Werner Lottermoser

Spectroscopic and structural properties of synthetic micas on the annite-siderophyllite binary: Synthesis, crystal structure refinement, Mössbauer, and infrared spectroscopy

Abstract The effect of the incorporation of Al-Tschermak’s molecule to the trioctahedral potassium mica annite {K}[Fe3]O10(OH)2 on local and average structure has been investigated by hydrothermal synthesis, structure refinement of X-ray powder diffraction data, Mössbauer and infrared spectroscopy. The various types of brackets indicate different structural sites. Samples with compositions {K}[Fe3-xAlx]O10(OH)2 were prepared by hydrothermal techniques. The maximum solubility of Al3+ is limited to x = 0.92 at 500 °C and to x = 0.82 at 700 °C. The main factor controlling the substitution limits is the ditrigonal distortion of the tetrahedral rings. Lattice parameters decrease linearly with increasing Al3+ content of the mica. A considerable decrease of M2-O and nearly no change of M1-O bond lengths with increasing Al3+ contents is indicative of preferred occupation of the M2 site by [Al3+]. Changes in K-O distances are also very pronounced and reflect the ditrigonal distortion of the tetrahedral sheet. The bimodal ferrous quadrupole splitting distribution (QSD) in annite, extracted from Mössbauer spectra, becomes narrower and more centered around 2.60 mm/s with increasing Al3+ contents, and its evolution suggests an increasing deviation from ideal octahedral coordination of Fe by O, illustrated by the increasing octahedral flattening angle y. The population of individual QSD components proves that it is impossible to resolve cis and trans M-sites in micas by Mössbauer spectroscopy. In the hydroxyl stretching region, up to 7 bands are observed in the infra-red spectra which correspond to OH groups adjacent to 3 Fe2+ (N-bands), to OH groups coordinated by Fe2+, Al3+, and Fe3+ (I-bands) and to configurations having one octahedral vacancy (V-bands). N- and I-type bands are shifted toward lower wavenumbers with increasing Al3+ content because of increasing OH···Otet interactions.

The role of Fe2+ and Fe3+ in synthetic Fe-substituted tetrahedrite

SummaryTetrahedrites with the composition between Cu12Sb4S13 and Cu10Fe2Sb4S13 were synthesized at 457 °C and 500 °C from the elements and carefully studied by Mössbauer spectroscopy of57Fe. Between Cu12Sb4S13 and Cu11Fe1Sb4S13 iron is predominantly ferric. Between Cu11Fe1Sb4S13 and Cu10Fe2Sb4S13 iron is predominantly ferrous and occupies the tetrahedral M1-sites.ZusammenfassungDie Rolle von Fe2+ und Fe3+ in synthetischen Tetraedriten mit Fe-Substitution Tetraedrite mit einer Zusammensetzung zwischen Cu12Sb4S13 and Cu10Fe2Sb4S13 wurden bei 457 °C und 500 °C aus den Elementen synthetisiert und sorgfdltig mit Mössbauer-Spektroskopie von57Fe untersucht. Zwischen Cu12Sb4S13 and Cu11Fe1Sb4S13 ist Eisen überwiegend dreiwertig. Zwischen Cu11Fe1Sb4S13 and Cu11Fe2Sb4S13 ist Eisen überwiegend zweiwertig und besetzt die tetraedrisch koordinierten M1-Plätze.

Octahedral cation partitioning in Mg,Fe 2+ -olivine. Mössbauer spectroscopic study of synthetic (Mg 0.5 Fe 2+ 0.5 ) 2 SiO 4 (Fa 50 )

The high-temperature partitioning of Fe 2+ and Mg between the two non-equivalent octahedral M1 and M2 sites in synthetic olivine (Fa 50 ) was studied by Mossbauer spectroscopy. Powder samples have been equilibrated in annealing experiments performed under reducing oxygen fugacity at temperatures between 500°C and 800°C followed by rapid quenching in order to prevent redistribution of cations. M-site ordering with Fe 2+ preferring M1, Mg preferring M2 sites increases continuously with rising equilibrium temperature. K D values increase from 1.21 at 500°C to 1.48 at 750°C. The results are consistent with both room temperature as well as in situ high temperature single crystal X-ray diffraction experiments of Heinemann et al. (2003a and b).

Single-crystal X-ray diffraction and temperature dependent 57Fe Mössbauer spectroscopy on the hedenbergite-aegirine (Ca,Na)(Fe2+,Fe3+)Si2O6 solid solution

Abstract Synthetic samples with different chemical compositions along the hedenbergite-aegirine (CaFe2+Si2O6-NaFe3+Si2O6) solid-solution series have been investigated by single-crystal X-ray diffraction and 57Fe Mössbauer spectroscopy. All compounds show C2/c symmetry, both at 298 K and at low temperature (100 K). The structural changes within the hedenbergite-aegirine series are dominated by the M1 site while the M2 site plays a minor role. Replacement of Fe2+ by Fe3+ increases the polyhedral distortion of the M1 site and causes an increased repulsion between neighboring M1 sites. The changes in M1-site geometry also induce distinct alterations within the kinking state of the tetrahedral chains, but the changes in tetrahedral bond lengths and angles are small. In addition to the single-crystal X-ray diffraction experiments, a large number of synthetic samples were investigated by 57Fe Mössbauer spectroscopy at 298 K and, for three selected compositions, between 80 and 700 K. Here, substantial line broadening of the Fe2+ resonance absorption was observed as an aegirine component is substituted. Two different groups of local distortion environments were observed for Fe2+ within the solid-solution series, which change relative proportions and numeric value of the quadrupole splitting as a function of chemistry and temperature. This line broadening cannot be ascribed to discrete next-nearest-neighbor (NNN) configurations of Ca and Na as has been done in the literature. Above ~250 K, additional resonance absorption appears in the spectra of samples with aegirine components between 20 and 75 mol%. This absorption gains intensity with increasing temperature, while the 57Fe hyperfine parameters approach values intermediate between Fe2+ and Fe3+. This effect is ascribed to fast electron delocalization between Fe2+ and Fe3+ at elevated temperature.

Synthesis and structural properties of clinopyroxenes of the hedenbergite CaFe2+Si2O6 – aegirine NaFe3+Si2O6 solid-solution series

Clinopyroxenes along the solid solution hedenbergite-aegirine M2[Ca2+1-xNa+xM1{Fe2+1-xFe3+x}Si2O6 were synthesized using hydrothermal techniques at 4 kbar. Different temperatures and redox conditions were used to determine optimum synthesis conditions and the stability range of individual compositions in the T - log fO2 field. Synthesized samples were characterized using microprobe analysis, X-ray powder diffraction and Mossbauer spectroscopy at 298 K and 80 K. The structure was refined in the C 2/ c space group by means of the Rietveld method. Along the solid-solution series between hedenbergite (a = 9.8448(6) A, b = 9.0296(6) A, c = 5.2452(4) A, β = 104.813) and aegirine endmembers (a = 9.6547(6) A, b = 8.7941(8) A, c = 5.2944(4) A, β = 107.398), the changes in unit cell dimensions show significant deviations from linearity. Mean and individual M1-O distances decrease linearly from hedenbergite to aegirine; mean M2-O and T-O distances do not change significantly, whereas individual length may vary. While in hedenbergite the coordination of the M2 site is 6+2, it is 4+4 in aegirine. The Mossbauer spectra of the solid-solution endmembers display narrowly split resonance absorption lines with hyperfine parameters typical for Fe2+ (δ = 1.18 mm/s, Δ = 2.25 mm/s at 298 K) and Fe3+ (δ = 0.38 mm/s, Δ = 0.30 mm/s at 298 K). Fe occupies only the M1 site. The Fe2+ resonance absorption is somewhat broadened in the 80 K spectra of the solid solution, which is due to a distribution of quadrupole splittings.

Fast Li-Ion-Conducting Garnet-Related Li7–3xFexLa3Zr2O12 with Uncommon I4̅3d Structure

Fast Li-ion-conducting Li oxide garnets receive a great deal of attention as they are suitable candidates for solid-state Li electrolytes. It was recently shown that Ga-stabilized Li7La3Zr2O12 crystallizes in the acentric cubic space group I4̅3d. This structure can be derived by a symmetry reduction of the garnet-type Ia3̅d structure, which is the most commonly found space group of Li oxide garnets and garnets in general. In this study, single-crystal X-ray diffraction confirms the presence of space group I4̅3d also for Li7–3xFexLa3Zr2O12. The crystal structure was characterized by X-ray powder diffraction, single-crystal X-ray diffraction, neutron powder diffraction, and Mößbauer spectroscopy. The crystal–chemical behavior of Fe3+ in Li7La3Zr2O12 is very similar to that of Ga3+. The symmetry reduction seems to be initiated by the ordering of Fe3+ onto the tetrahedral Li1 (12a) site of space group I4̅3d. Electrochemical impedance spectroscopy measurements showed a Li-ion bulk conductivity of up to 1.38 × 10–3 S cm–1 at room temperature, which is among the highest values reported for this group of materials.

Hagendorfite (Na,Ca)MnFe2(PO4)3 from type locality Hagendorf (Bavaria, Germany) : crystal structure determination and 57Fe Mössbauer spectroscopy

The crystal structure of hagendorfite (Na,Ca)MnFe 2 (PO 4 ) 3 from type locality has been determined and it was found to be isostructural with alluaudite. It accepts space group symmetry C2/c both at room temperature and at 100 K. At room temperature the lattice parameters are a = 11.9721(9) A, b = 12.5988(8) A, c = 6.5029(5) A, β = 114.841(8)° with Z = 4. For reasons of comparison the structure of an alluaudite sensu strictu (s.s.) from Buranga (Rwanda) was re-determined. The specific arrangement of M(1) and M(2) octahedral sites and of P(1) and P(2) tetrahedral sites gives rise to two different channels aligned along the crystallographic c-axis, containing the A(1) and A(2)’ sites. In both compounds the A(1) site is fully occupied and shows a mixed occupation of Na + , Ca 2+ and Mn 2+ (hagendorfite) and Ca 2+ and Na + (alluaudite s.s.). The A(2)’ is fully occupied by Na + in hagendorfite and partly filled by 0.14 Na + atoms per formula unit in Buranga alluaudite. The structural topology of hagendorfite is described in detail and structural parameters are compared to alluaudite s.s. and to other, mostly synthetic compounds also crystallizing in the alluaudite structure type. 57 Fe Mossbauer spectroscopy shows that iron is exclusively in the trivalent state in alluaudite s.s. while in hagendorfite about 2/3 of the total iron are in the divalent state. The ferrous iron resonance absorption contribution appears to be broad and can only be refined with four different Fe 2+ subcomponents, which are all ascribed to the M(2) site. The four different spectroscopic signals presumably arise from different next nearest neighbour occupations of adjacent M(2) and A(1) sites.

Electron efficiency of nZVI does not change with variation of environmental parameters.

Nanoscale zero-valent iron particles (nZVI) are already applied for in-situ dechlorination of halogenated organic contaminants in the field. We performed batch experiments whereby trichloroethene (TCE) was dehalogenated by nZVI under different environmental conditions that are relevant in practice. The tested conditions include different ionic strengths, addition of polyelectrolytes (carboxymethylcellulose and ligninsulphonate), lowered temperature, dissolved oxygen and different particle contents. Particle properties were determined by Mössbauer spectroscopy, XRD, TEM, SEM, AAS and laser obscuration time measurements. TCE dehalogenation and H2 evolution were decelerated by reduced ionic strength, addition of polyelectrolytes, temperature reduction, the presence of dissolved oxygen and reduced particle content. The partitioning of released electrons between reactions with the contaminant vs. with water (selectivity) was low, independent of the tested conditions. Basically out of hundred electrons that were released via nZVI oxidation only 3.1±1.4 were used for TCE dehalogenation. Even lower selectivities were observed at TCE concentrations below 3.5 mg l(-1), hence particle modifications and/or combination of nZVI with other remediation technologies seem to be necessary to reach target concentrations for remediation. Our results suggest that selectivity is particle intrinsic and not as much condition dependent, hence particle synthesis and potential particle modifications of nZVI particles may be more important for optimization of the pollutant degradation rate, than tested environmental conditions.

Powder- and single crystal Mössbauer spectroscopy on synthetic fayalite

Mössbauer measurements on synthetic iron orthosilicate Fe2SiO4 (fayalite) were carried out in the antiferromagnetic spin state below TN ≃65 K. The Mössbauer parameters isomer shift δ, inner magnetic field H(0), angle Θ between H(0) and the z-component of the electric field gradient (efg), quadrupole splitting QS and asymmetry parameter η were determined as a function of temperature. These parameters could be attributed to the two crystallographic sites M1 and M2.The smaller isomer shift on M1 with respect to M2 displays the more covalent character of the Fe-O bond on M1, which is supported by previous neutron diffraction experiments. H(0) shows a Brillouin-type behaviour with different fields on the two crystallographic sites (stronger on M1) and a small discontinuity at T = 23 K which corresponds with previous magnetic measurements. The quadrupole splitting is equal on both sites within error bars, in agreement with previous theoretical results and in contradiction to previous Mössbauer refinements published elsewhere.Single crystal Mössbauer spectroscopy revealed the orientation of the crystallographic c-axis (Pnma) with respect to the axes of the efg. A second parameter set which also holds true for the powder refinement yields the angle between c and H(0) on M1 to be temperature-dependent in quantitative agreement with previous neutron diffraction results and solves an old discrepancy discussed elsewhere. On M2, the angle between c and H(0) is found to be much smaller than 90° in contradiction to symmetry requirements.

Single crystal Mössbauer spectroscopy on the three principal sections of a synthetic fayalite sample in the antiferromagnetic state

The present work reports Mössbauer investigations for several temperatures below TN on fayalite single crystal sections cut perpendicularly to the crystallographic a and b-axis (Pnma). The previously detected correspondence between the c-component of the magnetic moment on M1 from neutron diffraction and our own Mössbauer measurements published elsewhere are confirmed for the other principal sections to a large extent. Small humps in the angular dependence of two components of the internal magnetic field H(0) on T below T=23 K are in good agreement with magnetometric and calorimetric data published elsewhere; a reinterpretation of single reflection neutron data has been possible by our results. Moreover, the axes of the electric field gradient (efg) are oriented within the crystallographic axes for the M1-site at low temperatures.The violation of symmetry on the M2 position as a result of our previous investigations could be confirmed for the section ⊥ a, but not with respect to b. A possible explanation in terms of saturation effects of large line intensities at the expense of the small ones is given in the context.