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Start Over Author "Studer, F" Journal journal of solid state chemistry
34 results on '"Studer, F"'

16. Spin State Transition in LaCoO3with Temperature or Strontium Doping as Seen by XAS

Author

Toulemonde, O., N'Guyen, N., Studer, F., and Traverse, A.

Abstract

An X-ray absorption spectroscopy study of the polycrystalline La1−xSrxCoO3(x=0, 0.25, 0.5) series at O K-edge was performed above and below the various magnetic transition temperatures. A crystal field analysis is proposed to explain the observed fine structure changes. Temperature-dependent O K-edge of LaCoO3shows evidence of smooth and continuous spin transition of Co3+from low to intermediate spin states. This result is confirmed by cobalt K-edge measurements. Upon strontium doping, O K-edges of the La1−xSrxCoO3(x=0, 0.25, 0.5) series suggested a mixed (LS–IS) Co3+/IS Co4+cobalt ion configuration. The fundamental mechanism governing the magnetotransport properties shown by the compounds is then discussed.

Published
2001
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17. Structural Aspects and Antiferromagnetic Ordering in the "123" Derivative LnSr2Cu2Ga1-xFexO7-δ(Ln= Y, Ho)

Author

Rykov, A., Caignaert, V., Van Tendeloo, G., Greneche, J.M., Studer, F., Nguyen, N., Ducouret, A., Bonville, P., and Raveau, B.

Abstract

The magnetic ordering in the orthorhombic phases LnSr2Cu2Ga1-xFexO7(Ln= Y, Ho; 0.03 ≤ x≤ 0.95) has been investigated by 57Fe Mössbauer spectroscopy. Iron occupies the Ga site (Cu(1) site) for small doping rates (x≈ 0.03), but occupies the Cu(1) and Cu(2) sites for iron-rich compositions (0.5 ≤ x≤ 0.95). For x= 0.9, the magnetic ordering takes place below TN≈ 420 K in both the Cu(1) and Cu(2) sites. The down concave curve of the temperature dependence of the hyperfine field in the Cu(1) site suggests a rather weak coupling between the Cu(1) and Cu(2) sites. The principal EFG axis VZZis directed in both sites along the longest axis of the orthorhombic cell, i.e., perpendicular to the alternating sheets. The sign of VZZis positive for the Cu(2) sites (ΔEQ= 0.6 mm/sec) and negative for the Cu(1) sites (ΔEQ= -1.9 mm/sec). In the distorted tetrahedral site, the EFG tensor is non-axial, however, its asymmetry is small (η < 0.4). The magnetic hyperfine field is perpendicular to the VZZaxis for both sites; i.e., H lies in the plane of the shortest axes. A structural analysis by electron microscopy reveals the presence of a large number of defects in the (Fe, Ga)O chain ordering as well as in the basic structure. The presence and the abundance of these defects is in agreement with the Mössbauer observations.

Published
1994
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33. Extension of the “1201” Family to Strontium-Rich Chromite and Ferrite, Bi0.4Sr2.5Cr1.1O4.9 and Bi0.4Sr2.5Fe1.1O5

Author

Allix, M., Pelloquin, D., Studer, F., Nguyen, N., Wahl, A., Maignan, A., and Raveau, B.

Subjects
*CHROMITE, *FERRITES, *STRONTIUM
Abstract

Two new Sr-rich “1201”-type oxides, Bi0.4Sr2.5Cr1.1O4.9 and Bi0.4Sr2.5Fe1.1O5 have been synthesized. These compounds, intergrowths of double rock-salt layers with single perovskite layers, show a 1:1 ordering between (Bi,M) and Sr species within the intermediate rock-salt layer [Bi0.4M0.1Sr0.5O]. The XANES study shows that bismuth is mainly trivalent, whereas iron is mixed valent containing 50% Fe3+ and 50% Fe4+ (also confirmed by Mo¨ssbauer), and chromium could be a mixture of Cr3+ and Cr6+ sitting in the perovskite and rock-salt-type sites, respectively. Both compounds exhibit antiferromagnetic interactions. The Cr-phase is a strong insulator, whereas the Fe-phase exhibits a semi-conductor-like resistivity whose value at room temperature is close to that of isotypic cobaltite. [Copyright &y& Elsevier]

Published
2002
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34. The oxycarbonates Sr4(Fe2−xMnx)1+y(CO3)1−3yO6(1+y): nano, micro and average structural approach

Author

Bréard, Y., Michel, C., Hervieu, M., Nguyen, N., Studer, F., Maignan, A., Raveau, B., and Bourée, F.

Subjects
*CARBONATES, *MANGANESE
Abstract

The possibility to synthesize layered oxycarbonates, with nominal composition Sr4Fe2−xMnxO6CO3 involving trivalent manganese, with 0≤x≤1.5, is reported for the first time. The structural study of Sr4FeMnO6CO3 using NPD, HREM, Mo¨ssbauer and XANES, shows that this phase is closely related to n=3 member of the Ruddlesden–Popper family. It derives from the latter by replacing the middle layer of transition metal octahedra by triangular CO3 groups, with two different “flag” and “coat hanger” configurations. The magnetic order is antiferromagnetic and fundamentally different from the magnetic behavior of Sr4Fe2O6CO3. [Copyright &y& Elsevier]

Published
2003
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