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Featured researches published by Stefan Marion.


Journal of Physics: Condensed Matter | 2000

Short-range ordering of oxygen vacancies in CaFexTi1 - xO3 - x/2 perovskites (0 < x < 0.4)

Catherine McCammon; Ana I. Becerro; Falko Langenhorst; Ross J. Angel; Stefan Marion; F. Seifert

We have synthesized samples in the system CaTi1 - x Fex O3 - x /2 with x = 0.05 - 0.38 at temperatures of 900 - 1320 °C under controlled conditions of oxygen fugacity. After drop quenching, samples were characterized by x-ray diffraction, transmission electron microscopy (TEM) and Mossbauer spectroscopy. Electrical conductivity measurements were made as a function of temperature, oxygen fugacity and sample composition. X-ray diffraction showed no long-range ordering of oxygen vacancies, which was confirmed by TEM measurements. Mossbauer spectroscopy showed Fe occurring in octahedral, tetrahedral and pentacoordinated sites. Analysis of area ratios indicates that a small number of Ti atoms must be located on non-octahedral sites, where the relative proportion of Ti on these sites decreases with increasing concentration of oxygen vacancies. Results are interpreted in terms of a model for short-range ordering of oxygen vacancies where clusters form chains of increasing length. The structure is dominated by oxygen vacancy monomers at low Fe concentrations, and the average number of vacancies per chain increases with increasing Fe concentration. Results are consistent with conductivity results which show high ionic conductivity at moderately low Fe concentrations.


American Mineralogist | 2001

Displacive components of the low-temperature phase transitions in lawsonite

Hinrich-Wilhelm Meyer; Stefan Marion; Peter Sondergeld; Michael A. Carpenter; Kevin S. Knight; Simon A. T. Redfern; Martin T. Dove

Abstract The phase transitions in deuterated lawsonite were investigated with high-resolution, time-offlight neutron diffraction between 2 and 500 K. From the analysis of spontaneous strain data, the thermodynamics of the phase transition at 273 K are not changed by the deuteration process. Shifts in atomic positions with temperature indicate continuous changes for a framework oxygen and for one of the deuterium atoms, whereas for the other deuterium atom, a more discontinuous behavior was observed in the average structure. Comparison of O···D and O···O bond lengths with IR data from a non-deuterated lawsonite permits a detailed analysis of assignments of O-H stretching modes


Ionics | 1999

Ionic and electronic conductivity in CaTi1−xFexO3−δ (x=0.1–0.3)

Stefan Marion; Ana I. Becerro; Truls Norby

Electrical conductivity measurements on CaTi1−xFexO3−δ (x=0.1, 0.2, 0.3) were performed on polycrystalline pressed and sintered tablets using the van der Pauw four point method in controlled atmospheres. The results were interpreted to reflect n-type, ionic and p-type conductivity at different oxygen partial pressures. An increasing iron content increases the number of oxygen vacancies and increases the ionic conductivity at high temperatures, but also increases the tendency of ordering, which suppresses the ionic conductivity at more moderate temperatures. These findings are in accordance with the phase diagram of the system CaTiO3-CaFeO2.5 based on X-ray and Mössbauer studies.


Solid State Ionics | 2001

Incorporation of water in strontium tantalates with perovskite-related structure

I. E. Animitsa; Truls Norby; Stefan Marion; Ronny Glöckner; Arkady Neiman

Abstract The electrical properties and water uptake of the oxygen-deficient perovskite-related compounds Sr 6 Ta 2 O 11 and Sr 5.92 Ta 2.08 O 11.12 have been investigated with AC conductivity, transport number measurements and thermogravimetry. In wet atmospheres, they become hydrated mainly around 500°C and exhibit mixed proton and oxygen ion conductivity. Part of the water uptake toward the theoretical hydration limit takes place in a step, probably a phase transition related to oxygen vacancy disordering or proton ordering, or both. The transition is accompanied also by steps and maxima in total conductivity, believed again to be related to order–disorder reactions.


Physical Chemistry Chemical Physics | 2000

The transition from short-range to long-range ordering of oxygen vacancies in CaFexTi1−xO3−x/2 perovskites

Ana Isabel Becerro; Falko Langenhorst; Ross J. Angel; Stefan Marion; Catherine McCammon; F. Seifert

The transition from isolated and finite oxygen vacancy clusters in disordered cubic perovskites to structures containing infinite chains of oxygen vacancies in an ordered arrangement was investigated by a combined Mossbauer spectroscopy, X-ray and electron diffraction and high-resolution transmission electron microscopic study. With increasing defect concentration (equivalent to an increase in the CaFeO2.5 component concentration) and/or decreasing annealing temperature, a disorder–order phase transition occurs leading to infinite chains of vacancies in the [101]cubic direction, arranged in sheets containing essentially tetrahedral iron. In the early stages of the ordering process and at low iron concentrations, these sheets exhibit orientational disorder with two different directions of vacancy chains in addition to stacking disorder with variable sequences of tetrahedral and octahedral layers. At higher vacancy concentrations, ordered layer sequences with proportions defined by stoichiometry are formed. The ordering of vacancies into infinite chains and sheets leads to a large and discontinuous decrease in ionic conductivity.


American Mineralogist | 2001

H2O-D2O exchange in lawsonite

Stefan Marion; Hinrich-Wilhelm Meyer; Michael A. Carpenter; Truls Norby

Abstract The mineral lawsonite, CaAl2Si2O7(OH)2⋅H2O, was deuterated to about 93% in a thermobalance by H2O-D2O gas-phase exchange at temperatures between 375 and 425 °C. The kinetics of this reaction are reported and diffusion coefficients for the H2O-D2O exchange are calculated from thermogravimetric measurements.


Phase Transitions | 1999

Ionic and electronic conductivity in CaTi0.9fe0.1o3-δ

Stefan Marion; Ana I. Becerro; T. Norby

Abstract The electrical conductivity of CaTi1−x Fe x O3-δ (x = 0.1) was measured by an alternating current van der Pauw technique versus oxygen partial pressure (10−30-1 atm) and temperature (450–1200°C). The results were interpreted to reflect n-type, ionic and p-type conductivity at respectively low, intermediate and high oxygen partial pressures. The apparent activation enthalpy for the ionic conductivity, interpreted to reflect the mobility of oxygen vacancies, was 0.87 eV. The enthalpy of intrinsic formation of electronic defects (apparent band gap E g) was 3eV. The results are compared with literature data for CaTi0.8Fe0.2O3-δ and with Fe-substituted SrTiO3 and discussed in terms of iron-oxygen vacancy association and ordering.


European Journal of Mineralogy | 2002

Kinetic effects associated with the low-temperature phase transitions in lawsonite

Stuart A. Hayward; R. Burriel; Stefan Marion; Hinrich-Wilhelm Meyer; Michael A. Carpenter

The specific heat of natural and deuterated lawsonite, CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O, has been measured in the temperature ranges 77 K-310 K, using ac calorimetry, and 230 K-320 K, using differential scanning calorimetry. Anomalies due to the C mcm - P mcn ( T c = 273 K) and P mcn - P 2 1 cn ( T c = 130 K in natural lawsonite, 143 K in deuterated lawsonite) transitions have been observed and analysed. By comparing these data with the results of earlier conduction calorimetry experiments, the kinetic behaviour of these two transitions was studied. For the C mcm - P mcn transition, the excess entropy only correlates with other measures of the order parameter (Δ S ∝ Q 2 ) in the situation where an ordered lawsonite is heated very slowly. However, the total excess entropy associated with the phase transition does not appear to vary significantly with the rate or direction of the temperature ramp. Extrapolation of the entropy vs . temperature curves to absolute zero implies a maximum excess entropy of 6 J K −1 mol −1 , consistent with the configurational entropy of a two-site mixing model, with a small excess. For the P mcn - P 2 1 cn transition, the various experiments give mutually consistent results, implying that the transition behaviour is limiting second order without kinetic anomalies.


Solid State Ionics | 2002

Water and protons in electrodeposited MnO2 (EMD)

Scott W. Donne; Frank H. Feddrix; Ronny Glöckner; Stefan Marion; Truls Norby

Abstract A set of solid/gas techniques have been applied to study protons and proton transport in electrodeposited MnO 2 (EMD) which is used in battery cathodes. The concentration of protons has been measured to be 0.4–0.5 mol H + per mol MnO 2 at 24 °C and p H 2 O=0.022 atm by in situ proton–deuteron exchange using heavy water vapour in a thermobalance. The proton content has furthermore been mapped versus T and p H 2 O, and it appears that about half of the protons are present in water loosely adsorbed in microporosity in the EMD. Electrical conductivity and transport number measurements show that EMD is mainly an electronic conductor, while a minor proton conductivity, probably mediated by the adsorbed water, increases strongly with decreasing temperature and increasing p H 2 O and is around 1% of the total conductivity at ambient, humid conditions.


MRS Online Proceedings Library Archive | 2002

Oxygen Vacancies in Perovskite and Related Structures: Implications for the Lower Mantle

Catherine McCammon; Ana I. Becerro; Stefan Lauterbach; Ulrich Bläß; Stefan Marion; Falko Langenhorst; Ross J. Angel; Peter A. van Aken; F. Seifert

The oxygen vacancy ordering process and displacive transitions have been characterised in the system CaTiO 3 -CaFeO 2.5 as a function of composition and temperature at atmospheric pressure using X-ray diffraction, Mossbauer spectroscopy, infrared spectroscopy, transmission electron microscopy, electron energy loss spectroscopy, neutron diffraction and electrical conductivity methods. With increasing concentration of vacancies the following sequence is observed: isolated defects → short defect chains → infinite chains in layers. Similar experiments at high pressures and temperatures have been conducted to determine the nature of oxygen vacancies in the lower mantle phases (Mg,Fe)(Si,Al)O 3-σ and Ca(Si,Fe)O 3-σ perovskite.

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F. Seifert

University of Bayreuth

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Kjell Wiik

Norwegian University of Science and Technology

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