Sven-Ulf Weber’s research while affiliated with Technische Universität Braunschweig and other places

What is this page?


This page lists works of an author who doesn't have a ResearchGate profile or hasn't added the works to their profile yet. It is automatically generated from public (personal) data to further our legitimate goal of comprehensive and accurate scientific recordkeeping. If you are this author and want this page removed, please let us know.

Publications (12)


Fig. 2. (a) A naked Fe 6 cluster in RE 15 [Fe 8 C 25 ] (the distances given for RE¼ Ho) and the structural fragments of 3 6 layers in the crystal structures of γ-Fe, ε-Fe 3 C and η-Fe 2 C. Distances shown in pm. (b) 3 6 Layer of Fe atoms in the crystal structure of ε-Fe 3 C (top) and the arrangement of Fe 6 clusters in the crystal structure of RE 15 [Fe 8 C 25 ] (bottom). For further details see text.
Fig. 3. (a) Representative 57 Fe Mössbauer spectra of Dy 15 [Fe 8 C 25 ] at selected temperatures between 10 K and 290 K. The measured spectra are fitted (black line) with three subspectra for FeA, FeB and Fe3 indicated by blue, red and green lines. (b) Temperature dependent magnetic hyperfine fields B hf at FeA, FeB and Fe3 are indicated by blue, red and green symbols.
RE 15 [Fe 8 C 25 ] (RE ¼ Dy and Ho): Fractional atomic coordinates, equivalent isotropic displacement parameters [10 4 pm 2 ] and site occupancy; estimated standard deviations are given in parentheses.
ChemInform Abstract: Crystal Structure and Moessbauer Studies of the Isotypic Fe6-Cluster Compounds RE15[Fe8C25], RE: Dy, Ho.
  • Article
  • Full-text available

June 2015

·

101 Reads

ChemInform

·

·

Guido Kreiner

·

[...]

·

Ruediger Kniep

The title compounds are prepared by arc melting of the elements followed by annealing (Ta ampules, 1373 K, 21 d).

Download

Fig. 2. (a) A naked Fe 6 cluster in RE 15 [Fe 8 C 25 ] (the distances given for RE¼ Ho) and the structural fragments of 3 6 layers in the crystal structures of γ-Fe, ε-Fe 3 C and η-Fe 2 C. Distances shown in pm. (b) 3 6 Layer of Fe atoms in the crystal structure of ε-Fe 3 C (top) and the arrangement of Fe 6 clusters in the crystal structure of RE 15 [Fe 8 C 25 ] (bottom). For further details see text.
Fig. 3. (a) Representative 57 Fe Mössbauer spectra of Dy 15 [Fe 8 C 25 ] at selected temperatures between 10 K and 290 K. The measured spectra are fitted (black line) with three subspectra for FeA, FeB and Fe3 indicated by blue, red and green lines. (b) Temperature dependent magnetic hyperfine fields B hf at FeA, FeB and Fe3 are indicated by blue, red and green symbols.
RE 15 [Fe 8 C 25 ] (RE ¼ Dy and Ho): Fractional atomic coordinates, equivalent isotropic displacement parameters [10 4 pm 2 ] and site occupancy; estimated standard deviations are given in parentheses.
Crystal structure and Mössbauer studies of the isotypic Fe6-cluster compounds RE15[Fe8C25], RE=Dy, Ho

May 2015

·

119 Reads

·

2 Citations

Journal of Solid State Chemistry

The carboferrates RE15[Fe8C25] (RE=Dy, Ho) were prepared from mixtures of the elements by arc-melting followed with subsequent annealing at 1373 K. The crystal structures were determined from single crystal X-ray diffraction data and revealed an isotypic relationship to Er15[Fe8C25] (hP48, P321). The main feature of the crystal structure is given by Fe6 cluster units characterized by covalent Fe-Fe bonding interactions. 57Fe Mössbauer spectra of Dy15[Fe8C25] were fitted by three subspectra with relative spectral weights of about 3:3:2 which is in general agreement with the crystal structure. Below 50 K, an onset of magnetic hyperfine fields at the three iron sites is observed which is supposed to be caused by dipolar fields arising from neighboring, slowly relaxing Dy magnetic moments.


Temperature-dependent 57Fe Mössbauer spectroscopy and local structure of the mullite-type Bi2(FexGa1−x)4O9 (0.1≤x≤1) solid solution

March 2014

·

82 Reads

·

11 Citations

Journal of Physics and Chemistry of Solids

The Bi2(FexGa1−x)4O9 oxide solid solution possessing a mullite-type structure has been investigated by 57Fe Mössbauer spectroscopy in dependence of composition (0.1≤x≤1) and temperature (293≤T/K≤1073). The spectra have been fitted with two doublets for tetrahedrally and octahedrally coordinated high-spin Fe3+ ions, respectively. The experimental areas of the subspectra were used to determine the distribution of iron on the two inequivalent structural sites. The fraction of iron cations occupying the octahedral site is found to increase with decreasing Fe content and the cation distribution is almost independent of temperature. The unusual temperature dependence of the quadrupolar splitting, QS, observed for the octahedral site with dQS/dT>0 is discussed in connexion with structural data for Bi2Fe4O9. The temperature dependence of Mössbauer isomer shifts and signal intensities is examined in the context of local vibrational properties of iron on the two inequivalent sites of the mullite-type lattice structure.


Fig. 1. Structure plot of the synthetic fayalite unit cell (Pnma), performed with CaRIne Ò . View along the c-axis. The Fe-ions on M1 are drawn in dark red, those on M2 in light red, oxygens in green and blue and silicon ions in gold. For comparison with Fig. 2, the relevant octahedra are displayed. The outer open polyhedron in dark red on M1 denotes the 6 A ˚ surroundings, the light red octahedron on M2 corresponds to the 4 A ˚ environment.  
Fig. 2. Deformation electron densities in synthetic fayalite according to the semi-quantitative method. The hyperareas collected for the calculation of the EFG are highlighted. a) 6 A ˚ section around the M1 Fe ion b) 4 A ˚ section around M2.
Fig. 3. Stereographic plot of the fayalite EFG with respect to the crystallographic axes (Pnma). Red crosses: Experimental from SCMBS; green squares: full quantitative from DFT calculation; blue triangles: semi-quantitative, this study. An open symbol denotes a vector direction towards the southern hemisphere. a) EFG on M1 b) EFG on M2.
A semi-quantitative approach to derive the electric field gradient, applied to synthetic fayalite, α-Fe2SiO4: a reappraisal

October 2012

·

53 Reads

·

2 Citations

European Journal of Mineralogy

Experimental and calculated structure factors from a previous synchrotron diffraction measurement on synthetic fayalite have been converted by an inverse Fourier transformation to difference electron (deformation) densities (DED). These were processed in a revised 3D-display program giving hyperareas of DED floating in space around the iron positions M1 and M2 within the fayalite unit-cell and spanning a cluster size of 6 and 4 Å, respectively. These relatively wide limits are due to the different site symmetries and had been proposed by earlier DFT (density functional theory) calculations. From the different hyperareas the supposed charges were integrated in space and processed to electric field gradients (EFG) on M1 and M2 using a point-charge model. The two EFGs were compared with respect to the system of crystallographic axes with those obtained from published single-crystal Mössbauer measurements (experimental EFGs), yielding excellent agreement within ±5° and surpassing even the DFT results. This study reports the procedure and the conditions of success of the underlying semi-quantitative method, which is halfway between theory (DFT) and experiment (diffractometry) and is promising valuable results on many other compounds. The term “nanoscope” for the graphical representation may be justified due to its high spatial resolution.


Temperature-dependent 57 Fe Mössbauer spectroscopy and local structure of mullite-type Bi 2 (Fe x Al 1 – x ) 4 O 9 (0.1≤ x ≤1) solid solutions

April 2012

·

99 Reads

·

15 Citations

International Journal of Materials Research (formerly Zeitschrift fuer Metallkunde)

Oxides of the chemical composition Bi 2(Fe xAl 1-x) 4O 9 (0.1 ≤ x ≤ 1) possessing a mullite-type structure have been investigated using 57Fe Mössbauer spectroscopy in the temperature range between room temperature and 1 073 K. The spectra have been fitted with two quadrupole doublets for Fe 3+ ions in octahedral and tetrahedral oxygen coordination. The distribution of iron on the two inequivalent structural sites has been determined from the relative areas of the subspectra. The site occupancy is found to be random within experimental error: it does not exhibit any preference of iron for the octahedral or tetrahedral sites, either as a function of temperature or with a dependence on the composition of the solid solutions.


Table 1 Chemical composition of chrysoberyl from Rio das Americanas, Minas Gerais, Brazil
Table 2 Chemical composition of sinhalite from Ocanpitiya, Sri Lanka
Fig. 3 Mössbauer spectra of natural sinhalite for the k-vector of the incident c-rays being parallel to the crystallographic axes a (a), b (b) and c (c), respectively. The measured spectra are denoted by black squares, the total calculated ones by red lines. The residuals between observed and calculated patterns are drawn at the bottom of each plot  
Table 5 Bond lengths (in A ˚ ) and polyhedral distortion parameters of the coordination polyhedra of natural chrysoberyl from Rio das Americanas, Minas Gerais, Brazil and sinhalite from Ocanpitiya, Sri Lanka
The electric field gradient in natural iron-doped chrysoberyl Al2BeO4 and sinhalite MgAlBO4 single crystals

December 2011

·

381 Reads

·

9 Citations

Physics and Chemistry of Minerals

This work reports on the evaluation of the electric field gradient (EFG) in natural chrysoberyl Al2BeO4 and sinhalite MgAlBO4 using two different procedures: (1) experimental, with single crystal Mossbauer spectroscopy (SCMBS) on the three principal sections of each sample and (2) a "fully quantitative" method with cluster molecular orbital calculations based on the density functional theory. Whereas the experimental and theoretical results for the EFG tensor are in quantitative agreement, the calculated isomer shifts and optical d-d-transitions exhibit systematic deviations from the measured values. These deviations indicate that the substitution of Al and Mg with iron should be accompanied by considerable local expansion of the coordination octahedra.


Solid Solutions in the System Fe1-xO/ZnO at Low Oxygen Partial Pressure

September 2010

·

28 Reads

·

2 Citations

Zeitschrift für anorganische und allgemeine Chemie

The maximum solubilities of Zn2+ in Fe1–xO and of Fe2+ in ZnO at temperatures from 1173–1473 K in the presence of metallic iron were determined. Mixed crystals Fe1–x–zZnzO with 0.04 ≤ z ≤ 0.19 were prepared at 1273 K. All samples were characterised by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy. The solubility of Fe2+ in wurtzite ZnO as well as the lattice parameter a of Zn1–yFeyO increases with reaction temperature whereas c remains virtually constant. The solubility of Zn2+ in Fe1–xO increases with temperature. The lattice parameter a of Fe1–x–zZnzO for 0.04 ≤ z ≤ 0.13 increases with zinc content. For0.13 ≤ z ≤ 0.19 a is constant, whereas for higher zinc contents a decreases. Mössbauer spectroscopy of the mixed crystal Fe1–x–0.09Zn0.09O yields a reduced amount of 6 % Fe3+ compared to wüstite Fe1–0.04O with 9 % Fe3+. Such reduction of Fe3+ content was previously observed in the mixed crystal systems Fe1–xO/MgO and Fe1–xO/MnO too. The increase of the lattice parameter a of Fe1–x–zZnzO for low concentrations is explained by removal of Fe3+ ions. At higher zinc contents the occupation with Zn2+ ions – with smaller radius than Fe2+ – balances or exceeds the effect of Fe3+ removal.



A Variable-Temperature Mössbauer Study into the Structure, Disorder and Formation Kinetics of Iron Nitrides

June 2010

·

9 Reads

Mössbauer spectra of the iron nitrides γ'-Fe 4 N 1+δ and ε-Fe 3 N 1+x have been measured at room tempera-ture and at 550 °C. γ'-Fe 4 N 1+δ is well ordered and close to ideal stoichiometry even at high tempera-tures. Its room temperature Mössbauer spectrum exhibits three magnetically split subspectra due to the fact that the structurally equivalent Fe(1) sites are split into two spectroscopically inequivalent sites, Fig. 1. This is consequence of the simultaneous presence of magnetic and quadrupolar interactions at Fe(1) sites, see Refs. [1,2] . In the case of ε-Fe 3 N 1+x , however, the stoichiometry-dependent magneti-cally-split room temperature spectra demonstrate the extensive disorder of nitrogen in the compound, Fig. 2 [2,3]. At 550 °C, spectra of ε-Fe 3 N 1+x have been measured at various defined nitrogen activities. The quadrupole-split spectra are discussed in relation to structure and disorder of the nonstoichiomet-ric material. In particular, it is shown that the composition dependent high-temperature spectra of ε-Fe 3 N 1+x can be modelled in the framework of a simple two-site model [2]. Fig. 1 Mössbauer spectrum of γ'-Fe 4 N at room temperature. The fit is based on three subspectra which are indicated by a solid line due to Fe(2) and by dotted and dashed lines for subspectra which are due to Fe(1a) and Fe(1b), respectively. -6 -4 -2 0 2 4 6 78 80 82 84 86 88 90 92 94 96 98 100 Transmission [%] v [mm/s] Weber et al., Mössbauer Study of Iron Oxides diffusion-fundamentals www.diffusion-online.org Diffusion Fundamentals 12 (2010) 83 © S.-U. Weber Fig. 2 a) Mössbauer spectrum of ε-Fe 3+x N at room temperature, b) Distribution of local magnetic fields according to the fit shown in Fig. 2a. By means of time-resolved Mössbauer measurements, the formation reaction of γ'-Fe 4 N 1+δ has been followed at in situ conditions at 550 °C. The formation kinetics have been found to obey a parabolic rate law. The nitrogen activity dependence of the experimental rate constant for the formation of γ'-Fe 4 N is discussed in relation to the point defect structure of the compound the atomic diffusion of ni-trogen. The analysis provides evidence for the diffusion of nitrogen by means of a vacancy and an interstitial mechanism in γ'-Fe 4 N 1+δ where the latter is dominating the formation kinetics of the com-pound.


57 Fe Mössbauer spectroscopy, X-ray single-crystal diffractometry, and electronic structure calculations on natural sinhalites

May 2009

·

118 Reads

·

3 Citations

Physics and Chemistry of Minerals

Natural sinhalites, MgAlBO4, from the Ratnapura District, Sri Lanka, and from Bodnar Quarry near Hamburg, Sussex Co., New Jersey, USA, have been characterized by 57Fe Mössbauer spectroscopy, electron microprobe, X-ray single-crystal diffractometry and by electronic structure calculations in order to determine the oxidation state and site occupancy of iron in the sinhalite structure. The samples contain about 3.35 and 1.46 wt% of total iron oxide, respectively. The structure refinement is successful and reproduces the total iron content provided that the substitution of Mg2+ by Fe2+ on the M2 position only is assumed. The 57Fe Mössbauer spectra at 77, 293, 573 and 773 K can be resolved into two doublets with hyperfine parameters common for octahedrally coordinated high-spin Fe2+. There is no evidence for iron in the tetrahedral site. Electronic structure calculations in local spin density approximation yield hyperfine parameters for Fe2+ on the M2-site at 0, 293, 573 and 773 K in quantitative agreement with experiments. Calculated spectroscopic properties for Fe2+ on the M1-site are at variance with the experimental data and, thus, indicate that substitution of Al3+ by Fe2+, if occurring at all, must be accompanied by considerable local expansion and distortion of the M1-octahedron.


Citations (8)


... Central quantity in this respect is the electric field gradient (EFG) whose main property consists in providing the link between crystallography and solid-state physics or between diffractometry and spectroscopy, respectively. Recently, this has been proved at a special example [1]. ...

Reference:

Application of a Difference Electron Nanoscope (DEN) - A computer hard- and software to display 3D electron densities and magnetical structures based on spectroscopic and diffractometric data
A semi-quantitative approach to derive the electric field gradient, applied to synthetic fayalite, α-Fe2SiO4: a reappraisal

European Journal of Mineralogy

... These Dy 3+ are coordinated by eight oxygen atoms forming distorted DyO 8 polyhedra linked to each other, building a 4-ring channel ( Figure 1). The infinite octahedral chains are the dominant (but not only 3 ) criterion for mullite-type phases which could be subdivided into so-called O8-phase [11][12][13][14] (e.g., PbMBO 4 11-13,15-17 and schafarzikites LM 2 O 4 [18][19][20][21][22][23][24][25] ), O9-phase (e.g., BiMO [26][27][28][29][30][31][32][33][34][35][36][37][38] and ABO [39][40][41][42][43][44], and O10-phase (e.g., BiMO 6,10,45,46 ). The DyMn 2 O 5 structure type belongs to the O10 phase which differs from the O9 BiMO phases with only an additional oxygen atom. ...

Temperature-dependent 57 Fe Mössbauer spectroscopy and local structure of mullite-type Bi 2 (Fe x Al 1 – x ) 4 O 9 (0.1≤ x ≤1) solid solutions

International Journal of Materials Research (formerly Zeitschrift fuer Metallkunde)

... Semiconducting materials showing photocatalytic activity by forming excitons in the visible electromagnetic spectrum are highly demanded nowadays [1][2][3][4][5]. In this regard mullite [6][7][8] and mullite-type materials of either O8 EMBO 4 [9][10][11][12][13][14] and Schafarzikite [15][16][17][18], O9 [19][20][21] or O10 [22][23][24] type, especially the multiferroic O9 phase Bi 2 Fe 4 O 9 [25][26][27][28], are widely used [29][30][31][32][33][34][35][36][37][38]. The respective pseudo-binary phase diagram Bi 2 O 3 -Fe 2 O 3 has been repeatedly investigated [39][40][41][42][43][44][45], providing descriptions of some thermodynamically stable phases such as perovskitetype BiFeO 3 [32,43], mullite-type Bi 2 Fe 4 O 9 [25,28,29,31,32,[34][35][36][37][38][46][47][48] and sillenite-type Bi 25 FeO 40 [40]. ...

Temperature-dependent 57Fe Mössbauer spectroscopy and local structure of the mullite-type Bi2(FexGa1−x)4O9 (0.1≤x≤1) solid solution
  • Citing Article
  • March 2014

Journal of Physics and Chemistry of Solids

... In the course of our systematic investigation of minerals with low iron content, it has been demonstrated that the electric field gradient (EFG) tensor is the most valuable quantity as supplying information on local distortions not amenable to conventional diffraction methods (Weber et al. 2009 and references therein). Moreover, it is a key quantity in studying the relationship between structural and chemical properties in solids because it provides a sensitive measure for the size and symmetry of the electric charge distribution around a given nucleus at a certain crystallographic site. ...

57 Fe Mössbauer spectroscopy, X-ray single-crystal diffractometry, and electronic structure calculations on natural sinhalites

Physics and Chemistry of Minerals

... In the case of an undoped Fe 1 − x O wüstite phase, such a presence of cationic vacancies results in a smaller lattice parameter "a". But as detailed in ref [39], the occupation with Zn 2+ ions in the wüstite network can induce an increase of the lattice parameter following a charge balance effect with Fe 3+ . According to this mechanism and considering the stoichiometric wüstite FeO (ICDD # 00-006-0615; a = 0.4307 nm) as a reference, the lattice parameter in the non-stoichiometric wüstite Fe 0.855 O should be less than 0.4307 nm due to the formation of cationic vacancies and Fe 3+ ions. ...

Solid Solutions in the System Fe1-xO/ZnO at Low Oxygen Partial Pressure
  • Citing Article
  • September 2010

Zeitschrift für anorganische und allgemeine Chemie

... Beryllium occupies tetrahedra and Al occupies two types of slightly distorted interstitial octahedral sites: B1 octahedra with Ci symmetry and B2 octahedra with Cs symmetry [9]. These octahedra have different volumes; additionally, B2 has an Al-O distance of 1.936 Å, which is larger than B1, with an Al-O distance of 1.890 Å [10]. Alexandrite is a type of chromium-doped bearing chrysoberyl in which Cr 3+ is substituted in the Al 3+ sites. ...

Fe-57 Mossbauer spectroscopy, X-ray single-crystal diffractometry, and electronic structure calculations on natural alexandrite
  • Citing Article
  • September 2007

Physics and Chemistry of Minerals

... We suspect that elevated paramagnetic iron concentration (our mineral is brown in color, which is likely caused by an O 2− → Fe 3+ charge transfer and Fe 2+ in octahedral coordination in sinhalite) led to increased 25 Mg nuclear spin relaxation rates and a distribution of Fermi contact shifts, inhibiting 25 Mg NMR spectral acquisition. 64 Grandidierite, MgAl 3 (BO 4 )(SiO 4 )O, crystallizes in orthorhombic space group Pbnm and features one crystallographically unique Mg atom in an [MgO 5 ] polyhedron. 60 Grandidierite has been studied previously by MacKenzie and Meinhold 7 and was one of the first crystalline compounds with a pentacoordinated Mg site to be investigated by 25 Mg MAS NMR spectroscopy. ...

The electric field gradient in natural iron-doped chrysoberyl Al2BeO4 and sinhalite MgAlBO4 single crystals

Physics and Chemistry of Minerals

... The extremely high sensitivity of Mössbauer spectroscopy and its ability to distinguish Fe 2+ from Fe 3+ is of great advantage in assessing the oxidation state of iron. Another advantage is also the ability of the method to analyze microcrystalline and amorphous substances containing iron [11][12][13]. It was shown that during the synthesis of CNTs a change occurred in the oxidation degree and in the binding of the iron atoms in the catalyst [14,15]. ...

57 Fe Mössbauer Spectroscopy on Multiwalled Carbon Nanotubes with Metal Filling
  • Citing Article
  • June 2006

The Journal of Physical Chemistry B