High-resolution electron microscopy of thermoelectric compounds Bi-(Sr,Ba)-Rh-O

Abstract

The crystal structures of thermoelectric (TE) compounds Bi_1.91Sr_2.0Rh_1.77O_x and Bi _2.12Ba_2.00Rh_1.95O_x are investigated by means of electron diffraction measurements and high-resolution electron microscopy. These compounds have layered structures and consist of two interpenetrating subsystems exhibiting incommensurated periodicities along b-axis. Both subsystems, CdI₂ -type RhO₂ sheets and a distorted four-layered rock-salt (RS)-type (Bi,(Sr,Ba))O blocks, have common a-, c-axes and beta;-angles. On the other hand, the ratios of two axis lengths, i.e., b ₁ (RhO₂ sheet) / b₂ (RS-type block), are irrational. Based on the cation ratio and b₁/b₂ value, the structural formulae are expressed as [(Bi_1-x>Rh_x)₂(Sr_1-yBi _y)₂O_4+delta]_0.63RhO₂ with x = 0.077, y = 0.016 and delta = 0.46 for the Sr-system and [(Bi_1-x>Rh_x)₂(Sr_1-yBi _y)₂O_4+delta]_0.56RhO₂ with x = 0.037, y = 0.047 and delta = 0.39 for the Ba-system, respectively. Based on the electron diffraction measurements, it is found that these compounds have irrational modulation vectors, q₁ = -a* + 0.63b₁* and q₂ = 0.17b₁* + c for Bi-Sr-Rh-O, and q₁ = -a* + 0.56b₁* and q ₂ = 0.11b₁* + 0.35c* for Bi-Ba-Rh-O, respectively, being the indications of (3+2) dimensional structures. High-resolution images taken with the incident electron beam parallel to the a- and c-axes clearly exhibit modulated atomic arrangements. These compounds are characterized by the displacement modulations in both RhO₂ layers and the RS-type blocks