Unexpected phase transition sequence in the ferroelectric BiTiO.

The high-temperature phase behaviour of the ferroelectric layered perovskite BiTiO has been re-examined by high-resolution powder neutron diffraction. Previous studies, both experimental and theoretical, had suggested conflicting structural models and phase transition sequences, exacerbated by the complex interplay of several competing structural instabilities. This study confirms that BiTiO undergoes two separate structural transitions from the aristotype tetragonal phase (space group 4) to the ambient-temperature ferroelectric phase (confirmed as monoclinic, 11). An unusual, and previously unconsidered, intermediate paraelectric phase is suggested to exist above with tetragonal symmetry, space group 4. This phase is peculiar in displaying a unique type of octahedral tilting, in which the triple perovskite blocks of the layered structure alternate between tilted and untilted. This is rationalized in terms of the bonding requirements of the Bi cations within the perovskite blocks.