Phase diagram and new phases in the Y-Ba-Cu-O system at high oxygen pressure

The Y-Ba-Cu-O phase diagram has been determined at 980° C at high oxygen pressure 200 bar=20 mPa. It shows several differences from the phase diagram at 1 bar. Highly oxidized phases are stabilized: BaO2 instead of BaO and YBa2Cu4O8 instead of YBa2Cu3O7-x. Two new phases have been observed: YBa5Cu2O10-x has a tetragonal unit cell with a=5.88 Å and c=8.04 Å , consistent with a four layered structure such as CuO2-BaO-(½ Y + ½ Ba)O-BaO, while YBa2Cu2O6-x appears to be isomorphous with the superconductor La1.6Sr0.4CaCu2O6. In addition, we find a high-pressure BaCuOx phase, its X-ray diffraction (XRD) pattern indicates a perovskite related layered structure which has a unit cell a=5.68 Å , c=14.40 Å . The formation of compounds on the YO1.5-BaO line: Y2Ba2O5, Y2BaO4 and Y2Ba4O7 is suppressed at high P(O2). Tie lines are observed to join CuO-2:1:1, 2:0:2-2:1:1, YO1.5−2:1:1, 2:1:1-BaO2, CuO-1:2:4 and 2:1:1-1:2:4

Studies on superconductivity in Y<SUB>1</SUB>La<SUB>x</SUB>Ba<SUB>2-x</SUB>Cu<SUB>3</SUB>O<SUB>7</SUB>± compositions

Compounds of the series Y1Ba2-xLaxCu3O7&#177; where La3+ ions substitute for the Ba2+ have been synthesised upto x=1 and their powder x-ray diffraction give no evidence of any extra phase. The structure changes from orthorhombic to tetragonal at x~0.4. All the compounds of this series are superconducting but the Tc decreases with increasing x. The oxygen content per unit cell increases with increasing La3+ in such a way that (Cu---O)1+ concentration remains unchanged at~0.7 for all compositions. Still the Tc falls abruptly from~90K for orthorhombic phase to &#x003C;50K for the tetragonal phases

Zero resistance at 132 K in lead-free Bi<SUB>1.9</SUB>Sb<SUB>0.1</SUB>Sr<SUB>2</SUB>Ca<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>y</SUB>

Zero resistance has been observed at 132 K in a material which has a nominal composition of Bi1.9Sb0.1Sr2Ca2Cu3Oy. X-ray results suggest predominance of the 2:2:2:3 phase but the well known transition at 105-110 K is absent, which apparently has been enhanced to 132 K

Superconductivity in compound Y<SUB>1&#8722;2x</SUB>Ce<SUB>x</SUB>Ba<SUB>2+x</SUB>Cu<SUB>3</SUB>O<SUB>6&#x00B7;5+&#x0431;</SUB> system

Partial substitution of Ce in place of Y has been achieved by taking care of charge compensation and compounds with the general formula Y1&#8722;2xCexBa2+xCu3O6&#x00B7;5+&#x0431; have been successfully prepared. X-ray powder diffraction analysis reveals that this substitution gives orthorhombic single phase for 0&#8804; x&#8804;0&#x00B7;3. The Tc remains nearly unchanged between 94-90 K

Calcium substitution at yttrium site in YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>y</SUB>

We have successfully synthesized single-phase superconducting Y1-xCaxBa2Cu3Oy for 0&#8804; x&#8804; 0.4 using a coprecipitation technique. All these compounds are orthorhombic with lattice parameters a~3.83 and c varying from 11.69 to 11.67 &#197; . The Tc remains unchanged around 90 K. It may be noted that in these compounds Ca substitutes for Y. On the other hand, it is known that in the compounds where Ca substitutes for Ba, the structure changes to tetragonal and Tc decreases rapidly. [See G. J. Baldha, R. B. Jotania, H. H. Joshi, H. N. Pandya, and R. G. Kulkarni, Solid State Commun. 71, 839 (1989)]. Furthermore, these compositions sinter much better than pure 1:2:3 and one gets high-density pellets (&#x22D9; 95% of total density) at relatively lower temperature (&#171; 950 &#176; C)

Enhanced critical current density in Ca-doped Y<SUB>1</SUB>Ba<SUB>2</SUB>Cu<SUB>4</SUB>O<SUB>8</SUB>

The Jc of Ca0.1Y0.9Ba2Cu4O8 (Ca-1:2:4) synthesized in elevated oxygen pressure is found to be &#x2273;106 A/cm2 at low temperatures and fields, two orders of magnitude greater than the Jc of undoped Y1Ba2Cu4O8 (1:2:4). The Jc (determined by magnetic hysteresis measurements) remains high (5&#x00D7;105 A/cm2) up to B=4 T at 5 K, and is still&#x2273;104 A/cm2 at 40 K and 4 T indicating strong flux pinning. At 77 K, Jc drops to 103 A/cm2 at B=1 T. Ca-1:2:4 was partially converted to 1:2:3+CuO by brief thermal treatment to generate additional flux pinning centers, but Jc did not improve further

Lithium substituted superconducting Y<SUB>1</SUB>Ba<SUB>2</SUB>Cu<SUB>4</SUB>O<SUB>8</SUB> formed in elevated oxygen pressure

Partial substitution of lithium in Y<SUB>1</SUB>Ba<SUB>2</SUB>Cu<SUB>4</SUB>O<SUB>8</SUB> has been accomplished by reacting the constituent oxides with LiOH&#x00B7;H<SUB>2</SUB>O at elevated oxygen pressure (200 bar). T<SUB>c</SUB> falls gradually at a rate of approximately 2 K/at.% of lithium added, up to the solubility limit of ~ 0.13 Li/formula unit. When Li content is increased further T<SUB>c</SUB> remains nearly constant at 57-58 K and the volume fraction of the superconducting phase decreases progressively

Investigations on the formation of new structural phases through stoichiometric deviations on Y sublattice in Y<SUB>1</SUB>Ba<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7</SUB>±

Attempts have been made to form new structural phases by introducing Y deficiency in YBa2Cu3O7&#177; structure and to see if the vacancies can be ordered to give new superstructures. Y0.5Ba2Cu3O7&#177; and Y.66Ba2Cu3O7&#177; have been successfully prepared with structure isomorphous to 1:2:3. These compounds are superconducting with Tc between 90-92K. No ordering is, however, observed; instead Ba and Cu ions move and occupy the vacant Y sites