T. Fries

System (AO)mM2B2n − 2FenO4n + z with m = 2 and n = 1,2 for A ≡ Bi, Pb and M, B ≡ Sr

Determination des parametres cristallins par diffraction RX pour BiPbSr 2 FeO 6,04 , Bi 1,5 Pb 0,5 Sr 4 5Fe 2 O 10,04

Magnetic properties and cation distribution in iron containing pyrochlores

Iron-containing pyrochlores, having ions with various valencies on the B-sites, were investigated by magnetic measurements and Mossbauer spectroscopy. The influence of the ionic radius and of the valence state of the different cations accommodated on A or B sites on the super-exchange interactions and local distortions was emphasized. Three local configurations of iron were given as evidence in agreement with a model based on a random distribution of ions over the B-sites.

Antiferromagnetism of the new non-superconducting 2201 stacking polytype BiPbSr2Fe1-xMxO6+z with M = Co, Ni and Morin-like spin-flop-transition in BiPbSr2FeO6+z

Abstract The magnetic properties and the spin structure of the BiPbSr2Fe1-xMxO6+z compounds with M = Co, Ni, isotypical with the members of the Bi2(T12)Sr2(Ba2)Can-1CunO2n+4 compositional series with n=1, were studied using magnetic susceptibility measurements and 57Fe Mossbauer spectroscopy. All the compounds investigated (with x=0, xCo=0.5 and xNi=0.25) are basically antiferromagnetic. The highest Neel temperature of 212 K is reached for BiPbSr2FeO6+z, where the hyperfine field at the Fe site has with 48.9 T a value close to the expectation for Fe3+ in a high spin state. A comparison between the values of the quadrupole splitting in the paramagnetic and in the magnetically ordered state shows that a spin-flop transition from the basal plane to the c-axis occurs in the temperature range 120-77 K in BiPbSr2FeO6.1. Such a behaviour is reminiscent of the Morin transition in α-Fe2O3, where the spin direction of the antiferromagnetic sublattices rotates from the plane to the trigonal axis. No spin-flop transition takes place in BiPbSr2Fe0.5Co0.5O6.01.

The system BiPbSr2Fe1−xTxO6+z(T≡Co,Ni)

Abstract The Substitution of cobalt or nickel for iron is reported for the system BiPbSr2Fe1−xTxO6 + z, (T ≡ Co, Ni). The average structure can be described by an orthorhombic subcell. The subcell of BiPbSr2FeO6 + z follows the general building principles of the series (AO)2M2Bn−1FenO3n + 1 + z and consists of perovskite-like slabs with one iron oxygen octahedron intercalated between AO double layers. The electrical resistivity is strongly influenced by the parameter x and decreases with increasing x.

Mössbauer spectroscopy study of (Bi, Pb)2Sr2Bin−1FenO3n+3+z stacking polytypes with n = 2, 3, 4

Abstract The (Bi, Pb)2Sr2Bin-1FenO3n+3+z compounds are strongly related to the members of the high-Tc superconducto r series (Bi, Pb)2Sr2Can-1CunO2n+4+z. In this study we show that the Fe compounds with n = 2, 3, 4 are antiferromagnets with Neel temperatures of 512, 597 and 620 K. The saturation hyperfine field and isomer shift values correspond to Fe ions in the 3+ oxidation state. For the n = 3 and 4 compounds, the different subspectra belonging to the inequivalent Fe lattice sites were measured, and the temperature dependence of their hyperfine fields was interpreted in the molecular field approximation.

Processing of Bi-2223 ceramics in the system BPSCCO

Abstract Systematic studies of the development of the superconducting properties for air or Ar/O2(8%) preparation of 2223 ceramics via a sol/gel, solid state or melt process indicate that (i) 2223 has the tendency to grow beneath a superficial 2212 layer, (ii) the growth rate is drastically accelerated in a reduced oxygen atmosphere, (iii) the sol/gel technique is superior to the solid state route, (iv) for melt processing the 2223 content of product and precursor is directly correlated.

Stacking polytypes in the systems Bi-Pb-Sr-Ca-T-O with T = Mn, Fe, Co, Ni, Cu

The influence of chemical substitution on the properties of the stacking polytypes (AO) m M 2 B n−1 T n O y (A, B, M = (Bi, PB), Sr, Ca) with T = Mn, Fe, Co, Ni, Cu is studied for the systems Cu-2212 (Bi 1 6 Pb 0.4 Sr 2−x Ca 1−y Nd x+y Cu 2 O 8+z Bi 2−x Pb x Sr 2 Ca 1−y Gd y Cu 2 O 8+z ); T-2201 (BiPbSr 2 TO 6+z TO 6+z ; T=Fe, Co, Ni) as well as for the series T-2212 and T-3212 with T = Mn, Fe.