Correlation Between Enhanced $T_{\rm c}$ , Orthorhombicity and the Volume of the Unit Cell in Argon Preheated $( \hbox{Y}_{1 - x}\hbox{Nd}_{x})\hbox{SrBaCu}_{3}\hbox{O}_{6 + z}$

We report on the preparation, X-ray diffraction with Rietveld refinement, ac magnetic susceptibility measurements, and effect of heat treatments in
$(\hbox{Y}_{1 - {\rm x}} \hbox{Nd}x)\hbox{SrBaCu}_{3}\hbox{O}_{6+{\rm z}}$
.(
$x = 0$
, 0.2, 0.4, 0.5, 0.6, 0.8, 1). Each sample was subject to two types of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. For each
$x$
, the [AO] heat treatment increases the orthorhombicity
$\varepsilon = (b - a)/(b + a)$
(for
$0 \leq x < 1$
), the distance
$d[\hbox{Cu}(1)-(\hbox{Sr/Ba})]$
for
$x < 0.25$
and
$T_{\rm c}$
(for
$x > 0.2$
). The latter increases by 9.8 to 77.2 K for
$x = 1[\hbox{AO}]$
. When
$x$
decreases from 1 to 0,
$T_{\rm c}$
increases with
$\varepsilon$
, with a transition from tetragonal to orthorhombic structure in the samples [O]. Remarkable correlations were observed between
$T_{\rm c}(x)$
and the volume of the unit cell
$V(x)$
; and between
$\delta T_{\rm c}(x) = T_{\rm c}[\hbox{AO}] - T_{\rm c}[\hbox{O}]$
and
$ \delta\varepsilon(x)$
. A combination of several factors such as decrease in
$d[\hbox{Cu}(1)-(\hbox{Sr/Ba})]$
; increase in cationic and chain oxygen ordering; the number
$p_{\rm sh}(x)$
of holes by
$\hbox{Cu}(2)-\hbox{O}_{2}$
superconducting planes and in-phase purity for the [AO] samples may account for the observed data.