123 Near equilibrium samples: Anomalies of the lattice parameters and some physical properties

Abstract Contrary to the literature, the dependence on nonstoichiometry of the lattice parameters of “nearly” equilibrated 123 samples (particularly the c- and b-axis), shows strong changes of slope in the nonstoichiometric range x = 6.8–6.9. At x = 7.0-6.9 the c-axis decreases with decreasing oxygen concentration. And at x ≅ 6.9 the well known trend sets on:increase of c-axis and decrease of b-axis with decreasing oxygen content. These changes are reflected in the physical properties investigated up to now: • The dependence of Tc on hydrostatic pressure is 6.9, increases strongly in the range x = 6.9-6.8 and shows a change of slope at x = 6.8 where it has reached the value dTc/dP = 0.35 K/kbar. At x = 6.5 is dTc/dP = 0.45 K/kbar. (Also the tc as a function of nonstoichiometry shows the well known decrease at x = 6.8). • The muon spin-depolarization rate as a function of nonstoichiometry shows also an unexpected strong decrease (25%) in the range x = 7.0-6.9. This suggests that the density of the superconducting carriers decreases strongly in this range. It seems possible from the lattice parameter effects that a threshold concentration of oxygen vacancies in the chains is necessary in order that the expansion of the apical bond sets on.