Lennart Häggström

Lithium extraction/insertion in LiFePO4: an X-ray diffraction and Mossbauer spectroscopy study

Abstract The extraction and insertion of lithium in solid-state synthesized LiFePO 4 has been followed by in situ X-ray diffraction and Mossbauer spectroscopy in ‘coffee-bag’ cells of type 〈Li-metal | liq. el. | LiFePO 4 〉 during the first cycle. Two-phase Rietveld refinement of the X-ray diffractograms gives the triphylite (LiFePO 4 ) to heterosite (FePO 4 ) phase-ratios as charging and discharging of the cell proceeds. The Fe 3+ /Fe 2+ ratios at each step, as measured by Mossbauer spectroscopy and X-ray diffraction, were in good general agreement with the amount of lithium calculated from the charge passed through the cell; there was, however, a slight tendency for the Mossbauer technique to record a higher concentration of the oxidized phase. The possible existence of a thin interface region at the phase boundary is discussed.

Thermal Stability of LiFePO4 ‐ Based Cathodes

The capacity and cyclability of solid-state synthesized LiFePO4-based laminate cells of type have been studied at 23, 40, and 60 degrees C. Larger capacities were obtained for cells cycled at the elevated temperatures. No evidenc

The lithium extraction/insertion mechanism in Li2FeSiO4

The lithium extraction and insertion mechanism in the cathode material Li2FeSiO4 has been monitored by in situ X-ray diffraction and Mossbauer spectroscopy during the first two cycles. The residual amounts of Li2FeSiO4 and LiFeSiO4 in the fully charged and discharged states are 5% and 10%, respectively, on the basis of both Mossbauer spectroscopy and powder XRD studies; this is also in good agreement with the results of electrochemical measurements. The observed lowering of the potential plateau from 3.10 to 2.80 V during the first cycle can be explained by a structural rearrangement in which some of the Li ions (in the 4b site) and Fe ions (in the 2a site) become interchanged.

Structural Transformations in Lithiated η′-Cu6Sn5 Electrodes Probed by In Situ Mössbauer Spectroscopy and X-Ray Diffraction

Structural Transformations in Lithiated n´-Cu 6 Sn 5 Electrodes Probed by In Situ Mossbauer Spectroscopy and X-Ray Diffraction

Mössbauer Study of Ordering in FeSi Alloys

The ordering in FeSi alloys with Si-content less than 25 at% has been examined. At low Si contents there is a relatively strong preference for large Si-Si inter-atomic distances. Furthermore a two phase region exists in the region from about 10 at% to about 20 at% Si, one of the phases probably of DO3 type.

First order magnetic transition, magnetic structure, and vacancy distribution in Fe2P

Abstract The para- to ferromagnetic transition in Fe 2 P has been studied using Mossbauer spectroscopy. The magnetic hyperfine fields drop abruptly from about half of their saturation values to zero at 214.5 K indicating a first order transition. The isomer shifts show a discontinuous change at the transition point. For some samples the transition takes place over a wide temperature range, probably due to impurities and other imperfections in the samples. From the magnetic hyperfine fields at 15 K the magnetic moments can be deduced to be 1.14 μ B and 1.78 μ B for Fe(1) and Fe(2), respectively. An assignment of the components in the Mossbauer spectra to the two crystallographically nonequivalent iron positions has been made from the temperature variation of the spectra. The ordering of metal vacancies has been investigated by a Mossbauer study of a nonstoichiometric Fe 2 P sample and by an X-ray diffraction study of a nonstoichiometric Mn 2 P crystal.

Lithium insertion into rhombohedral Li3Fe2(PO4)3

Abstract The lithium insertion process has been studied in rhombohedral Li 3 Fe 2 (PO 4 ) 3 (NASICON-type structure) by electrochemical and Mossbauer spectroscopic methods. The form of the discharge curve and the effective discharge capacity is found to depend on the mode of cathode preparation: two plateaus (one clear at ∼2.80 V and one less distinct at ∼2.65 V vs. Li/Li + ), corresponding to ca. 1.5–1.6 inserted lithium ions during the first cycle, are seen after more extreme grinding; milder treatment gave only the 2.8 V plateau and ca. 1.1 inserted lithium ions. Mossbauer spectra for the more extensively ground material show the Fe environments in R-Li 3 Fe 2 (PO 4 ) 3 to be highly symmetric; only a very narrow doublet with small quadrupolar splitting is observed, and the two crystallographically independent Fe-atoms cannot be distinguished. As lithium insertion proceeds, two doublets (average intensity ratio 1.5:1) appear, which can be assigned to two Fe 2+ sites. The average intensity ratio of 1.5:1 suggests that the extra lithium ions occupy sites closer to one of the Fe-atoms.

Mössbauer study of phosphides containing iron

Abstract A series of binary and ternary transition metal phosphides containing iron has been investigated by Mossbauer spectroscopy. Most of the compounds studied show complex magnetic behaviour, and interpretations are proposed for some cases. The crystallographic ordering mechanism in Me 2 P-type phosphides is discussed, and it is shown that great care must be taken in assigning the observed components of the Mossbauer spectra to the various crystallographic positions of the iron atoms.

Mössbauer and x-ray diffraction phase analysis of rusts from atmospheric test sites with different environments in Sweden

Abstract Rust samples from mild steel panels exposed at eight different atmospheric test sites in Sweden have been studied by transmission 57 Fe-Mossbauer spectroscopy and X-ray powder diffraction. All samples exhibited superparamagnetic effects in Mossbauer spectra recorded at room temperature, due to fine particles. The corrosion products were mainly identified in low temperature (∼10 K) Mossbauer spectra and with X-ray powder diffraction. In all samples, α-FeOOH and γ-FeOOH were the main constituents (>70%). Only in the marine samples were β-FeOOH and Fe 3-σ O 4 found. The α-FeOOH/γ-FeOOH ratio obtained was found to increase with the SO 2 concentration at the test sites.

A low-spin Fe( iii ) complex with 100-ps ligand-to-metal charge transfer photoluminescence

Transition-metal complexes are used as photosensitizers, in light-emitting diodes, for biosensing and in photocatalysis. A key feature in these applications is excitation from the ground state to a charge-transfer state; the long charge-transfer-state lifetimes typical for complexes of ruthenium 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 and copper being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs, it remains a formidable scientific challenge to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers. 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 σ-donor and π-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) d5 complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer (2LMCT) state that is rarely seen for transition-metal complexes. 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.