Crystal Structure of Thermoelectric Compound [Bi_(1.79)Sr_(1.98)O_y]_(0.63)[RhO_2]

摘要

We have studied the crystal structure of the misfit-layered crystal Bi_(1.91)Sr_2Rh_(1.77)O_x by electron diffraction and high-resolution electron microscopy. This compound consists of two interpenetrating subsystems of a RhO_2 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 A, c = 29.77 A, and β = 93.7°. On the other hand, the crystal structure is incommensurated parallel to the b-axis, b_1 = 3.07 A for the RhO_2 sheet and b_2 = 4.88 A for the (Bi,Sr)O block. The axis ratio, b_1/b_2 ～ 0.63, characterizes the structural analogue as [Bi_(1.79)Sr_(1.98)O_y]_(0.63)[RhO_2]. This compound has two modulation vectors, q_1 = a~* + 0.63b_1~* and q_2 = 0.17b_1~* + c~*, and the superspace group is assigned as the Cc(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.