[en] Phase transitions and twin lamellae of the (Bi,Pb)2223 phase have been studied by in situ heating experiments in a transmission electron microscope over a range of temperatures 293-800 K. In some crystal grains of the (Bi,Pb)2223 phase a modulation with the modulation wave vector q congruent with 0.137b* (Pb-type) was observed with 1st order satellites associated with the [0 0 1] zone. Furthermore, a monoclinic distortion was observed, as indicated by splitting of the reflections in the b* direction. The monoclinicity was confirmed by dark-field images, which showed (1 0 0) twin lamellae. Diffuse streaks along < h h 0> are present in some crystals already at room temperature, but appear in others at temperatures below 365 K. Their intensities sharpen with increasing temperature, and at 630 K a 2ax2b superstructure appears. At the same temperature the monoclinic distortion and the twin lamellae disappear gradually. The new 2ax2b superstructure and Pb-type modulations coexist until the phase starts to decompose. The change of these structural modulations and twin transformations are discussed in connection with oxygen ordering within the different structural layers

[en] A sol-gel method was successfully applied to synthesize RuSr₂GdCu₂O₈ (Ru-1212) as a single phase. The crystallization of Ru-1212 arises at 950 deg. C under O₂ flow with a small amount of secondary phase. The pure Ru-1212 phase is achieved after heating at 1020 deg. C in O₂ flow. The X-ray diffraction (XRD) pattern is refined (Rietveld) in the tetragonal space group P4/mmm with lattice parameters a=3.83904(9) Angst and c=11.5678(4) Angst. In situ high-temperature XRD and differential thermal analysis coupled with thermal-weight measurement show a structural decomposition of the phase at T_d=1050 deg. C followed by a partial melting at T_m=1118 deg. C. The decomposition produces crystallized Sr₂GdRuO₆, SrRuO₃ phases and a mixture rich in copper containing 'Sr, Gd, Cu, O' that does not diffract X-ray. This phase reduces to Cu⁺ at T_m with an important weight loss and a significant amount of liquid. The grain size and/or inappropriate grain boundaries of the pure phase treated below T_m does not permit to detect in sol-gel samples superconductivity otherwise observed in compounds prepared by solid-state reaction

[en] Single crystals of MgB₂ and Mg(B_1-xC_x)₂ have been grown using cubic anvil technique. T_c values vary in a wide range (39-9 K) with carbon content varying from 0% up to 16%. Using SiC as the precursor leads to C and not to Si substituted crystals. Micro-hardness measurements performed on MgB₂ single crystals give average value of 1100 kg/mm²

[en] Single crystals of MgB₂ have been grown with a high pressure cubic anvil technique. They grow via the peritectic decomposition of the MgNB₉ ternary nitride. The crystals are of a size up to 2 x 1 x 0.1 mm³ with a weight up to 230 μg. Typically they have transition temperatures between 38 and 38.6 K with a width of 0.3-0.5 K. Investigations of the P-T phase diagram prove that the MgB₂ phase is stable at least up to 2190 deg. C at high hydrostatic pressure in the presence of Mg vapor under high pressure. Substitution of aluminum for magnesium in single crystals leads to stepwise decrease of T_c. This indicates a possible appearance of superstructures or phases with different T_c's. The upper critical field decreases with Al doping

[en] Impact of Cs additions on the superconducting phases formation, microstructure and properties in the (Bi, Pb)-Sr-Ca-O system is studied. It is established that the Cs additions in the (Bi, Pb)-Sr-Ca-Cu-O systems essentially increase the rate of the 2223 phase formation, decrease the grains size and improve their transport properties

[en] New phase (Nd,Ce)_2+xCaCu₂O_6+y was prepared at a high-pressure/high-temperature condition of 6 GPa and 1300 deg. C. It had a nonstoichiometric composition close to Nd_2.16Ce_0.225CaCu₂O_6+y. According to X-ray diffraction pattern, the Nd_2.16Ce_0.225CaCu₂O_6+y phase has a tetragonal lattice with a 3.845(1) A, c = 19.349(5) A. However, electron microscopic observations revealed a complicated shear structure for this phase. Magnetic susceptibility and magnetic hysteresis measurements were performed for the Nd_2.16Ce_0.225CaCu₂O_6+y sample and it was found that the phase undergoes a weak ferromagnetic transition at 150 K. Below ∼40 K, complicated magnetic behavior was observed suggesting the presence of second weak ferromagnetic transition near 40 K

[en] New phases Sr₈ARe₃Cu₄O₂₄ (A=Sr,Ca) were discovered under high-pressure/high-temperature condition. X-ray powder diffraction and electron diffraction studies for these phases indicated that they have an ordered perovskite-type structure with cubic lattices of ∼8 A. They showed ferromagnetism at room temperature when they were synthesized under high-oxygen-pressure condition. The Ca-containing phase has a very high T_c of 440 K with a spontaneous magnetization of ∼1 μ_B/f.u

[en] X ray diffraction and electric conductivity measuring methods were applied to study the influence of synthesis time and temperature on formation of superconducting phases and their properties in Bi_1.6Pb_0.4Sr_1.98K_0.02Ca₂Cu₃F_0.8O_y specimens. It is stated that a 2223/2212 phase ratio having been formed during 240 h synthesis at 845 deg C is practically unchanged in the range of 845-855 deg C independently of the duration of additional synthesis. The synthesis at higher temperatures results in 2223 phase decomposition and an increase of 2212 and impurity phase contents. The specimens synthesized at 845 deg C for 240 h have maximal values of T_c (R = 0) equal to 112.8 K

No abstract available

No abstract available