Autosegregation and phase formation on the surface of calcium niobate crystals

Nanoscale changes in the surface morphology and elemental composition of calcium niobate, Ca₂Nb₂O₇, crystals in response to thermally stimulated surface autosegregation have been studied for the first time. The results demonstrate that, depending on annealing temperature and duration, the calcium niobate surface becomes niobium-enriched. This is accompanied by the formation of surface phases identical in composition to the host crystal. At 770 K, the surface phase consists of oriented nanocrystallites. With increasing annealing temperature and duration, the nanocrystallites transform into partially oriented microcrystallites identical in composition to the host crystal. Annealing at 1270 K, that is, well below the melting point of Ca₂Nb₂O₇ (1873 K) leads to disproportionation of the surface phase and formation of both niobium-enriched and niobium-deficient phases, including pure CaO. The high-temperature surface phases consist of individual crystallites and intergrowths, which form layers with a rather large surface coverage.