| 1. |
- Ganegoda, Hasitha, et al.
(författare)
-
Role of Fe Doping on Local Structure and Electrical and Magnetic Properties of PbTiO3
- 2021
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:22, s. 12342-12354
-
Tidskriftsartikel (refereegranskat)abstract
- The local structure and multiferroic properties of iron-doped lead titanate (PbTi1-xFexO3-delta) samples was investigated over the entire composition range (x = 0-1). Inherent polarization in PbTiO3 decreases due to Fe3+ incorporation up to the solubility limit (x similar to 0.3), although homogeneous doping persists only up to x = 0.1. Ti prefers highly distorted oxygen octahedra for any x value, while Fe prefers more symmetric O-deficient polyhedra (Fe-O-n). The charge compensating oxygen vacancies induce local tilting of the Fe-O-n polyhedra beyond a critical x value (x >= 0.2), promoting magnetic interaction between two adjacent Fe atoms. The strain induced by local heterogeneity could act as a coupling force between magnetic and ferroelectric properties. Fe-rich clusters evolve into ferromagnetic PbFe12O19 with increased Fe doping. PbTi1-xFexO3-delta ( x >= 0.3) samples therefore have separate origins for the ferroelectric (PbTi1-xFexO3-delta) and magnetic (PbFe12O19) phases.
|
|
| 2. |
- Hirsbrunner, Moritz, et al.
(författare)
-
Vibrationally-resolved RIXS reveals OH-group formation in oxygen redox active Li-ion battery cathodes
- 2024
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 26:28, s. 19460-19468
-
Tidskriftsartikel (refereegranskat)abstract
- Vibrationally-resolved resonant inelastic X-ray scattering (VR-RIXS) at the O K-edge is emerging as a powerful tool for identifying embedded molecules in lithium-ion battery cathodes. Here, we investigate two known oxygen redox-active cathode materials: the commercial LixNi0.90Co0.05Al0.05O2 (NCA) used in electric vehicles and the high-capacity cathode material Li1.2Ni0.13Co0.13Mn0.54O2 (LRNMC) for next-generation Li-ion batteries. We report the detection of a novel vibrational RIXS signature for Li-ion battery cathodes appearing in the O K pre-peak above 533 eV that we attribute to OH-groups. We discuss likely locations and pathways for OH-group formation and accumulation throughout the active cathode material. Initial-cycle behaviour for LRNMC shows that OH-signal strength correlates with the cathodes state of charge, though reversibility is incomplete. The OH-group RIXS signal strength in long-term cycled NCA is retained. Thus, VR-RIXS offers a path for gaining new insights to oxygen reactions in battery materials.
|
|
| 3. |
- Isaeva, Leyla, et al.
(författare)
-
Amorphous W-S-N thin films: the atomic structure behind ultra-low friction
- 2015
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 82, s. 84-93
-
Tidskriftsartikel (refereegranskat)abstract
- Amorphous W–S–N in the form of thin films has been identified experimentally as an ultra-low friction material, enabling easy sliding by the formation of a WS2 tribofilm. However, the atomic-level structure and bonding arrangements in amorphous W–S–N, which give such optimum conditions for WS2 formation and ultra-low friction, are not known. In this study, amorphous thin films with up to 37 at.% N are deposited, and experimental as well as state-of-the-art ab initio techniques are employed to reveal the complex structure of W–S–N at the atomic level. Excellent agreement between experimental and calculated coordination numbers and bond distances is demonstrated. Furthermore, the simulated structures are found to contain N bonded in molecular form, i.e. N2, which is experimentally confirmed by near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy analysis. Such N2 units are located in cages in the material, where they are coordinated mainly by S atoms. Thus this ultra-low friction material is shown to be a complex amorphous network of W, S and N atoms, with easy access to W and S for continuous formation of WS2 in the contact region, and with the possibility of swift removal of excess nitrogen present as N2 molecules.
|
|
| 4. |
- Jana, Somnath, et al.
(författare)
-
Charge disproportionate antiferromagnetism at the verge of the insulator-metal transition in doped LaFeO3
- 2019
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 99:7
-
Tidskriftsartikel (refereegranskat)abstract
- We explore the effects of electron doping in lanthanum ferrite, LaFeO3 by doping Mo at the Fe sites. Based on magnetic, transport, scanning tunneling spectroscopy, and x-ray photoelectron spectroscopy measurements, we find that the large gap, charge-transfer, antiferromagnetic (AFM) insulator LaFeO3 becomes a small gap AFM band insulator at low Mo doping. With increasing doping concentration, Mo states, which appear around the Fermi level, is broadened and become gapless at a critical doping of 20%. Using a combination of calculations based on density functional theory plus Hubbard U (DFT+U) and x-ray absorption spectroscopy measurements, we find that the system shows charge disproportionation (CD) in Fe ions at 25% Mo doping, where two distinct Fe sites, having Fe2+ and Fe3+ nominal charge states appear. A local breathing-type lattice distortion induces the charge disproportionation at the Fe site without destroying the antiferromagnetic order. Our combined experimental and theoretical investigations establish that the Fe states form a CD antiferromagnet at 25% Mo doping, which remains insulating, while the appearance of Mo states around the Fermi level is showing an indication towards the insulator-metal transition.
|
|
| 5. |
- Jana, Somnath, et al.
(författare)
-
Doping induced site-selective Mott insulating phase in LaFeO3
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Tailoring transport properties of strongly correlated electron systems in a controlled fashion counts among the dreams of materials scientists. In copper oxides, vary- ing the carrier concentration is a tool to obtain high- temperature superconducting phases. In manganites, dop- ing results in exotic physics such as insulator-metal tran- sitions (IMT), colossal magnetoresistance (CMR), orbital- or charge-ordered (CO) or charge-disproportionate (CD) states. In most oxides, antiferromagnetic order and CD phase is asssociated with insulating behavior. Here we re- port the realization of a unique physical state that can be induced by Mo doping in LaFeO3: the resulting metallic state is a site-selective Mott insulator where itinerant elec- trons evolving on low-energy Mo states coexist with local- ized carriers on the Fe sites. In addition, a local breathing- type lattice distortion induces charge disproportionation on the latter, without destroying the antiferromagnetic order. A state, combining antiferromangetism, metallic- ity and CD phenomena is rather rare in oxides and have utmost significance for future antiferromagnetic memory devices.
|
|
| 6. |
- Johansson, Malin B, 1972-, et al.
(författare)
-
Cesium Bismuth Iodide Solar Cells from Systematic Molar Ratio Variation of CsI and BiI3
- 2019
-
Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 58:18, s. 12040-12052
-
Tidskriftsartikel (refereegranskat)abstract
- Metal halide compounds with photovoltaic properties prepared from solution have received increased attention for utilization in solar cells. In this work, low-toxicity cesium bismuth iodides are synthesized from solution, and their photovoltaic and, optical properties as well as electronic and crystal structures are investigated. The X-ray diffraction patterns reveal that a CsI/BiI3 precursor ratio of 1.5:1 can convert pure rhombohedral BiI3 to pure hexagonal Cs3Bi2I9, but any ratio intermediate of this stoichiometry and pure BiI3 yields a mixture containing the two crystalline phases Cs3Bi2I9 and BiI3, with their relative fraction depending on the CsI/BiI3 ratio. Solar cells from the series of compounds are characterized, showing the highest efficiency for the compounds with a mixture of the two structures. The energies of the valence band edge were estimated using hard and soft X-ray photoelectron spectroscopy for more bulk and surface electronic properties, respectively. On the basis of these measurements, together with UV-vis-near-IR spectrophotometry, measuring the band gap, and Kelvin probe measurements for estimating the work function, an approximate energy diagram has been compiled clarifying the relationship between the positions of the valence and conduction band edges and the Fermi level.
|
|
| 7. |
- Man, Gabriel J., et al.
(författare)
-
A-site cation influence on the conduction band of lead bromide perovskites
- 2022
-
Ingår i: Nature Communications. - : Nature Research. - 2041-1723. ; 13:1
-
Tidskriftsartikel (refereegranskat)abstract
- Hot carrier solar cells hold promise for exceeding the Shockley-Queisser limit. Slow hot carrier cooling is one of the most intriguing properties of lead halide perovskites and distinguishes this class of materials from competing materials used in solar cells. Here we use the element selectivity of high-resolution X-ray spectroscopy and density functional theory to uncover a previously hidden feature in the conduction band states, the sigma-pi energy splitting, and find that it is strongly influenced by the strength of electronic coupling between the A-cation and bromide-lead sublattice. Our finding provides an alternative mechanism to the commonly discussed polaronic screening and hot phonon bottleneck carrier cooling mechanisms. Our work emphasizes the optoelectronic role of the A-cation, provides a comprehensive view of A-cation effects in the crystal and electronic structures, and outlines a broadly applicable spectroscopic approach for assessing the impact of chemical alterations of the A-cation on perovskite electronic structure. The A-cation influence on the mechanism of slow hot carrier cooling in perovskites is controversial. Here, Man et al. resolve a debated issue regarding A-cation influence on the electronic structure of lead halide perovskites.
|
|
| 8. |
|
|
| 9. |
- Mikheenkova, Anastasiia, et al.
(författare)
-
The Role of Oxygen in Automotive Grade Lithium-Ion Battery Cathodes : An Atomistic Survey of Ageing
- 2024
-
Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488 .- 2050-7496. ; 12:4, s. 2465-2478
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The rising demand for high-performance lithium-ion batteries, pivotal to electric transportation, hinges on key materials like the Ni-rich layered oxide LiNixCoyAlzO2 (NCA) used in cathodes. The present study investigates the redox mechanisms, with particular focus on the role of oxygen in commercial NCA electrodes, both fresh and aged under various conditions (aged cells have performed >900 cycles until a cathode capacity retention of ∼80%). Our findings reveal that oxygen participates in charge compensation during NCA delithiation, both through changes in transition metal (TM)–O bond hybridization and formation of partially reversible O2, the latter occurs already below 3.8 V vs. Li/Li+. Aged NCA material undergoes more significant changes in TM–O bond hybridization when cycling above 50% SoC, while reversible O2 formation is maintained. Nickel is found to be redox active throughout the entire delithiation and shows a more classical oxidation state change during cycling with smaller changes in the Ni–O hybridization. By contrast, Co redox activity relies on a stronger change in Co–O hybridization, with only smaller Co oxidation state changes. The Ni–O bond displays an almost twice as large change in its bond length on cycling as the Co–O bond. The Ni–O6 octahedra are similar in size to the Co–O6 octahedra in the delithiated state, but are larger in the lithiated state, a size difference that increases with battery ageing. These contrasting redox activities are reflected directly in structural changes. The NCA material exhibits the formation of nanopores upon ageing, and a possible connection to oxygen redox activity is discussed. The difference in interaction of Ni and Co with oxygen provides a key understanding of the mechanism and the electrochemical instability of Ni-rich layered transition metal oxide electrodes. Our research specifically highlights the significance of the role of oxygen in the electrochemical performance of electric-vehicle-grade NCA electrodes, offering important insights for the creation of next-generation long-lived lithium-ion batteries.
|
|
| 10. |
- Mikheenkova, Anastasiia, et al.
(författare)
-
The role of oxygen in automotive grade lithium-ion battery cathodes : an atomistic survey of ageing
- 2024
-
Ingår i: Journal of Materials Chemistry A. - 2050-7488 .- 2050-7496. ; 12:4, s. 2465-2478
-
Tidskriftsartikel (refereegranskat)abstract
- The rising demand for high-performance lithium-ion batteries, pivotal to electric transportation, hinges on key materials like the Ni-rich layered oxide LiNixCoyAlzO2 (NCA) used in cathodes. The present study investigates the redox mechanisms, with particular focus on the role of oxygen in commercial NCA electrodes, both fresh and aged under various conditions (aged cells have performed >900 cycles until a cathode capacity retention of ∼80%). Our findings reveal that oxygen participates in charge compensation during NCA delithiation, both through changes in transition metal (TM)–O bond hybridization and formation of partially reversible O2, the latter occurs already below 3.8 V vs. Li/Li+. Aged NCA material undergoes more significant changes in TM–O bond hybridization when cycling above 50% SoC, while reversible O2 formation is maintained. Nickel is found to be redox active throughout the entire delithiation and shows a more classical oxidation state change during cycling with smaller changes in the Ni–O hybridization. By contrast, Co redox activity relies on a stronger change in Co–O hybridization, with only smaller Co oxidation state changes. The Ni–O bond displays an almost twice as large change in its bond length on cycling as the Co–O bond. The Ni–O6 octahedra are similar in size to the Co–O6 octahedra in the delithiated state, but are larger in the lithiated state, a size difference that increases with battery ageing. These contrasting redox activities are reflected directly in structural changes. The NCA material exhibits the formation of nanopores upon ageing, and a possible connection to oxygen redox activity is discussed. The difference in interaction of Ni and Co with oxygen provides a key understanding of the mechanism and the electrochemical instability of Ni-rich layered transition metal oxide electrodes. Our research specifically highlights the significance of the role of oxygen in the electrochemical performance of electric-vehicle-grade NCA electrodes, offering important insights for the creation of next-generation long-lived lithium-ion batteries.
|
|
