Atomistic simulation of dopant substitution in YBa2Cu3O7

Abstract

A wide range of cation dopant substitutions in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ is investigated using computer-simulation techniques. Attention is focused on site selectivity and possible charge-compensation mechanisms. The calculated solution energies show strong systematic variations as a function of dopant ion radius. Our results suggest that ${\mathrm{Ni}}^{2+}$, ${\mathrm{Zn}}^{2+}$, and ${\mathrm{Cd}}^{2+}$ preferentially substitute for ${\mathrm{Cu}}^{2+}$ in the plane, whereas the alkaline-earth ions ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Sr}}^{2+}$ dissolve in the crystal at the ${\mathrm{Ba}}^{2+}$ site. We consider two extreme cases of localization for trivalent dopant substitution of copper with the compensating oxygen interstitial. The calculations predict that ${\mathrm{Al}}^{3+}$ and ${\mathrm{Fe}}^{3+}$ occupy the Cu(2) site for the delocalized model and the Cu(1) site for the more localized model. Substitution of rare-earth ions, such as ${\mathrm{La}}^{3+}$, is energetically most favorable at the barium site. Correlations between particular bond distances and $T_c$ are also discussed.