Vacancy ordering and oxygen dynamics in oxide ion conducting La{sub 1-x}Sr{sub x}Ga{sub 1-x}Mg{sub x}O{sub 3-x} ceramics: {sup 71}Ga, {sup 25}Mg and {sup 17}O NMR
The oxygen vacancies distribution in the rigid lattice and the thermally activated motion of oxygen atoms are studied in La{sub 1-x}Sr{sub x}Ga{sub 1-x}Mg{sub x}O{sub 3-x} (x=0.00; 0.05; 0.10; 0.15 and 0.20) compounds. For that {sup 71}Ga, {sup 25}Mg and {sup 17}O NMR was performed from 100 K up to 670 K, and ion conductivity measurements were carried out up to 1273 K. The comparison of the electric field gradients at the Ga- and Mg-sites evidences that oxygen vacancies appear exclusively near gallium cations as a species trapped below room temperature in local clusters, GaO{sub 5/2}-{open_square}-GaO{sub 5/2}. These clusters decay at higher temperature into mobile constituents of the structural octahedra Ga(O{sub 5/6{open_square}1/6}){sub 6/2}. At the same time, the nearest octahedral oxygen environment of magnesium cations persists at different doping levels. The case of two adjacent vacant anion sites is found highly unlikely within the studied doping range. The thermally activated oxygen motion starts to develop above room temperature as is observed from both the motional narrowing of {sup 17}O NMR spectra and the {sup 17}O nuclear spin-lattice relaxation rate. The obtained results show that two types of motion exist, a slow motion and a fast one. The former is a long-range diffusion whereas the latter is a local back and forth oxygen jumps between two adjacent anion sites. These sites are strongly differentiated by the probability of the vacancy formation, like the vacant apical site and the occupied equatorial site in the orthorhombic compositions x <0.15. -- Graphical abstract: Up to 700 K the vacant oxygen sites are near Ga but not near Mg. Display Omitted