Nucleation of a double diamond twin

Many twin boundary (TB) defects are found in the tubular network double diamond (DD) phase, suggesting these are readily formed, low energy structures. The DD phase consists of two inter-catenated networks comprised of basic loops of 6 tetrahedral nodes. The two networks are identical and achiral, just translationally shifted. The TB is a (222) plane with one network offset from the boundary plane while the other network has nodes on the mirror. The structure of the offset network is precisely the same as a (111) twin in carbon diamond, while for the other network of the DD, the TB causes formation of 2 new types of mirror symmetric nodes (alternating pentahedral and trihedral nodes) which link into a hexagonal mesh to form the TB.¹ Terminated TBs allow investigation of the formation of the twin. 3D reconstruction at the edge of the TB provides direct visualization of the nucleation pathway from DD crystal into DD twin. Observations show that one set of tetrahedral nodes on one network splits into two trihedral nodes, one of which then combines with another tetrahedral node to form a pentahedral node leaving a trihedral node in-between each pentahedral node. On the other network, each tetrahedral node again splits into two trihedral nodes, but then each of these trihedral nodes combines with another adjacent trihedral node arising from a different tetrahedral node to form a new tetrahedral node that connects across the boundary to a mirror symmetric partner.