Origin of self-aligned nano-domains in MgB2

Random arrangement of B atoms in the parent amorphous phase leads to a number of atomic defects, such as dislocations, formed in the reaction product of Mg and B. During crystallisation, dislocation walls form from random arrays of dislocations. Stress at the end of dislocation wall segments attract the surrounding edge dislocations, which are then incorporated and result in wall growth, forming small angle boundaries to connect well-ordered nano-domains. To minimise the energy of the system, the dislocations migrate to interdomain boundaries surrounding the nano-domains. These dislocation rearrangements result in rotation of adjacent nano-domains to form a contiguous crystal. By continuing this subgrain rotation process on neighbouring nano-domains, large (211) nano-domains can be aligned as observed by HRTEM. It is shown that the (211) plane may have the minimum surface energy in MgB2 and the (211) zone is the favoured orientation for crystal growth. 14 refs.