| 1. |
- Gualtieri, Magdalena Lassinantti, et al.
(author)
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Crack formation in α-alumina supported MFI zeolite membranes studied by in situ high temperature synchrotron powder diffraction
- 2007
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In: Journal of Membrane Science. - : Elsevier BV. - 0376-7388 .- 1873-3123. ; 290:1-2, s. 95-104
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Journal article (peer-reviewed)abstract
- Cracks are frequently formed in α-alumina supported MFI membranes during calcination. To better understand crack formation, in situ powder diffraction data were collected during calcination of a type of MFI membrane (ca. 1800 nm thick) which is known to crack reproducibly. In addition, data for MFI powder and a blank support were also collected. Both a synchrotron radiation facility and an in-house instrument were used. The unit cell parameters were determined with the Rietveld method, and the strain in the direction perpendicular to the film surface was calculated for the film as well as for the support. The microstrain in the support was also estimated. Based on the results obtained here, a model for crack formation in this type of MFI membrane was proposed. The lack of cracks in other types of MFI membranes (ca. 500 nm) prepared in our laboratory is also explained by the model. In thicker MFI films, the crystals are well intergrown. During heating, the MFI crystals contract and the α-alumina support expands. Consequently, a thermal stress develops in the composite which eventually leads to formation of cracks in the film and structural defects in the support. In thinner films, the crystals are less well intergrown and the thermal expansion mismatch leads to opening of grain boundaries rather than cracks.
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| 2. |
- Imam, N. G., et al.
(author)
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Probing the local atomic structure in CoLa0.15Fe1.85O4 as a function of the synthesis method by multi edge XAFS
- 2019
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In: Materials Research Express. - : IOP Publishing. - 2053-1591. ; 6:11
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Journal article (peer-reviewed)abstract
- We combined x-ray powder diffraction (XRD) and x-ray absorption fine structure spectroscopy (XAFS) to characterize the crystallographic and local atomic structure of nanosized La3+ substituted CoFe2O4 spinel ferrite of composition CoLa0.15Fe1.85O4 synthesized by different synthesis routes: standard ceramic technique, citric acid (citrate) precursor and urea assisted flash auto-combustion process. Rietveld refinement of XRD patterns allowed to describe the long-range atomic structure that was used as structural model for quantitative analysis of XAFS spectra collected at the Fe-K, Co-K and La-L3 and refined via a multi-edge approach, which provided details about local atomic structure and coordination chemistry around the absorbers. Combining the complementary information from XRD and XAFS it was possible to reliably describe the cation distribution over the two spinel sublattices (tetrahedral and octahedral) as a function of the synthesis routes. We demonstrated that the inversion parameter, the cation distribution, and the local atomic structure depend strongly on the synthesis method which in particular affects the La3+ lattice-site and vacancies. In particular the citrate synthesis appears to provide a pure single pure phase with highest coordination number of La and lowest inversion parameter.
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| 3. |
- Jana, Somnath, et al.
(author)
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Atomic-scale chemical fluctuation in LaSrVMoO6, a proposed half-metallic antiferromagnet
- 2010
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:18
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Journal article (peer-reviewed)abstract
- Half-metallic antiferromagnets (HMAFMs) have been proposed theoretically long ago but have not been realized experimentally yet. Recently, a double perovskite compound, LaSrVMoO6, has been claimed to be an almost real HMAFM system. Here, we report detailed experimental and theoretical studies on this compound. Our results reveal that the compound is neither a half-metal nor an ordered antiferromagnet. Most importantly, an unusual chemical fluctuation is observed locally, which finally accounts for all the electronic and magnetic properties of this compound.
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| 4. |
- Jana, Somnath, et al.
(author)
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LaSrVMoO6 : A case study for A-site covalency-driven local cationic order in double perovskites
- 2012
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:1
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Journal article (peer-reviewed)abstract
- An unusual atomic scale chemical fluctuation in LaSrVMoO6, in terms of narrow patches of La, V and Sr,Mo-rich phases, has been probed in detail to understand the origin of such a chemical state. Exhaustive tuning of the equilibrium synthesis parameters showed that the extent of phase separation can never be melted down below a unit cell dimension making it impossible to achieve the conventional B-site ordered structure, which establishes that the observed "inhomogeneous" patchlike structure with minimum dimension of few angstroms is a reality in LaSrVMoO6. Therefore another type of local chemical order, hitherto unknown in double perovskites, gets introduced here. X-ray diffraction, electron microscopy elemental mapping, magnetic, and various spectroscopic studies have been carried out on samples, synthesized under different conditions. These experimental results in conjunction with ab initio electronic structure calculation revealed that it is the energy stability, gained by typical La-O covalency as in LaVO3, that leads to the preferential La, V and Sr,Mo ionic proximity, and the consequent patchy structure.
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| 5. |
- Jana, Somnath, et al.
(author)
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Signature of an antiferromagnetic metallic ground state in heavily electron-doped Sr2FeMoO6
- 2012
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:5, s. 054433-
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Journal article (peer-reviewed)abstract
- Sr2FeMoO6 is a well-known double perovskite with exciting high-temperature magnetic properties. Through various magnetic and spectroscopic measurements, we collect compelling evidence here that this compound can be driven into a rare three-dimensional antiferromagnetic metallic state by heavy electron doping (70% Sr2+ substitution by La3+). Moreover, local structural study of these Sr2-xLaxFeMoO6 (1.0 <= x <= 1.5) compounds reveals unusual atomic scale phase distribution in terms of La,Fe- and Sr,Mo-rich regions driven by strong La-O covalency, a phenomenon hitherto undisclosed in double perovskites. The general trend of our findings is in agreement with theoretical calculations carried out on realistic structures having local chemical fluctuations, which reconfirms the relevance of the kinetic-energy-driven magnetic model.
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| 6. |
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| 7. |
- Nag, Abhishek, et al.
(author)
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Origin of the Spin-Orbital Liquid State in a Nearly J=0 Iridate Ba3ZnIr2O9
- 2016
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In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 116:9
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Journal article (peer-reviewed)abstract
- We show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of Ba3ZnIr2O9 is a realization of a novel spin-orbital liquid state. Our results reveal that Ba3ZnIr2O9 with Ir5+ (5d(4)) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive J = 0 state but each Ir ion still possesses a weak moment. Ab initio density functional calculations indicate that this moment is developed due to superexchange, mediated by a strong intradimer hopping mechanism. While the Ir spins within the structural Ir2O9 dimer are expected to form a spin-orbit singlet state (SOS) with no resultant moment, substantial frustration arising from interdimer exchange interactions induce quantum fluctuations in these possible SOS states favoring a spin-orbital liquid phase down to at least 100 mK.
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| 8. |
- Zenari, M., et al.
(author)
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Modeling the Electrical Degradation of Micro-Transfer Printed 845 nm VCSILs for Silicon Photonics
- 2024
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In: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 71:2, s. 1131-1138
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Journal article (peer-reviewed)abstract
- This article deals for the first time with the electrical degradation of novel 845 nm vertical-cavity silicon-integrated lasers (VCSILs) for silicon photonics (SiPh). We analyzed the reliability of these devices by submitting them to high current stress. The experimental results showed that stress induced: 1) a significant increase in the series resistance, occurring in two separated time-windows and 2) a lowering of the turn-on voltage. To understand the origin of such degradation phenomena, we simulated the $\textit{I}$ - $\textit{V}$ characteristics and the band diagrams by a Poisson-drift-diffusion simulator. We demonstrated that the degradation was caused by the diffusion of mobile species capable of compensating the p-type doping. The diffusing species are expected to migrate from the p-contact region in the top distributed Bragg reflector (DBR) towards the active layers.
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