At high temperature, Cu₃Au has a disordered fcc structure with space group Fm3m. On cooling, an ordering transition occurs and the low-temperature structure has space group Pm3m with Au at 0 0 0 (Wyckoff site 1a) and Cu at ½ ½ 0 (3c). 

(a) Sketch the low- and high-temperature structures. 

(b) Is this a translationengleiche or klassengleiche transition? 

(C) State the number of domain variants in the low-temperature structure and whether they will be related by twin or antiphase boundaries.