Crystal structure of misfit-layered compound [Bi_1.94Ba _1.83O_y]_0.56[RhO₂]

The modulated crystal structure of Bi_2.12Ba _2.00Rh_1.95O_x- has been studied by electron diffraction measurements and high-resolution electron microscopy. The crystal structure consists of two interpenetrating subsystems of a RhO₂ sheet and a distorted four-layered rock-salt-type (Bi,Ba)O block. Both subsystems have common a- and c-axes and a β-angle with a = 5.38 Å, c = 15.15 Å and β= 95.3°. On the other hand, the crystal structure is incommensurate along the b-axes; b₁ = 3.05 Å for the RhO ₂ sheet and b₂ = 5.45 Å for the (Bi,Ba) O block. The misfit ratio and the analytical composition give the chemical formula of [Bi_1.94Ba_1.83O_y]_0.56[RhO ₂]. This compound has two modulation vectors, q₁ = -a* + 0.56b*₁ and q2 = 0.11b*₁ + 0.35c*, and the superspace group is assigned as a C2/m(1aα0, 0βy)-type on the basis of the systematic absence of reciprocal lattice points shown in the electron diffraction patterns. This modulation vector q ₂ is different from the one in [Bi_1.79Sr _1.98O_y]_0.63[RhO₂], which has recently been studied by the authors. An additional modulation vector, q3 = 0.12a* + 0.10b*₁, is locally observed. High-resolution images taken with the incident electron beam parallel to the a- and c-axes clearly exhibit modulated atomic arrangements. This compound is characterized by not only the displacement modulation but also the concentration modulation in the rock-salt subsystem.