-delta (0.0 <= x <= 0.75). Analysis of X-ray powder diffraction (XRPD) data reveals that the unit cell parameter, a, increases linearly with an increased Indium concentration. XRPD data was also used to demonstrate the completion of sample hydration, which was reached when the materials showed a set of single-phase Bragg-peaks. Dynamic thermogravimetric analysis (TGA) data showed that approx. 89% of the total number of available oxygen vacancies can be filled in BaZr1-xInxO3-delta for x=0.50, and that the maximum water uptake occurs below 300 degrees C. Rietveld analysis of the room temperature neutron powder diffraction (NPD) data confirmed the average cubic symmetry (space group Pm-3m), and an expansion of the unit cell parameter after the hydration reaction. The strong O-H stretch band, 2500-3500 cm(-1), in the infrared absorbance spectrum clearly manifests the presence of protons in the hydrated material. Proton conductivity of hydrated BaZr1-xInxO3-delta, x=0.75 was investigated during heating and cooling cycles under dry argon atmosphere. The total conductivity during the heating cycle was nearly two orders of magnitude greater than that of cooling cycle at 300 degrees C, whilst these values were similar at higher temperatures i.e. T > 600 degrees C.
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| 10. |
- Andersson, A. K. E., et al.
(författare)
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Chemical Expansion Due to Hydration of Proton-Conducting Perovskite Oxide Ceramics
- 2014
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Ingår i: Journal of the American Ceramic Society. - : Wiley. - 0002-7820 .- 1551-2916. ; 97:8, s. 2654-2661
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Tidskriftsartikel (refereegranskat)abstract
- The crystal structures of proton-conducting BaZr1-xYxO3-x/2 (BZY05-BZY20) and BaCe0.8Y0.2O2.9 (BCY20) during hydration/dehydration has been studied by in situ high-temperature X-ray diffraction and thermal analysis. A contraction/expansion of the crystal lattice associated with dehydration/hydration was observed for all materials at elevated temperatures and the polymorphic phase transition temperatures of BaCe0.8Y0.2O2.9 were depressed by lowering the vapor pressure of water. A thermodynamic formalism is introduced to describe the chemical expansion associated with the hydration of oxygen vacancies in acceptor-doped oxides. A conventional point defect model was applied to describe the lattice strain associated with the hydration. The chemical expansion is discussed with respect to the available volumetric data on the hydration of proton-conducting oxide materials and its likely impact on ceramic fuel cells/hydrogen separation membranes utilizing a proton-conducting electrolyte.
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