A study of accurate charge density and bonding nature of CoSb3 at T = 10 K.

Interdisciplinary Graduate School of Science and Engineering, Tokyo Insititute of Technology* Department of Chemistry, University of Aarhus, Denmark** Department of Applied Physics, Nagoya University, Japan*** Materials and Structures Laboratory, Tokyo Institute of Technology, Japan****
â—‹Atsuko Ohno* Bo B. Iversen** Shinobu Aoyagi*** Eiji Nishibori*** Makoto Sakata*** Satoshi Sasaki****

Due to the high electrical and low thermal conductivities, the metal-doped CoSb3 is a good candidate of practical thermoelectric material. These physical properties must be affected by its bonding nature. It is, therefore, important to study chemical bonding of the metal-doped CoSb3. In this study, an accurate experimental charge density of CoSb3 has been determined as the first step by maximum entropy method (MEM) using high-resolution synchrotron powder diffraction data.
The synchrotron powder diffraction experiment was carried out at 10K by a large Debye-Scherrer camera installed at BL02B2, SPring-8. The wavelength of incident X-ray was 0.42066Å. The data were collected from 3 degree to 99 degree with 0.01 degree per step in 2θ. This covers the d-spacing up to 0.277 Å, which should be called high resolution data in real space. The collected data were firstly analysed by Rietveld refinement. The reliability factors based on weighted profile, Rwp, and, Bragg intensities, RI, were 6.0 and 2.5 %, respectively. Then, the MEM analyses were carried out using the structure factors derived from the Rietveld refinement.
The MEM charge density of CoSb3 has clearly revealed that there are three types bonding, which are two types Sb-Sb bonding in Sb4 ring and the rest is Co-Sb bonding. Judging from the charge density at bond mid-point, Co-Sb bond is stronger than any of two kinds of Sb-Sb bonds in Sb4 ring. The present results support the theoretical calculation [1] for the Co-Sb bond. The present study will be easily extended to cover various metal-doped CoSb3.

[1] Lefebvre-Devos et al., Phys. Rev. B 63, 125110 (2001).