Crystal Structure of Thermoelectric Compound [Bi1.79Sr1.98Oy]0.63[RhO2]

We have studied the crystal structure of the misfit-layered crystal Bi1.91Sr2Rh1.77O 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, b 1=3.07 Å for the RhO2 sheet and b 2=4.88 Å for the (Bi,Sr)O block. The axis ratio, b 1/b 2∼0.63, characterizes the structural analogue as [Bi1.79Sr1.98O y ]0.63[RhO2]. This compound has two modulation vectors, q1=a*+0.63b1 * and q2=0.17b1 *+c*, and the superspace group is assigned as the C c(1β0,0µ1) 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.