Atomic Simulations of Aurivillius Oxides: Bi\\r sub/3sub\\r TiNbO\\r sub/9sub\\r , Bi\\r sub/4sub\\r Ti\\r sub/3sub\\r O\\r sub/12sub\\r , BaBi\\r sub/4sub\\r Ti\\r sub/4sub\\r O\\r sub/15sub\\r and Ba\\r sub/2sub\\r Bi\\r sub/4sub\\r Ti\\r sub/5sub\\r O\\r sub/18sub\\r Doped with Pb, Al, Ga, In, Ta