That the presence of Sr on the Bi2 site increases the site preference of Se for the Se1 site is consistent with electrostatics. A lower Coulomb energy is achieved by Se 2-having Bi 3+ for its nearest neighbours, and Sr 2+ with Cl -than compared with the converse. Thus the confinement of Sr 2+ to the layer defined by the Bi2 site leads to the confinement of the Se 2-to the layer defined by the Bi1 site. The effect of doping on the structure is therefore nontrivial as the Sr and Se occupancies are correlated.