Nobuo Fukuoka

Single High-Tc Phase Region of the Bi-Pb-Sr-Ca-Cu-O System

The phase diagram in the superconducting Bi1.68PbvSrxCavCuzOw(v=0.28, 0.32) system was investigated by means of X-ray diffraction and electrical resistivity for samples calcined in air. The results revealed that the phase was classified into four regions, a single 110 K-phase region, two types of two-phase regions (110 K phase+Ca2CuO3 and 110 K phase+80 K phase) and a three-phase region (110 K phase+Ca2CuO3+80 K phase). The single 110 K phase with a Tc of about 106–108 K exists in a fairly wide composition range, especially in the Bi1.68Pb0.32Sr1.75CayCuzOw system, 1.75y1.85 and 2.65z2.85. This means that the single 110 K phase can be easily obtained, even by the usual calcination in air in the Bi-Pb-Sr-Ca-Cu-O system.

Magnetic and Electrical Properties of Manganese Telluride

The temperature dependence of the susceptibility of the antiferromagnetic compound MnTe was measured over a range of temperature between liquid nitrogen temperature and 720°C. An abrupt change in slope and a maximum of the susceptibility curve were found at 37°C and at 55°C respectively. These results are compared with the observed Neel temperatures previously reported by Squire and by Serre. A thermal hysteresis was also found above the Neel temperature, relating to the heat history of the specimen. It is shown that the susceptibility of MnTe 1+ x (0≤ x ≤1), where x is the excess content of tellurium, is explicable in terms of two phases, MnTe and MnTe 2 which posses different susceptibilities. The electrical properties, i.e., the resistivity, the thermo-emf and the Hall emf, were measured as functions of temperature. Evidence was found of anomalous behaviours at the Neel temperature for them. A large thermal hysteresis of the resistivity is also found above the Neel temperature, which suggests a change ...

Crystal Structure of Antiferromagnetic NiO Determined by X-Ray Topography

Spontaneous magnetostriction of NiO, which consists of two strain components, is reexamined in detail by Berg-Barrett method. The strain components thus determined are e x x -e y y =-5.4×10 -4 and e z x =0.91×10 -4 , respectively. These values are a few times as large as those found previously by Yamada et al. and their signs are in agreement with those expected theoretically. A precise crystal structure with monoclinic symmetry and a corresponding magnetic symmetry has been deduced from the experimental results.

Orthorhombic-Tetragonal Phase Boundary in YBa2Cu3O7-Δy

The orthorhombic-tetragonal phase boundary in the (Δy-T) diagram was determined by means of in situ X-ray diffraction methods for the YBa2Cu3O7-Δy. To cover the oxygen content region as widely as possible, two different methods were adopted. The first was performed under the fixed oxygen partial pressure and the second under the fixed oxygen content (deficiency Δy) of the samples. The obtained phase boundary showed a peculiar shape which differed from that proposed previously, suggesting the existence of another orthorhombic (ortho II) phase in low temperature region.

The Orthorhombic to Tetragonal Transition under Controlled Oxygen Content in High-Tc Superconductor YBa2Cu3Oy

The orthorhombic to tetragonal phase transition in YBa2Cu3Oy (y=6.9-Δy) compounds under fixed oxygen content and its relation to superconductivity were investigated by means of X-ray diffraction and resistivity measurement. In the quenching method, the phase transition was observed at about Δy=0.62 (800°C quenching temperature). In the region Δy=0.62–0.69, the high-temperature tetragonal phase transformed into the low-temperature orthorhombic phase by slow cooling, but at Δy>0.69 such a transformation was not observed. No essential difference in Tc was observed between different phases having the same oxygen content in the region Δy>0.5.

Phase transition of Y2BaCuO5

Phase transition of Y2BaCuO5 has been studied by X-ray diffraction, thermal gravimetric and differential thermal analyses. At temperatures higher than 1265°C in air, the transition has a body-centered cubic structure with a probable space group of Ia3 and the lattice constant of a=1.0602 nm. The transition temperature Tc(°) from the well known orthorhombic phase to the cubic one was changed depending heavily on the oxygen partial pressure P(atom) as Tc=1295+47.5 log P.

Effect of Composition Change on Superconducting Properties in a Y-Ba-Cu-O Compound System

The superconducting properties in the Y-Ba-Cu-O compound system were investigated by varying the composition ratio Y:Ba:Cu by means of electrical resistivity measurements and X-ray diffractometry. Many compounds exhibited superconductivity and these were located in a comparatively wide composition range, especially in the Cu-rich side, around the ratio of Y:Ba:Cu=1:2:3. The onset temperatures were nearly the same and the measured lattice parameters showed no great differences from those of YBa2Cu3Oy.