A Variable-Temperature Powder Neutron Diffraction Study of Ferroelectric Bi₄Ti₃O₁₂

Powder neutron diffraction has been used to pinpoint the nature of the ferroelectric-paraelectric phase transition in Bi 4 Ti 3 O 12 . Below T c (675 °C) the structure has been refined in orthorhombic space group B 2 cb [at 25 °C a = 5.4444(1) Å, b = 5.4086(1) Å, c = 32.8425(6) Å; at 650 °C a = 5.46183(5) Å, b = 5.44843(5) Å, c = 33.1742(3) Å]. At 800 °C the structure has been refined as tetragonal, I 4/ mmm a = 3.86334(2) Å, c = 33.2942(2) Å. Bond valence sum analysis of the coordination environments around each metal site as a function of temperature clearly demonstrate that the driving force for the ferroelectric transformation is the requirement for Bi in the perovskite-like A sites to optimize its bonding to neighboring oxygen atoms.