Crystal structure of thermoelectric compound [Bi_1.79Sr _1.98O_y]_0.63RhO₂

We have studied the crystal structure of the misfit-layered crystal Bi _1.91Sr₂Rh_1.77O_x by electron diffraction and high-resolution electron microscopy. This compound consists of two interpenetrating subsystems of a RhO₂ sheet and a distorted four-layered rock-salt-type (Bi,Sr)O block. Both subsystems have common a-axes, c-axes, and β-angles with a = 5.28 Å, c = 29.77 Å, and β = 93.7°. On the other hand, the crystal structure is incommensurated parallel to the b-axis, b1 = 3.07 Å for the RhO₂ sheet and b ₂=4.88 Å for the (Bi,Sr)O block. The axis ratio, b1/b2∼0.63, characterizes the structural analogue as [Bi _1.79Sr_1.98O_y]_0.63[RhO₂]. This compound has two modulation vectors, q₁ = a* + 0.63b ₁* and q₂ = 0.17b₁* + c*, and the superspace group is assigned as the Cc(1 β0, 0μ1) type from electron diffraction patterns. High-resolution images taken with the incident electron beam parallel to the a- and c-axes clearly exhibit modulated displacive as well as compositional atomic arrangements. A tentative structure model has been proposed, and the calculated image reproduces observed characteristic features reasonably well.