Past exam of the chemistry A course of the University of Cambridge / 2025 / Paper 2 / A9 / g

The sum of the three primitive vectors is $(a/2)(1,1,1)$, so the surface is a $(111)$ plane. Two in-plane translations areThey have length $L=\sqrt{2}a$ and included angle $60^{\circ }$, giving a triangular top-layer lattice. Body-centred sites lie in offset parallel layers, not as additional sites in this same plane. A primitive surface cell is the rhombus spanned by $t_1,t_2$, of area