Synthesis and first-principles calculations of the structural and electronic properties of type-I clathrates Sr8Ga16SnxGe30 − x

• Published in: physica status solidi (b) Vol. 249; no. 7; pp. 1423 - 1430
• Main Authors: Li, D. C., Fang, L., Deng, S. K., Kang, K. Y., Wei, W. H., Ruan, H. B.
• Format: Journal Article
• Language: English
• Published: Berlin WILEY-VCH Verlag 01.07.2012; WILEY‐VCH Verlag
• Subjects: clathrates; electronic structure; first-principle calculations; structure
• ISSN: 0370-1972; 1521-3951
• DOI: 10.1002/pssb.201147488

In the present work we report on the synthesis of type‐I clathrates Sr8Ga16SnxGe30 − x (0 ≤ x ≤ 12). To find out how the substitution of Sn for Ge affects structural stability and electronic structure, the structural and electronic properties for Sr8Ga16SnxGe30 − x (0 ≤ x ≤ 30) have been investigated by a first‐principles method based on the density‐functional theory (DFT). We found that the lattice constants of Sr8Ga16SnxGe30 − x series increase with increasing Sn content, which is consistent with X‐ray diffraction (XRD) results. Calculations indicate that the substitution of Sn for Ge leads to the change of the bulk modulus and the decrease of stability of the structure. It is found that these alloys are all indirect‐gap semiconductors and the bandgap decreases from about 0.36 eV in Sr8Ga16Ge30 to about 0.03 eV in Sr8Ga16Sn30 with increasing Sn content. The decrease of the bandgap is attributed to the increase of the free space for the Sr guest motion, which is accompanied by the guest's low‐energy modes and larger anharmonicity. These mean that an increase in the ratio of Sn‐to‐Ge can not only control the electrical properties of the materials, but also may reduce their thermal conductivity, suggesting that cage‐size tuning is one of the useful means to obtain high‐performance thermoelectric (TE) materials.