Polarized Octahedra in Barium Tetratitanate

Single-crystal x-ray diffraction studies show that BaTi4O9 is orthorhombic, space group Pmmn, with a = 14.53±0.02, b = 3.79±0.01, and c = 6.29±0.01 A, with two formula units per unit cell and calculated density 4.54 g cm—3. Each barium atom has four oxygen neighbors at 2.81 A, two at 2.96, and four at 3.09, at the corners of a pentagonal prism. Titanium atoms are in distorted octahedra of oxygen atoms. The Ti–O distances range from 1.77 to 2.32 A, with standard deviations of 0.03 A or less. Titanium atoms occur at points 0.30 and 0.21 A from the centers of gravity of the oxygen atoms of the two kinds of octahedra. This polarization of the two octahedra is similar to or greater in magnitude than that observed in the ferroelectric phases of BaTiO3 and PbTiO3.