Helium gas-bubble superlattice formation in molybdenum

The gas-bubble superlattice is a striking phenomenon where small helium bubbles of uniform size are regularly arranged on a space lattice having the same symmetry as the crystal lattice of the host metal but with a lattice spacing some twenty times greater. Typically, bubble diameters are ∼2 nm and bubble concentrations are ∼1 × 1025 m−3. Superlattices have been observed in all three metal types: bcc, fcc and hcp. The most common superlattice orientation is parallel with the host lattice. However, for the fcc metals, and the bcc metal vanadium the superlattice also contains structural variants – regions where the ordered bubble array has an orientation that is rational with, but different from, that of the crystal lattice of the host metal. Here we report on the structure of the bubble superlattice in the bcc metal molybdenum. Recently the proposal has been made that this superlattice could act as a photonic crystal for soft X-rays. An assessment is made of the quality of the superlattice in molybdenum in relation to this proposal.