Phase transitions in Bi₄Ti₃O₁₂

Bi₄Ti₃O₁₂ is a representative of the Aurivillius family of layered perovskites. These are high-temperature ferroelectric materials with prospects for applications in random-access memory and are characterized by an extremely confused interaction of their structural degrees of freedom. Using group-theoretical methods, structural distortions in the Bi₄Ti₃O₁₂ high-symmetry phase, caused by rotations of rigid octahedra and their displacements as a single unit, have been investigated, taking into account the connections between them. Within the Landau theory, a stable thermodynamic model of phase transitions with three order parameters has been constructed. It is shown that, according to the phenomenological phase diagram, the transition between the high-temperature tetragonal phase and the low-temperature ferroelectric can occur both directly and through intermediate states, including those observed experimentally. The role of improper ordered parameters and possible domain configurations in the structure formation of the low-temperature ferroelectric phase are discussed.