An initial structure of bulk Bi 2 TeO 2 was obtained from the Inorganic Crystal Structure Database (ICSD: 194944) and symmetrised using spglib. Structures of Bi 2 SO 2 and Bi 2 SeO 2 were derived from this by replacing the chalcogen. Each structure was fully optimised to tolerances of 10 -8 eV and 10 -3 eV Å-1 on the electronic total energy and forces.