THE STRUCTURES OF ALPHA-BI2TI4O11 AND BETA-BI2TI4O11

The phase transition and the crystal structures of Bi2Ti4O11 have been investigated by X-ray diffraction. The compounds possess the following parameters: alpha-Bi2Ti4O11: monoclinic, space group C2/c, a = 14.5999(6), b = 3.8063(2), c = 14.9418(8)Angstrom, beta = 93.129(4)degrees, V = 829.1(1)Angstrom(3), Z = 4, D-x = 6.29 g cm(-3), Mo K alpha, mu = 44.31 mm(-1), wR = 0.029 for 2361 reflections; beta-Bi2Ti4O11: monoclinic, space group C2/m, a' = 14.6412(12), b' = 3.8032(3), c' = 10.7824(6) Angstrom, beta = 136.135(5)degrees, V = 416.06(10) Angstrom(3), T = 573R, Z = 2, wR = 0.043 for 1204 reflections. The transformation at 506K leads from a paraelectric high-temperature modification (beta-Bi2Ti4O11), with the space group C2/m, to a C2/c antiferroelectric low-temperature phase (alpha-Bi2Ti4O11) and is induced by a displacement of the Bi atoms at ca 0.29 Angstrom along [010] in the channels of the Ti-O network. For the description of the transition, it is convenient to transform the unit cell of alpha-Bi2Ti4O11 according to a'' = a, b'' = b, c'' = -a + c. In this new setting, the position of superlattice reflections results in a cell doubling parallel [001], indicating a phonon condensation at the A-point [001/2] the boundary of the Brillouin zone of C2/m. A symmetry analysis has been performed based on the Landau formalism and projection operator methods. The displacements parallel and antiparallel to b were taken as the primary-order parameter, which transforms according to the irreducible representation A(2)(+).