Enhancement of ${\rm T}_{\rm c}$, Shielding and Irreversibility Line in Argon Preheated ${\rm Ln}({\rm SrBa}){\rm Cu}_{3}{\rm O}_{6+{\rm z}}$

We report here on the preparation, AC magnetic susceptibility and resistivity measurements of LnSrBaCu
₃
O
_6+z
(Ln = rare earth = Eu, Sm, Nd). Each sample was subject to two types of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. In the normal state, the heat treatment [AO] reduced considerably the linear resistivity parameters indicating a diminution of the interaction of carrier charges with phonons. For each ionic radius r(Ln
⁺³
), Tc increases remarkably by 10 K, 6 K and 6 K to T
_c
[AO] = 78 K, 85 K and 87 K, respectively for Ln = Nd, Sm and Eu. T
_c
(chi') and T
_c
(p = 0) were in good agreement. There was a remarkable improvement in the shielding effect in the case of the samples [AO] at all T < T
_c
and for any applied field. An enhancement of the irreversibility line was observed due to the argon treatment. A remarkable correlation was observed between T
_c
(r) and the number p
_sh
(r) of holes by Cu(2) - O
₂
superconducting planes. A combination of several factors such as the ionic size of the rare earth, increase in cationic and chain oxygen ordering; psh and in-phase purity for the [AO] samples may account for the observed data.