Substitutional doping and oxygen vacancies in La 2Zr 2O 7 pyrochlore oxide

Abstract

Density-functional calculations have been combined with thermodynamic modeling to study substitutional acceptor dopants and oxygen vacancies in La 2Zr 2O 7 pyrochlore oxide in equilibrium with an oxygen-containing atmosphere. Dilute-limit relative formation energy and corresponding relative concentration with respect to site selectivity and charge state have been calculated for twelve di- and trivalent dopants (Ba, Ca, Mg, Sr, Yb; In, Ga, Al, Nd, Gd, Sc, and Y) incorporated on the La- and Zr sites. Similarly, the formation energy of oxygen vacancies has been calculated, as well as dopant–vacancy pair interaction energies. The study showed that site selectivity of the dopants is correlated to dopant size while on-site charge state is largely determined by valence of the dopant and host cation. Furthermore, formation of oxygen vacancies at atmospheric oxygen partial pressure was shown to be expected only in acceptor-doped samples at elevated temperatures. According to the results, vacancies will then form predominantly on oxygen sites in crystallographic positions 48f due to greater lattice relaxation compared to vacancy formation on the 8b site. In the pair interaction between vacancies and dopants the results showed generally stronger vacancy attraction for dopants on the Zr site compared to the La site and correlation between dopant size and interaction strength was seen. In particular the importance of elastic mechanisms is pointed out. Finally, the effects of the substitutional doping on the concentration and mobility of oxygen vacancies are discussed.