Segregation in yttrium aluminum garnet: I, experimental determination

Single crystals of yttrium aluminum garnet (YAG) that were doped with various cations were annealed in air at different temperatures for varying amounts of time. Dopants were chosen to probe the effect of size, charge, and site occupancy on surface segregation. Of the dopants that were chosen for the study (calcium, silicon, neodymium, chromium, and strontium), calcium was the only one that consistently segregated upon annealing in air. Calcium enrichment to the (111) surface was measured using Auger electron spectroscopy, and the segregation enthalpy was determined to be Delta H-seg approximate to -32 +/- 10 kJ/mol. Enrichment occurred according to variations in valence, as opposed to variations in size; therefore, it is suggested that surface segregation is electrostatically driven. The results indicate that aliovalent substituents could be used for interface property tailoring, whereas isovalent dopants would not be useful.