Y. Muraoka

Ferromagnetism and large negative magnetoresistance in Bi1−xCaxMnO3 (x ≥ 0.8) perovskite

Abstract Polycrystalline perovskite manganites Bi 1−x Ca x MnO 3 (x ≥ 0.8) have been prepared by standard ceramic technique, and their magnetic and electronic property are examined down to 5 K. Ferromagnetic moment and reduced resistivity are observed for Bi 1−x Ca x MnO 3 (x ≥ 0.875), and also large negative magnetoresistances appear below T ′ c .

Preparation of hexagonal MoO3 by “Chimie Douce” reaction with NO2

Abstract The metastable hexagonal form of MoO 3 has been successfully prepared starting from the ammonium molybdate, using a “Chimie Douce” reaction with NO 2 gas. The lattice parameters of the as-prepared material were determined by XRD analysis to be: a = 10.633(2) A and c = 3.719(1) A. The composition of this material obtained by physical and chemical analysis is MoO 3 , 0.09 H 2 O, showing that a small amount of water molecules remained while there were no more ammonium ions in this structure. The structural transformation from hexagonal to stable orthorhombic α-MoO 3 phase was observed at 350°C. It was also shown that NO 2 reaction is a very useful method to remove topotactically ammonium ions from a parent structure at low temperature. This “Chimie Douce” process appears to be very promising for obtaining new metastable oxides with large tunnels or interlayer distances.

Structural and superconducting properties of iodine-intercalated Bi2Sr2Ca1−xYxCu2Oz

Abstract The structual and superconducting properties of Bi 2 Sr 2 Ca 1− x Y x Cu 2 O z intercalated with iodine between the Bi-O bila yers were studied. Formation of the stage-1 compound was verified by X-ray analysis, TG analysis and TEM observation. Expansion of the c -axis, which is caused by increase in the distance of the BiO-BiO layers, decreases with decreasing intercalated iodine concentration down to 60%. The expansion of the c -axis of fully intercalated compounds increases with increasing Y content from 7.2 A for x =0 to 7.5 A for x =1.0. The temperature dependence of resistivity showed that iodine-doped samples ( x =0 and 0.4) were more semiconductive than non-doped samples. The T c of the IBi 2 Sr 2 Ca 1− x Y x Cu 2 O z series in the over-doped region was reduced by 2–10 K compared with non-intercalated samples.

Synthesis and superconductivity of oxycarbonates of the Tl-1201 phase

Abstract TlBa x Sr 4- x Cu 2 (CO 3 )O y oxycarbonates with the Tl-1201 structure were synthesized over a wide range of x (1.0≤ x ≤3.2) and examined by X-ray powder diffraction, electron diffraction, high-resolution electron microscopy and magnetometry methods. New phases with the Tl-1201 structure having 6 and 7 times periodicities along the b -axis were isolated. The former was stable for a wide Ba-rich range (1.8≤ x ≤3.2), and the latter was observed for a smaller Ba content range (1.2≤ x ≤1.6). The phase stability was found to depend on the synthesis condition. The maximum T c of the as-synthesized specimen in this series occurred at about 70 K around x =2.0. The Meissner volume fraction of the as-synthesized specimen ( x =2.4) was 30% at 5 K.

Growth of high-quality Bi 2 Sr 2 CaCu 2 O z crystal and characterization

We report growth of high-quality Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub z} single crystals with dimensions 2{times}1{times}0.03mm{sup 3} with a melt and growth technique. The composition of this crystal is homogeneous and very close to the stoichiometric composition. Structure characterization of single crystal was realized by measuring the (001) x-ray diffraction. The lattice parameters of a typical crystal are a=5.414(2){Angstrom}, b=5.413(2){Angstrom} and c=30.823(10){Angstrom}. The superconducting temperature T{sub c}=95 K with a sharp transition width (10{endash}90{percent} level) between 6 and 10 K was determined from resistivity and dc susceptibility. We have measured the magnetization of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub z} single crystal under magnetic field applied along the c-axis. The anomalous second peak effect appeared between 20{endash}30 K. We discuss the qualities of these crystals. {copyright} {ital 1997 Materials Research Society.}

Effects of iodine intercalation on the superconducting properties of the Bi2Sr2−xLaxCuOz compound

Abstract Iodine intercalation of Bi 2 Sr 2− x La x CuO z (0.4 ⩽ x ⩽ 1.0) was studied. Transition from orthorhombic pristine phase to tetragonal intercalated phase was observed due to iodine intercalation. The expansion of the c -axis of the stage-1 intercalation compound increases slightly with increasing La content x . The T c of iodine-intercalated specimens with 0.4 ⩽ x ⩽ 0.6 decreases from that of the pristine, but in the range of 0.7 ⩽ x ⩽ 0.9, the T c of the intercalated specimen increases with respect to that of the pristine phase. Especially, the non-superconducting specimen with x = 0.9 became superconducting due to intercalation. This result demonstrates hole doping by iodine intercalation, i.e. the Cu valence of the pristine compound is increased.

AgI-intercalation into Bi2Sr2CaCu2Oz compound

Abstract Polycrystalline AgI-intercalated Bi 2 Sr 2 CaCu 2 O z is prepared and the structure and physical properties studied. The lattice parameters a and c are determined to be 5.40 and 22.8 A, respectively. The expansion of the lattice parameter c is 7.4 A, compared with that of a Bi 2 Sr 2 CaCu 2 O z (Bi2212) specimen ( c/2: 15.4 A ). This expansion is more than twice as large as that of iodine-intercalation (3.6 A). The EPMA measurement shows that the I/Ag ratio in the AgI-intercalated specimen is greater than 1.0. The T c is 69 K, which is lower than that of Bi2212 (75 K) and almost the same as that of IBi 2 Sr 2 CaCu 2 O z (IBi2212, 67 K). The effect of AgI-intercalation on the superconductivity is discussed.

Synthesis and superconductivity of ‘Hg2Cl2’-intercalated Bi2Sr2Can−1CunOzn = 1−3 compounds

Abstract We have found that ‘Hg 2 Cl 2 ’ can be intercalated into Bi 2 Sr 2 Ca n−1 Cu n O z ( n = −3 superconductors. Intercalated phases were examined by X-ray powder diffraction, electron diffractions, energy dispersive X-ray (EDX) analysis and magnetometry. The expansion along the c -axis upon interaction was estimated to be about 3.5A for n = 1−3 specimens. The presence of both Hg and Cl atoms was confirmed by EDX analysis. The intercalated phase with tetragonal symmetry of n = 1 has a superstructure of three times the peroskite unit along the a -axis, and has kept the staggered stacking of the basic building blocks of the host sample. The superconductivity of the n = 2 phase is also described.

New synthetic routes for preparing perovskites: Electrochemical oxidation and oxidation by NO2

Abstract Two new synthetic low temperature routes for preparing perovskite-related materials, especially metastable phases are depicted. The first one, the electrochemical oxidation, uses the electric field as the driving force for intercalating oxygen atoms within parent oxide networks. The reaction is achieved under anodic potential, in alkaline solution (1 M KOH or NaOH), at room temperature, in air. This process has been used for preparing various perovskite compounds such as AMO 3 (A=Sr, La; M=Fe 1− x Co x ) or A 2 MO 4+ δ (A=La, Nd, Sr). The most relevant results are reported. Potentiostatic and galvanostatic experiments have shown that the amount of intercalated oxygen can be controlled and that the process is reversible. Structural as well as electronic aspects of the oxygen intercalation are discussed and a reaction mechanism is proposed. The second route is based on the exothermic reaction of nitrogen dioxide NO 2 with NH 4 + ions at moderate temperatures (typically T 4 + cations in situ. Topotactic reactions are described for preparing new hexagonal forms of WO 3 or MoO 3 . The reaction process is discussed.

Growth of high-quality Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub z} crystal and characterization

We report growth of high-quality Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub z} single crystals with dimensions 2{times}1{times}0.03mm{sup 3} with a melt and growth technique. The composition of this crystal is homogeneous and very close to the stoichiometric composition. Structure characterization of single crystal was realized by measuring the (001) x-ray diffraction. The lattice parameters of a typical crystal are a=5.414(2){Angstrom}, b=5.413(2){Angstrom} and c=30.823(10){Angstrom}. The superconducting temperature T{sub c}=95 K with a sharp transition width (10{endash}90{percent} level) between 6 and 10 K was determined from resistivity and dc susceptibility. We have measured the magnetization of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub z} single crystal under magnetic field applied along the c-axis. The anomalous second peak effect appeared between 20{endash}30 K. We discuss the qualities of these crystals. {copyright} {ital 1997 Materials Research Society.}