J. Berthon

Crystal growth and characterization of the spin-ladder compound (Sr, Ca)14Cu24O41

Abstract High-quality centimetric size single crystals of Sr 14− x Ca x Cu 24 O 41 (0⩽ x ⩽12) have been successfully grown using the travelling solvent floating zone (TSFZ) method associated with an image furnace. We present the growth conditions together with a characterization of the single crystals by means of optical and electron microscopy, EDX, X-ray powder diffraction, Laue X-ray back-reflection and neutron diffraction.

Growth of wustite (Fe1−x O) crystals of various stoichiometries

Single crystals of wustite, Fe1−x o, with x varying between 0.92 and 0.95, have been grown in an arc furnace. The quality and stoichiometry of the crystals are reported.

Structure of metastable strontium ferrite

The metastable compound SrFe2O4 was prepared by crystallization at 630°C of a glass prepared by melt-spinning. It decomposes above 900°C. It has an orthorhombic unit cell at room temperature with a = 8.031(10) A, b = 18.208(18) A, c = 5.454(6) A, Z = 8, space group Pbc21.

Ferromagnetism and thermoelectric power of La1.5Ca1.5Mn2O7

The authors report on the preparation, X-ray diffraction, ac susceptibility, dc magnetization and thermoelectric power (TEP) of La{sub 1.5}Ca{sub 1.5}Mn{sub 2}O{sub 7}. The compound has a tetragonal structure with lattice parameters a = 3.886(1) {angstrom} and c = 19.34(1) {angstrom}. Magnetic measurements reveal a ferromagnetic transition at around {Tc} = 215 K. TEP measurements indicate hole conductivity and has a maximum at {Tc} indicating an insulator to metal transition. Some possible reasons for the recently reported observation of {Tc} being far above the maximum of resistivity are pointed out.

Delocalization of holes and restitution of superconductivity in Pr substituted at rare earth and Ba site in LnBa2Cu3O7(Ln=Nd, Sm)

Abstract Pr substituted at Sm site in SmBa2Cu3O7 did not induce any orthorhombic to tetragonal transition whereas 15% of Pr at Ba site did. Superconductivity was destroyed with 45% of Pr at Sm site or 15% of Pr at Ba site. However, Sr substitution at Ba site in both the cases restored it. Similar data were obtained in the case of Pr in NdBa2Cu3O7. Our data seem to suggest different mechanisms for hole delocalization operate depending on the site occupation of Pr.

Controlled reduction and oxidation of La0.85Sr0.15MnO3 single crystals

Abstract La 0.85 Sr 0.15 MnO 3 single crystals were grown by the floating zone method. They can be reversibly reduced under controlled oxygen partial pressure. Their defect chemistry is investigated together with the evolution of their cell parameters and that of their electric conductivity which is very sensitive to their oxygen content. On the other hand, stoichiometric crystals can be oxidized with a slow kinetics making possible the preparation of diphasic materials.

Effect of isovalent and non-isovalent substitutions on superconductivity in NdBa2−xPrxCu3O7

Abstract We have compared for the first time the structural and superconducting properties of (A) NdBa 2− x Pr x Cu 3 O 7 and (B) Nd 1− u Pr u Ba 2 Cu 3 O 7 . An orthorhombic to tetragonal transition was found in A for x > 0.2 whereas for B no such transition has been reproted. In both cases superconductivity was destroyed for x = 0.3 and u = 0.3 and could be restored by isovalent Sr substitution. However, Ca substitution at the Nd site restored superconductivity only in the case of A. Our results indicate that different mechanisms have to be invoked to account for the suppression of superconductivity in compounds A and B.

Preparation of glass ribbons in the fe2O3-SrO system by melt-spinning

Abstract Amorphous materials were prepared by the melt-spinning technique in the Fe 2 O 3 SrO system, for the composition 55 Fe 2 O 3 -45 SrO (mol%O) and 50 Fe 2 O 3 -50 SrO (mol%). The glasses were characterized by X-ray diffraction, DTA and magnetic measurements.

Insulator-superconductor transition in Tm0.4Pr0.6Ba2Cu3O7 by iso-valent substitution at Ba site

Abstract We report on the preparation, X-ray diffraction and a.c. susceptibility of Tm0.4Pr0.6Ba2−kSrkCu3O6+z. An orthorhombic to tetragonal transition was observed for k = 0.8. When the samples (for 0 ≤ k

XAFS study of sixfold coordinated arsenic(V) in LiAsO3 and CaAs2O6

Abstract Conversely to previously reported data, EXAFS indicate that the AsO distances are similar in LiAsO3 and CaAs2O6. The main features of the XANES spectrum of LiAsO3, except the intense white line, come from single and multiple scatterings on atoms found in a small cluster surrounding the absorbing As atom ( R A ).