| 1. |
- Alvi, Muhammad Rouf, et al.
(författare)
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Highly Efficient and Convenient Acid Catalyzed Hypersilyl Protection of Alcohols and Thiols by Tris(trimethylsilyl)silyl-N,N-dimethylmethaneamide
- 2012
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Tris(trimethylsilyl)silyl-N,N-dimethylmethaneamide, herein named hypersilylamide, is a convenient and efficient source of the hypersilyl group in the first widely applicable acid catalyzed protocol for silyl group protection of primary, secondary, tertiary alkyl as well as aryl alcohols and thiols in high yields. The sole by-product is N,N-dimethylformamide (DMF) and a range of solvents can be used, including DMF. A high selectivity in the protection of diols can be achieved, also for diols with very small differences in the steric demands at the two hydroxyl groups. Moreover, in the protection of equivalent alcohol and thiol sites the protection of the alcohol is faster, allowing for selective protection in high yields. Quantum chemical calculations at the M062X hybrid meta density functional theory level give insights on the mechanism for the catalytic process. Finally, the hypersilyl group is easily removed from all protected alcohols and thiols examined herein by irradiation at 254 nm.
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| 2. |
- Denoel, Fernand, et al.
(författare)
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Eu Doping in the GdCd7.88 Quasicrystal and Its Approximant Crystal GdCd6
- 2024
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 63:11, s. 5040-5051
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Tidskriftsartikel (refereegranskat)abstract
- The effect of Eu doping in the Tsai quasicrystal (QC) GdCd7.88 and its periodic 1/1 approximant crystal (AC) GdCd6 are investigated. This represents the first synthesis of Eu-containing stable QC samples, where three samples with the final composition Gd1-xEuxCd7.6±α at Eu doping concentrations x = 0.06, 0.13, and 0.19 are obtained (α ∼ 0.2). They are compared to two 1/1 ACs with compositions Gd1-xEuxCd6 (x = 0.12, 0.16). In addition, a new type of 1/1 AC, differing only by the inclusion of extra Cd sites unique to the Eu4Cd25 1/1 AC, has been discovered and synthesized for the concentrations Gd1-xEuxCd6+δ (x = 0.25, 0.33, 0.45, 0.69, 0.73, and 0 < δ ≤ 0.085). Due to the preferred cube morphology of its single grains, we refer to them as c-type 1/1 ACs and to the conventional standard ones as s-type. In both QCs and s-type ACs, the Eu content appears to saturate at a concentration of similar to 20%. On the other hand, any Gd| Eu ratio is allowed in the c-type ACs, varying continuously between GdCd6 and Eu4Cd25. We describe and contrast the changes in composition, atomic structure, specific heat, and magnetic properties induced by Eu doping in the quasicrystalline phase and the s-type and c-type 1/1 ACs. By comparing our results to the literature data, we propose that the occupancy of the extra Cd sites can be used to predict the stability of Tsai-type quasicrystalline phases.
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| 3. |
- Denoel, Fernand, et al.
(författare)
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Magnetic frustration and glassiness in an icosahedral i-Tb-Cd quasicrystal
- 2024
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 109:18
-
Tidskriftsartikel (refereegranskat)abstract
- The dynamical magnetic properties of the icosahedral i-Tb-Cd quasicrystal (QC) were investigated by means of squid magnetometry. At low temperatures, below T ∼ 4.5 K, we observe conventional spin glass features including aging, memory, and rejuvenation. Interestingly, from T ∼ 4.5 to 10 K, the spin configuration is also found to evolve with time (i.e., age as in spin glass), yet without memory. This suggests a reentrantlike behavior for the i-Tb-Cd QC, where upon cooling from high temperatures, the system first displays a magnetically frustrated phase before turning into a spin glasslike one at the lowest temperatures. We discuss the nature and possible origin of this magnetic behavior, in the light of the results obtained in the i-Gd-Cd QC, and ternary Tb-based quasicrystal approximants.
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| 4. |
- Fang, Hailiang, et al.
(författare)
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Insights into phase transitions and magnetism of MnBi crystals synthesized from self-flux
- 2019
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Ingår i: Journal of Alloys and Compounds. - : ELSEVIER SCIENCE SA. - 0925-8388 .- 1873-4669. ; 781, s. 308-314
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Tidskriftsartikel (refereegranskat)abstract
- To effectively synthesize high purity ferromagnetic low temperature phase (LTP) MnBi with optimal microstructure is still a challenge that needs to be overcome for the system to reach its full potential. Here, the phase transitions and magnetic properties of MnBi crystals are reported. The phase transition between the low and high temperature structure of MnBi was systematically investigated at different heating/cooling rates using in situ synchrotron radiation X-ray diffraction. The material crystallizes in a layered hexagonal structure giving a platelike microstructure. The magnetic characterization of the crystals reveal that the saturation magnetization varies from 645 kA/m at 50 K to 546 kA/m at 300 K. Magnetization measurements also show that the sample upon heating becomes non-magnetic and transforms to the high temperature phase (HTP) at similar to 640 K, and that it regains ferromagnetic properties and transforms back to the LTP at similar to 610 K upon subsequent cooling.
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| 5. |
- Gebresenbut, Girma Hailu, 1982-, et al.
(författare)
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Atomic-Scale Tuning of Tsai-Type Clusters in RE-Au-Si Systems (RE = Gd, Tb, Ho)
- 2020
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 59:13, s. 9152-9162
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Tidskriftsartikel (refereegranskat)abstract
- Tsai-type quasicrystals and approximants are distinguished by a cluster unit made up of four concentric polyhedral shells that surround a tetrahedron at the center. Here we show that for Tsai-type 1/1 approximants in the RE-Au-Si systems (RE = Gd, Tb, Ho) the central tetrahedron of the Tsai clusters can be systematically replaced by a single RE atom. The modified cluster is herein termed a pseudo-Tsai cluster and represents, in contrast to the conventional Tsai cluster, a structural motif without internal symmetry breaking. For each system, single-phase samples of both pseudo-Tsai and Tsai-type 1/1 approximants were independently prepared as millimeter-sized, faceted, single crystals using the self-flux synthesis method. The full replacement of tetrahedral moieties by RE atoms in the pseudo-Tsai 1/1 approximants was ascertained by a combination of single-crystal and powder diffraction studies, as well as energy dispersive X-ray spectroscopy (EDX) analyses with a scanning electron microscope (SEM). Differential scanning calorimetry (DSC) studies revealed distinctly higher decomposition temperatures, by 5-35 K, for the pseudo-Tsai phases. Furthermore, the magnetic properties of pseudo-Tsai phases are profoundly and consistently different from the Tsai counterparts. The onset temperatures of magnetic ordering (T-mag) are lowered in the pseudo-Tsai phases by similar to 30% from 24 to 17 K, 11.5 to 8 K, and 5 to 3.5 K in the Gd-Au-Si, Tb-Au-Si, and Ho-Au-Si systems, respectively. In addition, the Tb-Au-Si and Ho-Au-Si systems exhibit some qualitative changes in their magnetic ordering, indicating decisive changes in the magnetic state/structure by a moment-bearing atom at the cluster center.
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| 6. |
- Gebresenbut, Girma Hailu, 1982-, et al.
(författare)
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Chemical speciation in Gd-Cd-M (M=Zn, Au) quasicrystal approximants
- 2021
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Ingår i: Zeitschrift für Anorganische und Allgemeines Chemie. - : John Wiley & Sons. - 0044-2313 .- 1521-3749. ; 647:2-3, s. 86-90
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the effect of partial replacement of Cd by M=Au and Zn in the crystal structure of the 1/1 Tsai-type quasicrystal approximant (AC) GdCd6. Compositionally homogeneous single crystal samples Gd(Cd0.87Zn0.13(1))(6) and Gd(Cd0.80Au0.20(1))(6) were grown from melts Gd-5(Cd0.8Zn0.2)(100) and Gd-1(Cd0.9Au0.1)(100), respectively, and isolated by centrifugation. The M for Cd substitution in GdCd6 is accompanied with a sizeable reduction of the cubic unit cell parameter, from 15.514(2) angstrom to 15.329(1) angstrom (Zn) and 15.314(1) angstrom (Au). Site preferences were established from single crystal X-ray diffraction data. A clear preference of atomic sites for Au and Zn is noted which is compared to earlier reported Yb(Cd0.75Mg0.25)(6). Three and two out of in total seven crystallographic sites defining the Cd partial structure accept preferably metals more and less electronegative than Cd, respectively, and are classified as negatively and positively polarized sites in the binary 1/1 AC.
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| 7. |
- Gebresenbut, Girma Hailu, 1982-, et al.
(författare)
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Effect of pseudo-Tsai cluster incorporation on the magnetic structures of R-Au-Si (R = Tb, Ho) quasicrystal approximants
- 2022
-
Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 106:18
-
Tidskriftsartikel (refereegranskat)abstract
- In cluster-based quasicrystals, tetrahedra located in conventional Tsai clusters may be replaced by single rare-earth (R) ions at the cluster centers (pseudo-Tsai clusters). In this study, we investigate the effect of the pseudo-Tsai cluster incorporation on the magnetic structures of two approximants, the Tsai-type Tb-Au-Si [denoted TAS(0)] and Ho-Au-Si [denoted HAS(52)] with partial replacement of conventional Tsai clusters by pseudo-Tsai clusters, up to 52%. The mixture of Tsai and pseudo-Tsai clusters can be considered a different source of randomness/disorder other than the conventional chemical mix sites (Au/Si). The effect of the latter has been previously discussed regarding the origin/cause of spin-glass-like ordering and Anderson localization of electronic states in quasicrystals and approximant crystals. Single crystal neutron diffraction experiments at 2 K were performed and bulk physical properties (magnetization and specific heat) were investigated. In addition, earlier collected powder neutron diffraction data of TAS(14) with 14% replacement was reanalyzed in light of the results on TAS(0) and HAS(52). We find that the arrangement of ordered magnetic spins in the icosahedral shells of these phases is similar, while the cluster-center R magnetic states are different. In the case of TAS(14), the cluster-center Tb magnetic moments seem to affect the arrangement of surrounding icosahedral magnetic moments, and the magnetic structure of the icosahedral shell deviates from that of TAS(0). In the case of HAS(52), however, the icosahedral R magnetic moments are less affected by the cluster-center R, while the averaged cluster-center R magnetic moments are significantly diminished. We discuss these results considering the magnetic ordering effect on the bulk physical properties.
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| 8. |
- Gómez, Cesar Pay, 1974-, et al.
(författare)
-
ANALYSIS OF STRUCTURE AD CHEMICAL ORDER I A TERNARY Yb12Mg52Cd36 QUASICRYSTAL
- 2010
-
Ingår i: Philosophical Magazine. - Sapporo.
-
Konferensbidrag (refereegranskat)abstract
- The atomic structure of the ternary Yb12Mg52Cd36 quasicrystal has been refined from single crystal X-ray data. The ternary quasicrystal is related to the binary i-Yb16Cd84 parent phase,[1, 2] and suffers from chemical disorder mainly due to mixing between Mg and Cd. Studies on related ternary approximants however indicate that this chemical disorder is only partial, and that there are strong selection rules in several ternary systems that govern the choice of a particular atom at a specific site. The purpose of this work is thus to elucidate the chemical order between the constituent elements in the ternary Yb12Mg52Cd36 quasicrystal and its relation to ternary approximants in other REMg-Cd (RE=Rare Earth) systems. The structure refinements performed on the Yb12Mg52Cd36 quasicrystal clearly indicate that it is composed of similar atomic clusters as the binary i-Yb16Cd84 phase and that there is a strong selection rule that mainly determines the chemical order at the cluster level. Similar observations have also been made in related ternary approximant phases. The structure refinement is the first of its kind performed on a ternary Yb-Cd-related quasicrystal, and the results indicate that the structures and chemical order of ternary quasicrystals can be understood by extracting and combining information from structure refinements on both quasicrystals and related approximants.
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| 9. |
- Huang, Yu-Chin, et al.
(författare)
-
Structural Analysis of the Gd-Au-Al 1/1 Quasicrystal Approximant Phase across Its Composition-Driven Magnetic Property Changes
- 2023
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 62:36, s. 14668-14677
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Tidskriftsartikel (refereegranskat)abstract
- Gd14AuxAl86–x Tsai-type 1/1 quasicrystal approximants (ACs) exhibit three magnetic orders that can be finely tuned by the valence electron concentration (e/a ratio). This parameter has been considered to be crucial for controlling the long-range magnetic order in quasicrystals (QCs) and ACs. However, the nonlinear trend of the lattice parameter as a function of Au concentration suggests that Gd14AuxAl86–x 1/1 ACs are not following a conventional solid solution behavior. We investigated Gd14AuxAl86–x samples with x values of 52, 53, 56, 61, 66, and 73 by single-crystal X-ray diffraction. Our analysis reveals that increasing Au/Al ordering with increasing x leads to distortions in the icosahedral shell built of the Gd atoms and that trends observed in the interatomic Gd–Gd distances closely correlate with the magnetic property changes across different x values. Our results demonstrate that the e/a ratio alone may be an oversimplified concept for investigating the long-range magnetic order in 1/1 ACs and QCs and that the mixing behavior of the nonmagnetic elements Au and Al plays a significant role in influencing the magnetic behavior of the Gd14AuxAl86–x 1/1 AC system. These findings will contribute to improved understanding towards tailoring magnetic properties in emerging materials.
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| 10. |
- Joshi, Deep C., et al.
(författare)
-
Memory and rejuvenation in a quasicrystal
- 2020
-
Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 132:2
-
Tidskriftsartikel (refereegranskat)abstract
- The glassy features of a single crystal of the icosahedral quasicrystal i-GdCd7.5 were investigated by means of squid magnetometry. The temperature-dependent zero-field-cooled magnetization was recorded on re-heating from low temperatures after halts in the cooling. The results evidence dynamical features akin to those of archetypal spin glasses, such as aging, memory, and rejuvenation. The results are compared to those of model spin glasses with different spin dimensionality, suggesting a qualitative similarity to the behaviour of metallic RKKY "Heisenberg" spin glasses. Copyright (C) 2020 The author(s)
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| 11. |
- Menon, Ashok S., et al.
(författare)
-
A Crystallographic Reinvestigation of Li1.2Mn0.6Ni0.2O2
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Despite substantial research interest, the crystallography of the promising Li-ion positive electrode material, Li1.2Mn0.6Ni0.2O2, remains disputed. The dispute is predicated on the description of the cationic arrangement in the structure, and multiple structure models have been proposed. This study attempts to provide a fresh perspective to this debate through a multi-scalar structural characterisation of Li1.2Mn0.6Ni0.2O2. Combining Bragg diffraction, transmission electron microscopy and magnetic measurements with reverse Monte Carlo analysis of total scattering data, a quantitative structural description of Li1.2Mn0.6Ni0.2O2 is developed and the existing single- and multi-phase structural descriptions of this compound have been unified. Furthermore, the merits and drawbacks of each technique is evaluated with respect to the crystallography of Li1.2Mn0.6Ni0.2O2 to explain the factors that have contributed to the lack of clarity pervading the structural description of this material. It is envisioned that a better understanding of the crystallography of Li1.2Mn0.6Ni0.2O2 contributes to harnessing the electrochemical potential of this compound.
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| 12. |
- Menon, Ashok S., et al.
(författare)
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Influence of Synthesis Routes on the Crystallography, Morphology, and Electrochemistry of Li2MnO3
- 2020
-
Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 12:5, s. 5939-5950
-
Tidskriftsartikel (refereegranskat)abstract
- With the potential of delivering reversible capacities of up to 300 mAh/g, Li-rich transition-metal oxides hold great promise as cathode materials for future Li-ion batteries. However, a cohesive synthesis-structure-electrochemistry relationship is still lacking for these materials, which impedes progress in the field. This work investigates how and why different synthesis routes, specifically solid-state and modified Pechini sol-gel methods, affect the properties of Li2MnO3, a compositionally simple member of this material system. Through a comprehensive investigation of the synthesis mechanism along with crystallographic, morphological, and electrochemical characterization, the effects of different synthesis routes were found to predominantly influence the degree of stacking faults and particle morphology. That is, the modified Pechini method produced isotropic spherical particles with approximately 57% faulting and the solid-state samples possessed heterogeneous morphology with approximately 43% faulting probability. Inevitably, these differences lead to variations in electrochemical performance. This study accentuates the importance of understanding how synthesis affects the electrochemistry of these materials, which is critical considering the crystallographic and electrochemical complexities of the class of materials more generally. The methodology employed here is extendable to studying synthesis-property relationships of other compositionally complex Li-rich layered oxide systems.
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| 13. |
- Menon, Ashok S., et al.
(författare)
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Synthesis-structure relationships in Li- and Mn-rich layered oxides : phase evolution, superstructure ordering and stacking faults
- 2022
-
Ingår i: Dalton Transactions. - : Royal Society of Chemistry. - 1477-9226 .- 1477-9234. ; 51:11, s. 4435-4446
-
Tidskriftsartikel (refereegranskat)abstract
- Li- and Mn-rich layered oxides are promising positive electrode materials for future Li-ion batteries. The presence of crystallographic features such as cation-mixing and stacking faults in these compounds make them highly susceptible to synthesis-induced structural changes. Consequently, significant variations exist in the reported structure of these compounds that complicate the understanding of how the crystallographic structure influences its properties. This work investigates the synthesis-structure relations for three widely investigated Li- and Mn-rich layered oxides: Li2MnO3, Li1.2Mn0.6Ni0.2O2 and Li1.2Mn0.54Ni0.13Co0.13O2. For each compound, the average structure is compared between two synthetic routes of differing degrees of precursor mixing and four annealing protocols. Furthermore, thermodynamic and synthesis-specific kinetic factors governing the equilibrium crystallography of each composition are considered. It was found that the structures of these compounds are thermodynamically metastable under the synthesis conditions employed. In addition to a driving force to reduce stacking faults in the structure, these compositions also exhibited a tendency to undergo structural transformations to more stable phases under more intense annealing conditions. Increasing the compositional complexity introduced a kinetic barrier to structural ordering, making Li1.2Mn0.6Ni0.2O2 and Li1.2Mn0.54Ni0.13Co0.13O2 generally more faulted relative to Li2MnO3. Additionally, domains with different degrees of faulting were found to co-exist in the compounds. This study offers insight into the highly synthesis-dependent subtle structural complexities present in these compounds and complements the substantial efforts that have been undertaken to understand and optimise its electrochemical properties.
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| 14. |
- Menon, Ashok S., et al.
(författare)
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Synthetic Pathway Determines the Nonequilibrium Crystallography of Li- and Mn-Rich Layered Oxide Cathode Materials
- 2021
-
Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 4:2, s. 1924-1935
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Tidskriftsartikel (refereegranskat)abstract
- Li- and Mn-rich layered oxides show significant promise as electrode materials for future Li-ion batteries. However, an accurate description of its crystallography remains elusive, with both single-phase solid solution and multiphase structures being proposed for high performing materials such as Li1.2Mn0.54Ni0.13Co0.13O2. Herein, we report the synthesis of single- and multiphase variants of this material through sol-gel and solid-state methods, respectively, and demonstrate that its crystallography is a direct consequence of the synthetic route and not necessarily an inherent property of the composition, as previously argued. This was accomplished via complementary techniques that probe the bulk and local structure followed by in situ methods to map the synthetic progression. As the electrochemical performance and anionic redox behavior are often rationalized on the basis of the presumed crystal structure, clarifying the structural ambiguities is an important step toward harnessing its potential as an electrode material.
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| 15. |
- Mikheenkova, Anastasiia, 1995-
(författare)
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Investigating ageing mechanisms in electric vehicle batteries : A multiscale approach to material analysis
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electrifying passenger transport is a key strategy in combating global warming, with Li-ion batteries (LIBs) being the current go-to technology. Despite LIB’s satisfactory performance and carbon-neutral operation, lifetime and safety are still public concerns. A thorough understanding of battery ageing is crucial for improving LIBs and advancing the overall sustainability of LIB technology. This thesis bridges a gap between academic and industrial research by combining commercial battery investigation with a multiscale approach using a combination of in-house and synchrotron characterization methods used with the implementation of method development to study commercial batteries.The multiple degradation mechanisms were identified at various scales in the aged commercial cells. Specifically, the results show that the studied cells exhibit significant and distinct ageing heterogeneity in prismatic and cylindrical cell formats, where the area with the highest degradation is found on the side of the positive tab, where the current and temperature gradients are expected to be the strongest. After decoupling the performance on the electrode level, the Ni-rich layered oxide positive electrodes show a significant increase in Li+ diffusion resistance in the aged materials as a function of the State of Charge (SoC) range and temperature. Furthermore, heterogeneity is an issue relevant also on a secondary particle scale, where identified SoC gradients ranging from the centre to the surface of the particle might induce kinetic limitations and cause an increase in Li+ diffusion resistance. On a single particle level, the formation of a large number of voids within the grains was found. Such degradation can additionally contribute to the resistance increase in the material by changing tortuosity for Li-ions. Finally, at the atomic level, Ni was found to be the dominant charge compensator, which can decrease up to 25% of the redox activity after ageing. Compared to Ni, Co was found to be less redox-active, but more involved in charge compensation through changes in hybridization with the oxygen atom. The oxygen, in turn, was revealed to participate in anionic redox reactions at low SoC by both hybridization to TM and also through the formation of molecular oxygen at lower potentials than previously reported. The observed decrease in oxygen anion redox activity follows with material losing performance.The results presented in the thesis demonstrate the importance of the multiscale approach in order to form a more complete understanding of the degradation processes which have effects within different scales.
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| 16. |
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| 17. |
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| 18. |
- Pay Gómez, Cesar, Docent, 1974-
(författare)
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Quasicrystals
- 2011
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Ingår i: Nordic IYC Chemistry teacher conference, Royal Academy of Sciences (KVA), Sweden. Invited speaker. October 28-29, 2011. - Stockholm.
-
Konferensbidrag (populärvet., debatt m.m.)
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| 19. |
- Pay Gómez, Cesar, Docent, 1974-, et al.
(författare)
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Quasicrystals for Hydrogen Storage
- 2012
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Much attention is now being focused on Quasicrystals in the light of the recent Nobel Prize in chemistry. Although these exotic materials have been known to us since 1982, little has been known concerning the detailed atomic structures of icosahedral quasicrystals until recently. Based on the constituent atomic clusters in icosahedral quasicrystals, they can be classified into one of three families: Bergman-, Mackay- and Tsai-type quasicrystals. The Bergman-type family, also referred to as the Frank-Kasper type family (FK-type) is by definition structurally related to other well-known FK-type alloys such as LaNi5 and MgZn2 which are already being used commercially in high-capacity electrodes and hydrogen fuel cells. In spite of this fact, few investigations have been made in order to explore the hydrogen storage properties of quasicrystals and their related approximants. In this work we will give an introduction to the crystal chemistry of quasicrystals and approximants, highlighting their structural relation to simple FK-type alloys and their potential use as hydrogen storage materials.
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| 20. |
- Pay Gómez, Cesar, Docent, 1974-
(författare)
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Structure analysis of icosahedral quasicrystals and approximants by X-ray diffraction
- 2016
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this tutorial lecture we will give an introduction to the various aspects and procedures involved in structure analysis of complex metallic alloys such as quasicrystals, approximants and other related phases. Special attention will be given to pitfalls related to positional and chemical order/disorder phenomena along with useful tips on how to avoid them. The examples will be based on binary and ternary phases related to the icosahedral Yb-Cd quasicrystal.
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| 21. |
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| 22. |
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| 23. |
- Pay Gómez, Cesar, Docent, 1974-
(författare)
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Structures of Icosahedral Quasicrystals and Approximants
- 2012
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Structure determination of quasicrystals (QCs) has been one of the most challenging tasks since their discovery by Shechtman et al.[1] In this work we will assess the present state of the art concerning structure determination of icosahedral quasicrystals and approximants. In particular we will discuss the structure of the binary stable i-YbCd5.7 quasicrystal,[2] and how our knowledge of this phase has affected our overall understanding of icosahedral quasicrystals.[3] The binary i-YbCd5.7 phase could be characterized due to a fortuitous combination of circumstances: the quasicrystal is stable and high quality singlecrystals can be grown in equilibrium with a melt of the same composition. It is binary and chemically well-ordered, and furthermore two closely related approximant phases are known in close compositional vicinity to the quasicrystal. By carefully studying the structures of these related approximants and collecting singlecrystal data on the quasicrystal the structure of the i-YbCd5.7 phase could be characterized. However, far from all quasicrystals benefit from stability, chemical order and the availability of closely related approximants, in fact the i-YbCd5.7 QC is a rare exception.Fortunately,muchofwhatwehavelearntfromthis phasecanbeappliedalsotootherquasicrystals.Primarilythis applies to the related phases of the same family, since the i-YbCd5.7phaseistheparentstructureforthelargestfamilyof icosahedral quasicrystals and approximants. Most of these phases however, are ternary and suffer from chemical disorder which is yet to be well understood.[1] Shechtman et al., Phys. Rev. Lett. 53, 1951 (1984). [2] Tsai et al., Nature 408, 537 (2000).[3] Takakura et al., Nature Materials. 6, 58 (2007).
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| 24. |
- Pereira de Carvalho, Rodrigo, et al.
(författare)
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Structure-property relationships in organic battery anode materials : exploring redox reactions in crystalline Na- and Li-benzene diacrylate using combined crystallography and density functional theory calculations
- 2021
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Ingår i: Materials Advances. - : Royal Society of Chemistry. - 2633-5409. ; 2:3, s. 1024-1034
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Tidskriftsartikel (refereegranskat)abstract
- Organic-based materials are potential candidates for a new generation of sustainable and environmentally friendly battery technologies, but insights into the structural, kinetic and thermodynamic properties of how these compounds lithiate or sodiate are currently missing. In this regard, benzenediacrylates (BDAs) are here investigated for application as low-potential electrodes in Na-ion and Li-ion batteries. Aided by a joint effort of theoretical and experimental frameworks, we unveil the structural, electronic and electrochemical properties of the Na(2)BDA and Li(2)BDA compounds. The crystal structure of these systems in their different sodiated and lithiated phases have been predicted by an evolutionary algorithm interplayed with density functional theory calculations. Due to difficulties in obtaining useful single crystals for the BDA salts, other methods have been explored in combination with the computational approach. While the predicted structure of the pristine Na(2)BDA compound has been experimentally confirmed through the 3D Electron Diffraction (3DED) technique, the hydrated version of Li(2)BDA is analysed through single crystal X-ray diffraction. The calculated cell voltages for the sodiation (0.63 V vs. Na/Na+) and lithiation (1.12 V vs. Li/Li+) processes display excellent quantitative agreement with experimental findings. These results validate the developed theoretical methodology. Moreover, fundamental aspects of the electronic structures and their relationship with the reaction thermodynamics are discussed. The results suggest a possible disproportionation between the sodiated phases of Na(2)BDA, supporting a two-electron process, and also unveil major differences for the two employed cations: Na+ and Li+.
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| 25. |
- Renman, Viktor, et al.
(författare)
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Investigation of the Structural and Electrochemical Properties of Mn2Sb3O6CI upon Reaction with Li Ions
- 2017
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:11, s. 5949-5958
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Tidskriftsartikel (refereegranskat)abstract
- The structural and electrochemical properties of a quaternary layered compound with elemental composition Mn2Sb3O6Cl have been investigated upon reaction with lithium in Li half cells. Operando XRD was used to investigate the potential impact of this particular layered structure on the lithiation process. Although the results suggest that the material is primarily reacted through a conventional conversion mechanism, they also provide some hints that the space between the slabs may act as preferential entry points for lithium ions but not for the larger sodium ions. Cyclic voltammetry, galvanostatic cycling, HRTEM, SAED, and EELS analyses were performed to unravel the details of the reaction mechanism with the lithium ions. It is found that two pairs of reactions are mainly responsible for the reversible electrochemical cycling of this compound, namely, the alloying of Li-Sb and the conversion of MnxOy to metallic Mn with concomitant formation of Li2O upon lithium uptake. A moderate cycling stability is achieved with a gravimetric capacity of 467 mAh g(-1) after 100 cycles between 0.05 and 2.2 V vs Li+/Li despite the large particle sizes of the active material and its nonoptimal inclusion into composite coatings. The electrochemical activity of the title compound was also tested in Na half cells between 0.05 and 2 V vs Ne/Na. It was found that a prolonged period of electrochemical milling is required to fully gain access to the active material, after which the cell delivers a capacity of 350 mAh CI. These factors are demonstrated to clearly limit the ultimate performances for these electrodes.
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| 26. |
- Renman, Viktor, et al.
(författare)
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Manganese Hexacyanomanganate as a Positive Electrode for Nonaqueous Li-, Na-, and K-Ion Batteries
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 123:36, s. 22040-22049
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Tidskriftsartikel (refereegranskat)abstract
- K2Mn[Mn(CN)(6)] is synthesized, characterized, and evaluated as possible positive electrode material in nonaqueous Li-, Na-, and K-ion batteries. This compound belongs to the rich and versatile family of hexacyanometallates displaying distinctive structural properties, which makes it interesting for ion insertion purposes. It can be viewed as a perovskite-like compound in which CN-bridged Mn(CN)(6) octahedra form an open framework structure with sufficiently large diffusion channels able to accommodate a variety of insertion cations. By means of galvanostatic cycling and cyclic voltammetry tests in nonaqueous alkali metal half-cells, it is demonstrated that this material is able to reversibly host Li+, Na+, and K+ ions via electrochemical insertion/deinsertion within a wide voltage range. The general electrochemical features are similar for all of these three ion insertion chemistries. An in operando X-ray diffraction investigation indicates that the original monoclinic structure is transformed into a cubic one during charging (i.e., removal of cations from the host framework) and that such a process is reversible upon subsequent cell discharge and cation reuptake.
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| 27. |
- Renman, Viktor, et al.
(författare)
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Manganese pyrosilicates as novel positive electrode materials for Na-ion batteries
- 2018
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Ingår i: Sustainable Energy & Fuels. - : Royal Society of Chemistry (RSC). - 2398-4902. ; 2:5, s. 941-945
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Tidskriftsartikel (refereegranskat)abstract
- A carbon-coated pyrosilicate, Na2Mn2Si2O7/C, was synthesized and characterized for use as a new positive-electrode material for sodium ion batteries. The material consists of 20-80 nm primary particles embedded in a approximate to 10 nm-thick conductive carbon matrix. Reversible insertion of Na+ ions is clearly demonstrated with approximate to 25% of its theoretical capacity (165 mA h g(-1)) being accessible at room temperature at a low cycling rate. The material yields an average potential of 3.3 V vs. Na+/Na on charge and 2.2 V on discharge. DFT calculations predict an equilibrium potential for Na2Mn2Si2O7 in the range of 2.8-3.0 V vs. Na+/Na, with a possibility of a complete flip in the connectivity of neighboring Mn-polyhedra - from edge-sharing to disconnected and vice versa. This significant rearrangement in Mn coordination (approximate to 2 angstrom) and large volume contraction (>10%) could explain our inability to fully desodiate the material, and illustrates well the need for a new electrode design strategy beyond the conventional down-sizing/coating procedure.
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| 28. |
- Renman, Viktor, et al.
(författare)
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Structural-electrochemical relations in the aqueous copper hexacyanoferrate-zinc system examined by synchrotron X-ray diffraction
- 2017
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753 .- 1873-2755. ; 369, s. 146-153
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Tidskriftsartikel (refereegranskat)abstract
- The storage process of Zn2+ in the Prussian blue analogue (PBA) copper hexacyanoferrate (Cu[Fe(CN)(6)](2/3)-nH(2) O-CuHCF) framework structure in a context of rechargeable aqueous batteries is examined by means of in operando synchrotron X-ray diffraction. Via sequential unit-cell parameter refinements of time-resolved diffraction data, it is revealed that the step-profile of the cell output voltage curves during repeated electrochemical insertion and removal of Zn2+ in the CuHCF host structure is associated with a non-linear contraction and expansion of the unit-cell in the range 0.36 < x < 1.32 for Znx/3Cu[Fe(CN)(6)](2/3)-nH(2)O. For a high insertion cation content there is no apparent change in the unit-cell contraction. Furthermore, a structural analysis with respect to the occupancies of possible Zn2+ sites suggests that the Fe(CN)(6) vacancies within the CuHCF framework play an important role in the structural-electrochemical behavior of this particular system. More specifically, it is observed that Zn2+ swaps position during electrochemical cycling, hopping between cavity sites to vacant ferricyanide sites.
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| 29. |
- S. Menon, Ashok, et al.
(författare)
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Synthesis–Structure Relationships in Li- and Mn-rich Layered Oxides
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Li- and Mn-rich layered oxides are promising positive electrode materials for future Li-ion batteries. The coexistence of complex crystallographic features like cation-mixing and stacking faults make them highly susceptible to synthesis-induced crystallographic changes. Consequently, this has resulted in significant variations in the reported structure of these materials and exacerbated the difficulty in understanding the crystallography of these materials. Here, the effect of synthesis methods and annealing parameters on the average structure of three Li- and Mn-rich layered oxides—Li2MnO3, Li1.2Mn0.6Ni0.2O2 and Li1.2Mn0.54Ni0.13Co0.13O2—have been systematically investigated. Each compound is synthesized through two methods using four annealing protocols and the resultant structural changes are studied, to improve our understanding of the synthesis–structure relationships in these materials. Furthermore, synthesis-specific thermodynamic and kinetic factors governing the equilibrium crystallography of each composition are also explored. Improving our understanding of how the synthesis affects the pristine structure of these materials is an important step in developing these material systems for use as future positive electrode materials.
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| 30. |
- Shiino, Takayuki, et al.
(författare)
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Examination of the critical behavior and magnetocaloric effect of the ferromagnetic Gd-Au-Si quasicrystal approximants
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:17
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the critical behavior and magnetocaloric effects of the Gd-Au-Si (GAS) ferromagnetic quasicrystal approximants, Gd13.7 Au72.7 Si13.6 [referred to as GAS(0)] and Gd15.4 Au68.6 Si16.0 [GAS(100)]. The former is a conventional Tsai-type 1/1 approximant crystal, while the latter has a slightly different atomic decoration from the Tsai type (thus referred to as “pseudo-Tsai” type). Their critical exponents at the ferromagnetic transitions are close to those of the mean-field theory. Both GAS systems exhibit an interesting magnetic-field dependence of the specific heat, which is reflected in the behavior of their magnetocaloric effect (MCE). The MCE is characterized by an adiabatic cooling (heating) effect over a relatively broad temperature range below ∼30 K, which stems from a broad feature in the specific heat.
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| 31. |
- Shiino, Takayuki, et al.
(författare)
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Nonequilibrium dynamical behavior in noncoplanar magnets with chiral spin texture
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:18
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Tidskriftsartikel (refereegranskat)abstract
- We observe nonequilibrium dynamical magnetic behavior in the magnetically ordered phase of a Tsai-type Tb-Au-Si quasicrystal approximant system. The magnetic texture in the ordered phase is found to exhibit scalar spin chirality (SSC) order, inferring that SSC is the order parameter of the present magnetic system. We further find that the introduction of “pseudo-Tsai” clusters, associated with additional Tb atoms in the structure, induces spin-glass dynamics. We discuss the observed dynamical magnetic behavior in the Tb-Au-Si systems, considering the effect of the pseudo-Tsai clusters on the magnetic configuration and local spin chirality.
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| 32. |
- Shiino, Takayuki, et al.
(författare)
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Singular magnetic dilution behavior in a quasicrystal approximant
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:22
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Tidskriftsartikel (refereegranskat)abstract
- We report the effect of magnetic dilution on the physical properties of (Gd1−xYx)Cd6 approximant crystals (ACs), close siblings of their corresponding quasicrystal (QC). Compared to the pure system GdCd6, we observe remarkable changes in the thermodynamic and magnetic bulk properties near and below the static-ordering temperatures from diluting the magnetic Gd atoms with nonmagnetic Y atoms by only 1–3% (x=0.01–0.03). On the other hand, the corresponding QC system exhibits a monotonic change in its spin-glass behavior upon the magnetic dilution. We discuss the origin of the magnetic-dilution behavior in the present AC system in terms of possible magnetic frustration and short-range magnetic correlation that can be linked to its peculiar structure.
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| 33. |
- Yamada, Tsunetomo, et al.
(författare)
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Atomic structures of the Sc-Zn and R-Cd icosahedral quasicrystals
- 2017
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Ingår i: Acta Crystallographica Section A. - 2053-2733. ; A73:Suppl., s. C1317-C1317
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Tidskriftsartikel (refereegranskat)abstract
- We present the refinement results of atomic structures of binary icosahedral (i) ScZn7.3, GdCd7.9, DyCd7.5 and TmCd7.3 quasicrystals (QCs) [1, 2] and the study of phason modes. Bragg peak intensities data collection has been carried out on the CRISTAL beamline at the synchrotron SOLEIL, using an incoming X-ray energies equal to 25.5 keV (Sc-Zn) and 24.2 keV (R-Cd), and a CCD camera located at 8 cm from the samples. For all iQC samples a high redundancy (average 50) has been obtained resulting in 4057 (Sc-Zn) and 4871 ~ 5130 (R-Cd) unique reflections having intensity larger than 3 sigma and internal R factors around 8 %. The atomic structures of the iQCs were solved using the measured Bragg intensity data based on a six-dimensional model that is isostructural to the i-YbCd5.7 one [3] and the refinement program named qcdiff by A. Yamamoto, resulting in R factors equal to 10.9 % (Sc-Zn) and 8.9 ~ 10.9 % (R-Cd). The resulting structures are described with a quasiperiodic packing of large Tsai-type rhombic triacontahedron clusters (RTH) and double Friauf polyhedra (DFP), both resulting from a close-packing of large (Sc, R) and small (Zn, Cd) atoms. The significant difference in alloy composition between i-ScZn7.3, i-RCd~7.9 and the ideal model of i-YbCd5.7 was found to lay mainly on the DFP where one of the two large atom site (Sc, R) is replaced by a small atom (Zn, Cd) resulting in a significant distortion of the DFP. Residual disorder with relative occupancies of Sc(R)/Zn(Cd)=80/20 was also found on the icosahedral site. This illustrates that a detailed understanding of the atomic structure can now be achieved in QCs. The stabilization mechanism for these binary iQCs and the microscopic origins to explain the phason fluctuations will be discussed in this presentation. (1) Canfield, P. C., Caudle, M. L., Ho, C. S., Kreyssig, A., Nandi, S., Kim, M. G., Lin, X., Kracher, A., Dennis, K. W., McCallum, R. W. & Goldman, A. I. (2010) Phys. Rev. B, Solution growth of a binary icosahedral quasicrystal of Sc12Zn88, 81, 020201. (2) Goldman, A. I., Kong, T., Kreyssig, A., Jesche, A., Ramazanoglu, M., Dennis, K. W., et al. (2013). Nature Materials, A family of binary magnetic icosahedral quasicrystals based on rare earths and cadmium, 12(8), 714–718. (3) Takakura, H., Gómez, C. P., Yamamoto, A., de Boissieu, M., & Tsai, A. P. (2006). Nature Materials, Atomic structure of the binary icosahedral Yb–Cd quasicrystal, 6(1), 58–63.
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| 34. |
- Yamada, Tsunetomo, et al.
(författare)
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Diffuse scattering and phason modes in the Zn-Sc icosahedral quasicrystal
- 2012
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Konferensbidrag (refereegranskat)abstract
- Recently, a new binary icosahedral quasicrystal Zn-Sc has been obtained by Canfield et. al. [1]. Because of the chemical order and the x-ray contrast between Zn and Sc, this phase is a nice system for the structure refinement. On the other hand, a large amount of diffuse scattering can be seen on the x-ray diffraction pattern [1]. In this study, we carried out an absolute scale measurement of the x-ray diffuse scattering of i-ZnSc to study the possible presence of phason modes (phason diffuse scattering) and estimate phason elastic constats K1 and K2. Millimeter size single grain of i-ZnSc was obtained by slowly cooling from the melt. Systematic Q-scans and diffuse scattering maps have been measured on the D2AM beamline (ESRF) using an incoming x-ray energy equal to 9.3 keV. Compared with i-ZnMgSc quasicrystal [2], we find that the amount of diffuse scattering is larger in i-ZnSc sample. Also, the maximal Qperp value necessary for indexing the diffraction pattern was found to be less than 3 (r.l.u.) i.e. much smaller than for i-ZnMgSc for which it was found to be 7 [2]. In addition, as for other quasicrystals [3] a characteristic diffuse intensity distribution due to phason fluctuations around strong Bragg reflections is clearly visible on the systematic reciprocal space map. The ratio K2/K1 is found to be close to the three-fold instability limit, which results in the strong elongation of the diffuse scattering along directions parallel to a three-fold axis. Finally, the simulation is carried out based on the elastic theory and reproduces well the observed anisotropic shape of the diffuse scattering. [1] P. C. Canfield, et. al., Phys. Rev. B, 2010, 81, 020201. [2] M. de Boissieu, et. al., Phys. Rev. Lett., 2005, 95, 105503. [3] T. Janssen, et. al., Aperiodic Crystals. Oxford University Press, 2007
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| 35. |
- Yamada, Tsunetomo, et al.
(författare)
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Diffuse Scattering in the Zn6Sc 1/1 cubic approximant.
- 2010
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Konferensbidrag (refereegranskat)abstract
- The low-temperature phase transition of the Zn6Sc cubic 1/1 approximant [1] has been investigated by X-ray diffraction. It is a crystalline approximant to a recently discovered quasicrystal Zn88Sc12 [2] and has a bcc lattice composed of a Tsai-type cluster with an orientationally disordered Zn tetrahedra at the centers [3]. The phase transition undergoes below 160K and has been attributed to the orientational ordering of the Zn tetrahedra along [110] direction of hightemperature phase [1, 4]. In order to obtain an insight into the orientational ordering occurring at the phase transition, diffuse scattering has been measured. Well above the phase transition at 220K (i.e. 60 K above) we observed a broad diffuse scattering intensity which sharpen and merges into a Bragg peak as the temperature decreases and reaches the transition temperature (Tc). This signature of short-range ordering above Tc will be discussed in details and compared to refined average structures obtained between Tc and room temperature by measuring Bragg peaks. [1] R. Tamura et al., Phys. Rev. B 71, 092203, (2005). [2] P. C. Canfield et al., Phy. Rev. B 81, 020201(R) (2010). [3] Q. Lin and J. D. Corbett, Inorg. Chem., 43, 1912, (2004). [4] T. Ishimasa et al., Phil. Mag., 87, 2887, (2007).
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| 36. |
- Yanis, Sassi, et al.
(författare)
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Structure of the Au4.85Si1.324Ho1.04(100) surface : an experimental and theoretical study
- 2018
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Konferensbidrag (refereegranskat)abstract
- The Au4.85Si1.324Ho1.04 compound belongs to the family of the Au-SM-REquasicrystalline approximants (SM=Si,Ge and RE=rare earth), which unveiled longrange magnetic ordering at low temperatures [1]. Its structure (Im3 space group, a=14.55 Å) is described by a stacking of Tsai-type clusters (Fig. 1).The Au4.85Si1.324Ho1.04(100) surface was studied by a combination of surface sciencetechniques and theoretical calculations based on the Density Functional Theory. Weused a structural model for the bulk crystal adapted from the one deduced from XRay Diffraction, without any partial occupancy nor dynamical effects. The cohesiveenergies of several configurations were calculated.Experimentally, the (100) surface was prepared by cycles of sputtering and annealing.The surface presents a terrace and step morphology with a unique step height equal tohalf the cell parameter, showing a surface plane selection. Low Energy ElectronDiffraction identifies a reconstruction for the surface annealed at 853 K, while a (1x1)pattern is visible for the surface annealed at 798 K.Several models were built to determine the Au4.85Si1.324Ho1.04(100) surface structure.Their surface energies were calculated, highlighting 3 possible surface models. Thecorresponding Scanning Tunneling Microscopy images were simulated. Finally, ourresults are discussed and compared to the experimental observations.
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