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Häggström, Lennart
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Publications (10 of 36) Show all publications
Kjmer, K. S., Kaul, N., Prakash, O., Chabera, P., Rosemann, N. W., Honarfar, A., . . . Wärnmark, K. (2019). Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime. Science, 363(6424), 249-253
Open this publication in new window or tab >>Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime
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2019 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 363, no 6424, p. 249-253Article in journal (Refereed) Published
Abstract [en]

Iron's abundance and rich coordination chemistry are potentially appealing features for photochemical applications. However, the photoexcitable charge-transfer states of most iron complexes are limited by picosecond or subpicosecond deactivation through low-lying metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of photoluminescence. In this study, we show that octahedral coordination of iron(Ill) by two mono-anionic facial tris-carbene ligands can markedly suppress such deactivation. The resulting complex [Fe(phtmeimb)(2)](+), where phtmeimb is {phenyl[tris(3-methylimidazol-1-ylidene)]borate}(-), exhibits strong, visible, room temperature photoluminescence with a 2.0-nanosecond lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal charge-transfer ((LMCT)-L-2) state to the doublet ground state. Reductive and oxidative electron-transfer reactions were observed for the (2)LMCTstate of [Fe(phtmeimb)(2)](+) in bimolecular quenching studies with methylviologen and diphenylamine.

Place, publisher, year, edition, pages
AMER ASSOC ADVANCEMENT SCIENCE, 2019
National Category
Theoretical Chemistry Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-376883 (URN)10.1126/science.aau7160 (DOI)000456140700029 ()30498167 (PubMedID)
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilSwedish Energy AgencyKnut and Alice Wallenberg FoundationSwedish National Infrastructure for Computing (SNIC)Stiftelsen Olle Engkvist ByggmästareCarl Tryggers foundation Wenner-Gren FoundationsThe Crafoord Foundation
Available from: 2019-02-12 Created: 2019-02-12 Last updated: 2019-02-12Bibliographically approved
Cedervall, J., Andersson, M., Delczeg-Czirjak, E. K., Iusan, D., Pereiro, M., Roy, P., . . . Deen, P. P. (2019). Magnetocaloric effect in Fe2P: Magnetic and phonon degrees of freedom. Physical Review B, 99(17), Article ID 174437.
Open this publication in new window or tab >>Magnetocaloric effect in Fe2P: Magnetic and phonon degrees of freedom
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 17, article id 174437Article in journal (Refereed) Published
Abstract [en]

Devices based on magnetocaloric materials provide great hope for environmentally friendly and energy efficient cooling that does not rely on the use of harmful gasses. Fe2P based compounds are alloys that have shown great potential for magnetocaloric devices. The magnetic behavior in Fe2P is characterized by a strong magnetocaloric effect that coexists with a first-order magnetic transition (FOMT). Neutron diffraction and inelastic scattering, Mossbauer spectroscopy, and first-principles calculations have been used to determine the structural and magnetic state of Fe2P around the FOMT. The results reveal that ferromagnetic moments in the ordered phase are perturbed at the FOMT such that the moments cant away from the principle direction within a small temperature region. The acoustic-phonon modes reveal a temperature-dependent nonzero energy gap in the magnetically ordered phase that falls to zero at the FOMT. The interplay between the FOMT and the phonon energy gap indicates hybridization between magnetic modes strongly affected by spin-orbit coupling and phonon modes leading to magnon-phonon quasiparticles that drive the magnetocaloric effect.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-387585 (URN)10.1103/PhysRevB.99.174437 (DOI)000469324500011 ()
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research , EM16-0039
Available from: 2019-06-26 Created: 2019-06-26 Last updated: 2019-06-26Bibliographically approved
Somerville, J. W., Sobkowiak, A., Tapia-Ruiz, N., Billaud, J., Lozano, J. G., House, R. A., . . . Bruce, P. G. (2019). Nature of the "Z"-phase in layered Na-ion battery cathodes. Energy & Environmental Science, 12(7), 2223-2232
Open this publication in new window or tab >>Nature of the "Z"-phase in layered Na-ion battery cathodes
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2019 (English)In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706, Vol. 12, no 7, p. 2223-2232Article in journal (Refereed) Published
Abstract [en]

Layered sodium transition metal oxides with the P2 structure, e.g. Na-2/3[Ni1/3Mn2/3]O-2, are regarded as candidates for Na-ion battery cathodes. On charging, extraction of Na destabilizes the P2 phase (ABBA oxide ion stacking) in which Na+ is in trigonal prismatic coordination, resulting in layer gliding and formation of an O2 phase (ABAC stacking) with octahedral coordination. However, many related compounds do not exhibit such a simple P2 to O2 transition but rather form a so called Z-phase. Substituting Ni by Fe in Na-2/3[Ni1/3Mn2/3]O-2 is attractive as it reduces cost. The Fe containing compounds, such as Na-2/3[Ni1/6Mn1/2Fe1/3]O-2, form a Z-phase when charged above 4.1 V vs. Na+/Na. By combining ex situ and operando X-ray diffraction with scanning transmission electron microscopy and simulated diffraction patterns, we demonstrate that the Z-phase is most accurately described as a continuously changing intergrowth structure which evolves from P2 to O2 through the OP4 structure as an intermediate. On charging, Na+ removal results in O-type stacking faults within the P2 structure which increase in proportion. At 50% O-type stacking faults, the ordered OP4 phase forms and on further charging more O-type stacking faults are formed progressing towards a pure O2 structure. This gives the superficial appearance of a solid solution. Furthermore, in contrast to some previous studies, we did not detect Fe migration at any state-of-charge using Fe-57-Mossbauer spectroscopy. It was, however, found that the Fe-substitution serves to disrupt cation ordering in the material.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY, 2019
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-392868 (URN)10.1039/c8ee02991a (DOI)000477950000008 ()
Available from: 2019-09-26 Created: 2019-09-26 Last updated: 2019-09-26Bibliographically approved
Cedervall, J., Nonnet, E., Hedlund, D., Häggström, L., Ericsson, T., Werwinski, M., . . . Sahlberg, M. (2018). Influence of cobalt substitution on the magnetic properties of Fe5PB2. Inorganic Chemistry, 57(2), 777-784
Open this publication in new window or tab >>Influence of cobalt substitution on the magnetic properties of Fe5PB2
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2018 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 57, no 2, p. 777-784Article in journal (Refereed) Published
Abstract [en]

In this study the effects of cobalt substitutions in Fe5PB2 have been studied. An increased cobalt content reduces the magnetic exchange interactions. This has been concluded from a large, linear decrease in both the Curie temperature as well as the saturated magnetic moment. At high cobalt concentrations, cobalt prefers to order at the M(2) position in the crystal structure. A tunable Curie transition like this shows some prerequisites for magnetic cooling applications.

The substitutional effects of cobalt in (Fe1–xCox)5PB2 have been studied with respect to crystalline structure and chemical order with X-ray diffraction and Mössbauer spectroscopy. The magnetic properties have been determined from magnetic measurements, and density functional theory calculations have been performed for the magnetic properties of both the end compounds, as well as the chemically disordered intermediate compounds. The crystal structure of (Fe1–xCox)5PB2 is tetragonal (space group I4/mcm) with two different metal sites, with a preference for cobalt atoms in the M(2) position (4c) at higher cobalt contents. The substitution also affects the magnetic properties with a decrease of the Curie temperature (TC) with increasing cobalt content, from 622 to 152 K for Fe5PB2 and (Fe0.3Co0.7)5PB2, respectively. Thus, the Curie temperature is dependent on composition, and it is possible to tune TC to a temperature near room temperature, which is one prerequisite for magnetic cooling materials.

National Category
Inorganic Chemistry Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-331758 (URN)10.1021/acs.inorgchem.7b02663 (DOI)000422810900030 ()29298054 (PubMedID)
Funder
Swedish Research CouncilSwedish Energy AgencyEuropean Regional Development Fund (ERDF)
Available from: 2017-10-19 Created: 2017-10-19 Last updated: 2018-10-19Bibliographically approved
Chabera, P., Liu, Y., Prakash, O., Thyrhaug, E., El Nahhas, A., Honarfar, A., . . . Warnmark, K. (2017). A low-spin Fe(III) complex with 100-ps ligand-to-metal charge transfer photoluminescence. Nature, 543(7647), 695-+
Open this publication in new window or tab >>A low-spin Fe(III) complex with 100-ps ligand-to-metal charge transfer photoluminescence
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2017 (English)In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 543, no 7647, p. 695-+Article in journal (Refereed) Published
Abstract [en]

Transition-metal complexes are used as photosensitizers(1), in light-emitting diodes, for biosensing and in photocatalysis(2). A key feature in these applications is excitation from the ground state to a charge-transfer state(3,4); the long charge-transfer-state lifetimes typical for complexes of ruthenium(5) and other precious metals are often essential to ensure high performance. There is much interest in replacing these scarce elements with Earth-abundant metals, with iron(6) and copper(7) being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs(6,8-10), it remains a formidable scientific challenge(11) to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered(12) photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers(13-15). Here we present the iron complex [Fe(btz)(3)](3+) (where btz is 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)), and show that the superior sigma-donor and pi-acceptor electron properties of the ligand stabilize the excited state sufficiently to realize a long charge-transfer lifetime of 100 picoseconds (ps) and room-temperature photoluminescence. This species is a low-spin Fe(III) d(5) complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer ((LMCT)-L-2) state that is rarely seen for transition-metal complexes(4,16,17). The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces. These findings suggest that appropriate design strategies can deliver new iron-based materials for use as light emitters and photosensitizers.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2017
National Category
Chemical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-320033 (URN)10.1038/nature21430 (DOI)000397619700051 ()28358064 (PubMedID)
Funder
Knut and Alice Wallenberg FoundationSwedish Energy AgencyThe Crafoord FoundationSwedish National Infrastructure for Computing (SNIC)Stiftelsen Olle Engkvist Byggmästare
Available from: 2017-04-13 Created: 2017-04-13 Last updated: 2017-04-18Bibliographically approved
Cedervall, J., Häggström, L., Ericsson, T. & Sahlberg, M. (2016). Mössbauer study of the magnetocaloric compound AlFe2B2. Paper presented at International Conference on the Applications of the Mössbauer Effect (ICAME 2015), 13-18 September 2015, Hamburg, Germany. Hyperfine Interactions, 237, Article ID 47.
Open this publication in new window or tab >>Mössbauer study of the magnetocaloric compound AlFe2B2
2016 (English)In: Hyperfine Interactions, ISSN 0304-3843, E-ISSN 1572-9540, Vol. 237, article id 47Article in journal (Refereed) Published
Abstract [en]

Mössbauer spectroscopy in the ferromagnetic AlFe2B2 reveals Tc=299 K and shows good agreement with magnetic measurements. The crystals are plate-shaped. The flakes are found from X-ray diffraction to be in the crystallographic ac-plane in the orthorhombic system. The axes of the principle electric field gradient tensor are, by symmetry, colinear with the crystal a-, b- and c-axes. By using information about the quadrupole splitting and line asymmetry in the paramagnetic regime together with the quadrupole shift of the resonance lines in the ferromagnetic regime the magnetic hyperfine field direction is found to be in the ab-plane having an angle =40° to the b-axis.

National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-312463 (URN)10.1007/s10751-016-1223-7 (DOI)000372730100020 ()
Conference
International Conference on the Applications of the Mössbauer Effect (ICAME 2015), 13-18 September 2015, Hamburg, Germany
Available from: 2017-01-10 Created: 2017-01-10 Last updated: 2018-04-09Bibliographically approved
Ojwang, D. O., Grins, J., Wardecki, D., Valvo, M., Renman, V., Häggström, L., . . . Svensson, G. (2016). Structure Characterization and Properties of K-Containing Copper Hexacyanoferrate. Inorganic Chemistry, 55(12), 5924-5934
Open this publication in new window or tab >>Structure Characterization and Properties of K-Containing Copper Hexacyanoferrate
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2016 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 55, no 12, p. 5924-5934Article in journal (Refereed) Published
Abstract [en]

Copper hexacyanoferrate, Cu-II[Fe-III(CN)(6)](2/3)center dot nH(2)O, was synthesized, and varied amounts of IC ions were inserted via reduction by K2S2O3 (aq). Ideally, the reaction can be written as Cu-II[Fe-III(CN)(6)](2/3)-nH(2)O + 2x/3K(+) + 2x/3e(-)K(+) <-> K-2x/3 Cu-II[Fe-x(II).Fe-1-x(II),(CN)(6)](2/3)-nH(2)O. Infrared, Raman, and Mossbauer spectroscopy studies show that Fe-II is continuously reduced to Fell with increasing x, accompanied by a decrease of the a-axis of the cubic Fn (3) over barm unit cell. Elemental analysis of K by inductively coupled plasma shows that the insertion only begins when a significant fraction similar to 10% of the Fe-III, has already been reduced. Thermogravimetric analysis shows a fast exchange of water with ambient atmosphere and a total weight loss of similar to 26 wt % upon heating to 180 degrees C, above which the structure starts to decompose. The crystal structures of Cu-III[Fe-III(CN)(6)](2/3)center dot nH(2)O and K2/3Cu[Fe(CN)(6)](2/3)center dot nH(2)O were refined using synchrotron X-ray powder diffraction data. In both, one-third of the Fe(CN)(6) groups are vacant, and the octahedron around Cull is completed by water molecules. In the two structures, difference Fourier maps reveal three additional zeolitic water sites (8c, 32f, and 48g) in the center of the cavities formed by the-Cu-N-C-Fe- framework. The K-containing compound shows an increased electron density at two of these sites (32f and 48g), indicating them to be the preferred positions for the K+ ions.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-299716 (URN)10.1021/acs.inorgchem.6b00227 (DOI)000378369900028 ()27258790 (PubMedID)
Funder
Swedish Research Council
Available from: 2016-07-26 Created: 2016-07-26 Last updated: 2017-11-28Bibliographically approved
Blidberg, A., Häggström, L., Ericsson, T., Tengstedt, C., Gustafsson, T. & Björefors, F. (2015). Structural and Electronic Changes in Li2FeP2O7 during Electrochemical Cycling. Chemistry of Materials, 27(11), 3801-3804
Open this publication in new window or tab >>Structural and Electronic Changes in Li2FeP2O7 during Electrochemical Cycling
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2015 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 27, no 11, p. 3801-3804Article in journal (Refereed) Published
National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:uu:diva-258346 (URN)10.1021/acs.chemmater.5b00440 (DOI)000356202800004 ()
Available from: 2015-07-13 Created: 2015-07-13 Last updated: 2017-12-04Bibliographically approved
Sobkowiak, A., Ericsson, T., Edström, K., Gustafsson, T., Björefors, F. & Häggström, L. (2014). A Mössbauer spectroscopy study of polyol synthesized tavorite LiFeSO4F.. Paper presented at Proceedings of the 32nd International Conference on the Applications of the Mössbauer Effect (ICAME 2013) held in Opatija, Croatia, 1–6 September 2013.. Hyperfine Interactions, 226(1-3), 229-236
Open this publication in new window or tab >>A Mössbauer spectroscopy study of polyol synthesized tavorite LiFeSO4F.
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2014 (English)In: Hyperfine Interactions, ISSN 0304-3843, E-ISSN 1572-9540, ISSN 0304-3843, Vol. 226, no 1-3, p. 229-236Article in journal (Refereed) Published
Abstract [en]

The tavorite polymorph of LiFeSO4F has attracted considerable attention as a cathode material for lithium ion batteries due to interesting structural and electrochemical characteristics. For the analysis of such iron-based electrode materials, Mössbauer spectroscopy has become an important and highly useful tool. In this work, we perform a detailed Mössbauer study of pristine tavoriteLiFeSO4F prepared by an optimized synthesis in tetraethylene glycol as reaction media. In contrast to many reported results, we demonstrate the use of an asymmetric fitting model for the inner doublet of the spectrum, which is coupled to the structural properties of the compound. Moreover, we discuss a new approach of ascribing the Fe2 + -doublets to the two distinct crystallographic iron sites of tavorite LiFeSO4F by comparing the Mössbauer signal intensities with the expected f-factors for the corresponding iron atom.

Keywords
lithium ion battery, tavorite LiFeSO4F, Mössbauer spectroscopy
National Category
Materials Chemistry
Research subject
Physics with specialization in Nuclear Physics; Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-241039 (URN)10.1007/s10751-013-0935-1 (DOI)
Conference
Proceedings of the 32nd International Conference on the Applications of the Mössbauer Effect (ICAME 2013) held in Opatija, Croatia, 1–6 September 2013.
Funder
StandUp
Available from: 2015-01-08 Created: 2015-01-08 Last updated: 2017-12-30
Sobkowiak, A., Roberts, M. R., Häggström, L., Ericsson, T., Andersson, A. M., Edström, K., . . . Björefors, F. (2014). Identification of an Intermediate Phase, Li1/2FeSO4F, Formed during Electrochemical Cycling of Tavorite LiFeSO4F. Chemistry of Materials, 26(15), 4620-4628
Open this publication in new window or tab >>Identification of an Intermediate Phase, Li1/2FeSO4F, Formed during Electrochemical Cycling of Tavorite LiFeSO4F
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2014 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 26, no 15, p. 4620-4628Article in journal (Refereed) Published
Abstract [en]

Many compounds adopting the tavorite-type crystal structure have attracted considerable attention as cathode materials for lithium ion batteries due to the favorable structural characteristics, facilitating promising electrochemical performance. Recent reports have highlighted the complex mechanism of lithium insertion/extraction in some of these compounds, such as the stabilization of intermediate phases in the LiFeSO4OH and LiVPO4F systems. In the case of tavorite LiFeSO4F, reported density functional theory (DFT) calculations have suggested the possibility of a similar behavior, but thus far, no experimental verification of such a process has, to the best of our knowledge, been successfully demonstrated. In this work, we investigate the structural evolution of LiFeSO4F upon extraction/insertion of lithium ions from/into the host framework. By thorough ex situ characterizations of chemically and electrochemically prepared LixFeSO4F-samples (0 ≤ x ≤ 1), we demonstrate the stabilization of an intermediate phase, Li1/2FeSO4F, for which one possible structural model is proposed. However, results indicating charge ordering on the iron-sites, suggesting the formation of a super structure with a larger unit cell, are also highlighted. Moreover, the degree of formation of Li1/2FeSO4F is shown to be highly dependent on the rate of lithium extraction as a result of an exceptionally small potential separation (similar to 15 mV during charging) of the two subsequently occurring biphasic processes, LiFeSO4F/Li1/2FeSO4F and Li1/2FeSO4F/FeSO4F. Finally, the intermediate phase is shown to be formed both on charge and discharge during battery cycling, even though an apparent asymmetrical electrochemical trace suggests the contrary.

National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-232001 (URN)10.1021/cm502104q (DOI)000340346300038 ()
Funder
StandUpVINNOVA, HVV
Available from: 2014-09-15 Created: 2014-09-12 Last updated: 2017-12-30
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