Vacancy association energy in scandium doped ceria: 45Sc MAS NMR and 2D exchange spectroscopy study

Abstract

Oxygen vacancy exchange between Sc ions in 0.5% Sc-doped ceria Sc0.005Ce0.995O0.9975 is investigated by means of 2D exchange spectroscopy (EXSY). 45Sc NMR spectrum of Sc-doped ceria contains two peaks—one corresponding to Sc ions coupled with an oxygen vacancy (Sc-7), and the other corresponding to Sc ions in a regular lattice site surrounded by eight oxygen ions (Sc-8). The vacancy exchange between these two Sc sites generates the cross-peaks in the EXSY spectrum. Relative amplitudes of the cross-peaks provide direct values of the exchange frequency at a given temperature. Arrhenius analysis of the exchange frequency gives the activation energy Ea=1.18eV for the vacancy hopping between Sc sites. Most of this energy barrier is due to association energy which binds the vacancy to Sc3+. Large dopant–vacancy association energy in Sc doped ceria is demonstrated by 45Sc NMR spectrum of La/Sc doubly doped sample, ScxLaxCe1−2xO2−x, x=0.005, where the only line of Sc-7 site shows that all vacancies are bound by Sc3+ ions.