Unexpected phase transition sequence in the ferroelectric Bi₄Ti₃O₁₂

The high-temperature phase behaviour of the ferroelectric layered perovskite Bi₄Ti₃O₁₂ 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. Here, we confirm that Bi₄Ti₃O₁₂ undergoes two separate structural transitions from the aristotype tetragonal phase (space group I4/mmm) to the ambient temperature ferroelectric phase (confirmed as monoclinic, B1a1). An unusual, and previously unconsidered, intermediate paraelectric phase is suggested to exist above T_C, with tetragonal symmetry, space group P4/mbm. 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 rationalised in terms of the bonding requirements of the Bi³⁺ cations within the perovskite blocks.