J. Primot

Role of lead substitution in the production of 110-K superconducting single-phase Bi-Sr-Ca-Cu-O ceramics

We have systematically investigated the influence of the three parameters (temperature of sintering, lead content, and duration of the sintering) on the growth of the (2:2:2:3) phase in ceramics of nominal composition Bi2−xPbxSr2Ca2Cu3O10−x/2, and have determined the compositions and thermal conditions leading to ceramics with Tc≊110 K and containing more than 95% of this phase. We show that this result can be obtained by usual calcination near 880°C in air, without recurring to the introduction of excess in the constituants, providing that intermediate grinding are effected, in order to reactivate the formation of the (2:2:2:3) phase which otherwise saturates for long durations of sintering. The suitable nominal compositions (0.3≤x≤0.4) correspond to those that yield inside the grains a lead content in the vicinity of the limit of solubility for lead. From the results of the preceding investigations and of experiments effected on lead‐substituted single crystals of the (2:2:1:2) phase, we have clarified ...

Etude dilatometrique de la transition ferroelastique de l'orthophosphate de plomb monocristallin

Abstract Single crystals of lead phosphate Pb3 (PO4)2 have been grown by Czochralski pulling. Thermal expansion has been investigated in this material by standard dilatometry and by X-ray powder diffraction, between room-temperature and 250° C, across the 180° C ferroelastic phase transition. Behaviour of the spontaneous strain, a most important quantity for ferroelastics, is deduced from thermal expansion measurements along the b and c monoclinic axes. The large room-temperature value of the strain component corresponding to a shear in the (100) plane of the crystal, 22 × 10−3, agrees with the value derived from lattice parameters. A jump of the spontaneous strain is observed at the Curie point which discloses a first order transition. This result, confirmed by differential thermal analysis measurements, is also consistent with recent birefringence data. The estimated transition heat is 0.17 cal/g.

Preparation, cristallogenese et caracterisation des bromures mixtes de lanthanide, de sodium et de cesium Cs2 NaLn (III) Br6

Single crystals of the new perovskites Cs2Na Ln (III) Br6 (with Ln (III) = Ce, Nd, Gd, Y) have been grown by the Bridgmann-Stockbarger method, using sealed silica-glass ampoules. They have been characterised by analytical and standard X-Ray methods; they crystallised with cubicface centered lattices, the parameters of which are: a = 11.508 - 11.421 - 11.370 - 11.304 A for Ln = Ce - Nd - Gd - Y.

Crystal growth and structural particularities of (BaF2)1−x (Y,LnF3)x solid solutions

Abstract Highly transparent single crystals of (BaF 2 ) 1−x (Y,LnF 3 ) x′ with Ln = Yb, Nd, are grown by the Bridgman technique. Lattice parameter variations are given, and the compensation by interstitial fluorine ions of the structure is verified from the crystal density.

Influence of lead substitution on the properties of the superconducting materials Bi2 − xPbxSr2Can − 1CunO2n + 4

Abstract It is shown that the structural modifications induced by the partial substitution of bismuth by lead in Bi-Sr-Ca-Cu-O is to establish bonding between the building blocks of the structure of these material, these blocks being almost independent in the lead-free compounds. On this basis one can understand two important aspects concerning the lead-subtituted materials: 1. i) the possibility of obtaining 110K superconducting single phase ceramics, 2. ii) the increase of the percolation threshold for the 110K zero-resistivity transition in thin films, with respect to the situation which is observed in lead-free thin films.

Phase characterization and superconductivity transitions in fluorinated YBaCuO

We present the results of an investigation of ceramics of nominal composition YBa2Cu3OxFy. X-ray diffraction and electron microscopy have determined the presence of BaF2, CuO, Cu Y2O6 and YBaCu0 phases. The occurence of a resistivity anomaly at 150K in y = 1.5 ceramics is atiributed to an important presence of insulating layers of BaF2 which strongly pertubs the electrical measurements.

NMR study of 17o in the superconductor Bi2Sr2CaCu2O8+x

Abstract 17O NMR measurements in a powder sample of Bi2Sr2CaCu2O8+x in the normal as well as in the superconducting phase have allowed us to assign the two lines of the spectrum to the oxygen site in the SrO planes and in the CuO2 planes. The absence of the expected BiO line is accounted for on the basis of the incommensurate superstructure existing in these compounds. The spectra obtained for T < Tc are analyzed.