Temperature-dependent crystal structure of ferroelectric Ba2LaTi2Nb3O15

The crystal structure of Ba2LaTi2Nb3O15 was determined using high resolution powder neutron diffraction data collected at 100 K and 400 K. Both structures, refined in space groups P4bm (100 K) and P4/mbm (400 K), are closely related tetragonal tungsten bronzes with Ba in large 15-coordinate sites, La in 12-coordinate sites and Nb,Ti disordered over octahedral sites. The structural origin of the low temperature ferroelectricity is attributed primarily, from variation in bond lengths with temperature, to off-centre displacement of Ti/Nb atoms from their octahedral sites. At room temperature, selected area electron diffraction showed a weak superstructure leading to a doubling of a and c but which is incommensurate parallel to [110].