Giant Crystal Grain Rotation in Au-Bombarded NiO

This work was stimulated by the large shape changes of ion-bombarded NiO layers found by Bolse [1]. His observations were somewhat reminiscent of ion-hammering of amorphous materials, an effect of shear stress relaxation in liquid ion tracks [2]. Schattat et al. [3] provided evidence for liquid ion tracks in NiO. On the other hand, first x-ray diffraction measurements revealed that NiO remained crystalline during swift heavy ion bombardment and any shear stress relaxation within the liquid ion tracks should be annihilated during subsequent recrystallization. This contribution aims at a clarification of the ioninduced deformation mechanism. Previous irradiations of NiO single crystals at 90 and 300 K with a 600 MeV Au beam at ISL, Berlin, had suggested that the deformation direction was set by the beam direction and that thermally activated migration was unimportant. Under normal irradiation (angle = 0 between the NiO slab normal and ion beam), the single crystal exhibited a slight mosaicity and a large in-plane compressive stress of ~ 3.5 GPa. For 0, however, the NiO single crystals rapidly fragmented into nanocrystals of ~30 nm diameter and the bombarded layer underwent shear flow. Inaugurating the newly installed on-line x-ray diffractometer at the M2-branch at the UNILAC, NiO single crystals were bombarded at ~40°C with 940-MeV Au ions. The following results were obtained: i) The shear flow originated from a combined rotation and translation of the nanocrystals. ii) The rotation angle increases with increasing ion fluence t. No saturation has been found (see fig. 1). iii) The rotation is reversible (cf. fig. 1).