Substitution for copper inYBa2Cu3Oy: The first 3%

A systematic study of published data on the effects of substitution of transition and nontransition metal ions for copper in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ suggests that: (a) limiting behavior can be deduced for its effect on both the critical temperature $T_c$ and the spontaneous strain (SS) from studies on closed-shell dopants (Ga/Al,Zn); (b) the cross correlation between initial values (0%-3%) for $\frac{d{T}_c}{d(\%M)\mathrm{}}$ and $\frac{d\left(\mathrm{SS}\right)}{d(\%M)\mathrm{}}$ provides the definitive discrimination between preferential occupation of the Cu(1) and Cu(2) sites; and, (c) selective doping applies to open-shell [${\mathrm{Fe}}^{3+}$, ${\mathrm{Co}}^{3+}$ in the Cu(1) site and ${\mathrm{Ni}}^{2+}$ in the Cu(2) sitel ions as well as closed-shell [${\mathrm{Ga}}^{3+}$ and ${\mathrm{Al}}^{3+}$ in the Cu(1) site and ${\mathrm{Zn}}^{2+}$ in the Cu(2) sitel ions. It can be further suggested that charge selectivity may well be at the root of the site selectivity expressed in these chemically complex materials.