an antiphase domain
When a disordered alloy orders on cooling, ordering does not start everywhere at once. It nucleates in many separate spots, and each little ordered region has to choose which sublattice the gold (say) will occupy. Neighbouring regions can make opposite choices — both perfectly ordered, but out of step with each other, like two people tiling the same floor in the same black-and-white pattern but starting on opposite colours. Each such correctly-ordered but mutually-shifted region is an antiphase domain.
Where two antiphase domains meet, the pattern is out of register: cross the boundary and you suddenly find like atoms where the order wanted unlike neighbours (an A-A or B-B contact instead of A-B). That mismatched seam is an antiphase boundary (APB), described by a shift vector — the fractional lattice translation needed to bring the two domains back into step. As the material anneals, domains grow and swallow their neighbours, and the APBs move and shorten, much like grain growth.
Antiphase boundaries are not just curiosities: in ordered intermetallics they strongly affect how dislocations move (a dislocation gliding through an ordered crystal drags an APB behind it, which raises the strength), and superlattice reflections in diffraction are broadened or streaked when the ordered domains are small. Antiphase domains are the ordering counterpart of grains, and their size and boundary energy help set the mechanical behaviour of ordered alloys.
As ordered Cu3Au nucleates, separate ordered patches each choose which corner sites gold takes; where these antiphase domains meet, the seam has the wrong Cu-Cu or Au-Au neighbours instead of Cu-Au — an antiphase boundary. Imaged in dark-field TEM using a superlattice reflection, the antiphase domains show up as a mosaic of patches.
An antiphase domain: a fully ordered patch that is out of step with its neighbours; the seam between them is an antiphase boundary (a fractional lattice shift).
An antiphase domain is itself fully ordered, merely out of step with its neighbours; the seam where two meet is an antiphase boundary described by a fractional lattice-translation vector. A dislocation passing through an ordered crystal drags an APB behind it, which raises the strength.