Ordering of oxygen vacancies and excess charges in reduced bulk ceria

G. E. MURGIDA; V. FERRARI; A. M. LLOIS; M. V. GANDUGLIA PIROVANO

Zaragoza

Workshop; Reducible oxide chemistry, structure and functions COST Action CM1104; 2014

COST Action CM1104

Ordering of oxygen vacancies and excesscharges in reduced bulk ceria G. E. Murgida1,2, M. V. Ganduglia Pirovano3, V. Ferrari,1,2 and A. M. Llois1,2 1Centro Atómico Constituyentes, GIyANN, CNEA, San Martín, Buenos Aires, Argentina 2Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina 3Instituto de Catálisis y Petroleoquímica - CSIC, Madrid, Spain murgida@tandar.cnea.gov.ar The in-depth understanding and control of the density, and distribution of oxygen vacancies in ceria provide a means to influence the electronic structure and to tailor the systems? functionality. Using the DFT+U method we investigated the ordering of oxygen vacancies and excess charge localization driving the Ce4+Ce3+ reduction in bulk CeO2 for defect concentrations ranging from 1.6% to 25%. The isolated vacancy was modeled using a unit cell of Ce32O64 composition. We computed the total energy for structures with Ce3+ ions at nearest neighbor (NN), next-nearest neighbor (NNN), third- and fourth- nearest neighbor cation sites to the vacancy. We found that the energies of all structures with either NN-NN, NN-NNN or NNN-NNN localizations lie within a narrow range of 0.1 eV, whereas those of structures with Ce3+ ions further away from the defect, have energies that are at least 0.3 eV above the lowest energy structure which is of the NNN-NNN type. To analyze the interaction between oxygen vacancies, as well as the ordering of defects and Ce3+ions, we used a smaller unit cell with Ce8O16 composition and considered one and two first, second, third and fourth neighboring defects in the oxygen sublattice with all possible configurations for the two and four Ce3+ions, respectively. We found correlations between vacancyvacancy as well as vacancyCe3+relative positions and total energies, providing clear indication of the proneness of both vacancy and Ce3+ for particular relative positions.Specifically, we found that: (i) The energy is a decreasing function of the average coordination number of the Ce3+ ions, which varies between 6 (all Ce3+NN) and 8 (all Ce3+NNN), (ii) The vacancies prefer to be second neighbors, and (iii) Two oxygen vacancies tend to avoid having common first neighboring Ce ions. For this particular cell at a high vacancy concentration (12.5%), we modeled the total energy as a sum of independent contributions related to the above preferences and fitted the ab initio energies in order to quantify the various terms. This simple model though sound for the specific vacancy concentration and distribution fitted to, its predictive power for more dilute cases is fairly limited. In addition, we computed the averaged defect formation energy leading to the experimentally stable structures corresponding to Ce7O12(14%), Ce11O20 (9%) and Ce2O3 (25%) compounds and calculated the thermodynamically most stable CexOy phases as a function of oxygen pressure and temperature. Defect-induced lattice relaxation effects are found to play a crucial role in stabilizing the stable reduced structures.