Correlation Between Enhanced $T_{\rm c}$ , Orthorhombicity and the Volume of the Unit Cell in Argon Preheated

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.