Spherical solid He nanometer bubbles in an anisotropic complex oxide

Abstract

We show, using room temperature, high-resolution electron microscopy studies, that implanted He in ${\text{LiNbO}}_{3}$ nucleates and accumulates as bubbles. These He inclusions are at $\ensuremath{\sim}20\text{ }\text{GPa}$ pressure and most probably in the solid phase. In addition, the energetically favored shape of the inclusions in their as-implanted form is spherical and not oblate; this spherical shape is due to the fact that their diameter is below a critical radius for balancing the surface and elastic energies as predicted by elastic theory. When annealed, the characteristic length scale of the He inclusions increases, forming faceted bubbles. Annealing also causes the He inclusions to migrate and accumulate into strings due to the preferred ${10\overline{1}4}$-pyramidal-twinning planes.