Path Integral Treatment of Proton Transport Processes in BaZrO3

Nuclear quantum effects on proton transfer and reorientation in BaZrO3 is investigated theoretically using the ab initio path-integral molecular-dynamics simulation technique. The result demonstrates that adding quantum fluctuations has a large effect on, in particular, the transfer barrier. The corresponding rates and diffusion coefficient are evaluated using the path-centroid transition state theory. In contrast with what is found assuming classical mechanics for the nuclear motion, the reorientation step becomes rate limiting below 600 K

Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

The dissociation of molecules, even the most simple hydrogen molecule, cannot be described accurately within density functional theory because none of the currently available functionals accounts for strong on-site correlation. This problem led to a discussion of properties that the local Kohn-Sham potential has to satisfy in order to correctly describe strongly correlated systems. We derive an analytic expression for the nontrivial form of the Kohn-Sham potential in between the two fragments for the dissociation of a single bond. We show that the numerical calculations for a one-dimensional two-electron model system indeed approach and reach this limit. It is shown that the functional form of the potential is universal, i.e., independent of the details of the two fragments.We acknowledge funding by the Spanish MEC (Grant No. FIS2007-65702-C02-01), “Grupos Consolidados UPV/EHU del Gobierno Vasco” (Grant No. IT-319-07), and the European Community through e-I3 ETSF project (Grant Agreement No. 211956).Peer reviewe

Substitutional doping and oxygen vacancies in La₂Zr₂O₇ pyrochlore oxide

Density-functional calculations have been combined with thermodynamic modeling to study substitutional aacceptor dopants and oxygen vacancies in La2Zr2O7 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, 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 is discussed