Crystal Structure of Misfit-Layered Compound [Bi1.94Ba1.83Oy]0.56[RhO2]

Abstract

The modulated crystal structure of Bi2.12Ba2.00Rh1.95Ox 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 RhO2 sheet and b2=5.45 Å for the (Bi,Ba)O block. The misfit ratio and the analytical composition give the chemical formula of [Bi1.94Ba1.83Oy]0.56[RhO2]. This compound has two modulation vectors, q1=-a*+0.56b1* and q2=0.11b1*+0.35c*, and the superspace group is assigned as a C2/m(1α0,0βγ)-type on the basis of the systematic absence of reciprocal lattice points shown in the electron diffraction patterns. This modulation vector q2 is different from the one in [Bi1.79Sr1.98Oy]0.63[RhO2], which has recently been studied by the authors. An additional modulation vector, q3=0.12a*+0.10b1*, 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.