Anomalous grain growth in the surface region of a nanocrystalline CeO${}_2$ film under low-temperature heavy ion irradiation

Grain growth and phase stability of nanocrystalline ceria are investigated under ion irradiation at different temperatures. Irradiations at temperatures of 300 and 400 K result in uniform grain growth throughout the film. Anomalous grain growth is observed in thin films of nanocrystalline ceria under 3-MeV Au${}^{+}$ irradiation at 160 K. At this low temperature, significant grain growth is observed within 100 nm from the surface, and no obvious growth is detected in the rest of the films. While the grain growth is attributed to a defect-stimulated mechanism at room temperature and above, a defect diffusion-limited mechanism is significant at low temperatures with the primary defect responsible being the oxygen vacancy. The nanocrystalline grains remain in the cubic phase regardless of defect kinetics.

Figure 1

(a) XTEM, (b) SAED, and (c) HREM of the as-deposited nanostructured ceria film.

Figure 2

TEM, SAED, and HREM images of the NSC film irradiated at 160 K at doses of (a)–(c) 0.54, (d)–(f) 3.62, and (g)–(i) 10.8 dpa, respectively.

Figure 3

(a) Graph showing irradiation-induced grain growth as a function of dose, as calculated by GIXRD, for the samples irradiated at 160, 300, and 400 K. (b) Magnified region of the low-dose 160-K irradiation-induced grain growth demonstrating the rapid growth with low doses, followed by less rapid growth. The lines serve to guide the eye.

Figure 4

TEM, SAED, and HREM images of the NSC film irradiated at 300 K at doses of (a)–(c) 0.54 and (d)–(f) 35 dpa, respectively.

Figure 5

Normalized lattice fringe intensity profiles for the as-deposited, near-surface 160-K 10.8-dpa and near interface 160-K 10.8-dpa samples. The insets are the average $d_{111}$ spacing as measured by the intensity profiles of, at least, five grains.