Mineo Kogachi

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.

MAGNETIC SUSCEPTIBILITY OF YBa2Cu3O7−Δy

Magnetic properties of YBa2Cu3O7−Δy was investigated by dc- and ac-magnetic susceptibility as a function of oxygen deficiency Δy. The susceptibility in the normal state at low temperatures is dominated by the Curie-Weiss law, while the susceptibility at high temperatures deviates from the Curie-Weiss law. The deviations were successfully explained by assuming the triplet spin-pair excitation from the antiferromagnetically coupled Cu2+ ions in the ground state.Magnetic properties of YBa/sub 2/Cu/sub 3/O/sub 7/-/Delta//nu/ was investigated by dc- and ac-magnetic susceptibility as a function of oxygen deficiency /Delta/y. The susceptibility in the normal state at low temperatures is dominated by the Curie-Weiss law, while the susceptibility at high temperatures deviates from the Curie-Weiss law. The deviations were successfully explained by assuming the triplet spin-pair excitation from the antiferromagnetically coupled Cu/sup 2+/ ions in the ground state.

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.

Axial Ratio Change of the L10-Type CuAu1-yPdy Quasibinary Alloys

The composition dependence of the lattice parameters a and c of CuAu1-yPdy (0≤y<0.53) with the L10-type structure was investigated by X-ray diffraction method. The axial ratio c/a increases with increasing y from 0.926 of the terminal alloy CuAu at first, takes a maximum value of 0.936 at y=0.08, and then decreases almost linearly to 0.856 at y=0.53. The internal energy of a quasibinary system derived by using the Morse potential function was applied to explain the change in the axial ratio with the composition of present alloy system.

Order-Disorder Transformation in Cu3Au–Cu3Pd and Cu3Au–Cu3Pt Quasibinary Systems

The phase relations and kinetics of ordering in fcc-type Cu3Au1-xPdx and Cu3Au1-xPtx (0≤x≤1) were studied. The L12-phase, which is stable in the three terminal binary alloys, also exists in both these quasibinary systems over the whole composition range. The transition temperature from the L12-phase to the Al-phase has a maximum at x=0.55 for the former system and at x=0.56 for the latter system. The dependence of the long-range order parameter upon temperature observed for the stoichiometric alloy Cu3Au0.5Pd0.5 indicates that the transition is of the first order. The superlattice formation was treated thermodynamically by the Bragg-Williams approximation. The transition temperatures from the L12-phase to the disordered Al-phase were calculated for various compositions. The calculated results agree fairly well the experiments for both systems.