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 Bi4Ti3O12 Below T-c(675 degrees C) the structure has been refined in orthorhombic space group B2cb [at 25 degrees C a = 5.4444(1) Angstrom, b = 5.4086(1) Angstrom c = 32.8425(6) Angstrom; at 650 degrees C a = 5.46183(5) Angstrom, b = 5.44843(5) Angstrom, c = 33.1742(3) Angstrom]. At 800 degrees C the structure has been refined as tetragonal, I4/mmm a = 3.86334(2) A, c = 33.2942(2) Angstrom. 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.</p