Phase equilibria, crystal chemistry, electronic structure and physical properties of Ag–Ba–Ge clathrates

• Published in: Acta materialia 2011-04, Vol.59 (6), p.2368-2384
• Main Authors: Zeiringer, I., Chen, MingXing, Bednar, I., Royanian, E., Bauer, E., Podloucky, R., Grytsiv, A., Rogl, P., Effenberger, H.
• Format: Article
• Language: English
• Subjects: Clathrate; Clathrates; Electronic structure; Fermi surfaces; Germanium; Lattice vacancies; n-Type; Phase diagram; Silver; Site preference (crystals); Solid solutions; Transport properties; X-rays
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2010.12.033

In the Ag–Ba–Ge system the clathrate type-Ι solid solution, Ba 8Ag x Ge 46− x − y □ y , extends at 800 °C from binary Ba 8Ge 43□ 3 (□ is a vacancy) to Ba 8Ag 5.3Ge 40.7. For the clathrate phase (1 ⩽ x ⩽ 5.3) the cubic space group Pm 3 ¯ n was established by X-ray powder diffraction and confirmed by X-ray single-crystal analyses of the samples Ba 8Ag 2.3Ge 41.9□ 1.8 and Ba 8Ag 4.4Ge 41.3□ 0.3. Increasing the concentration of Ag causes the lattice parameters of the solid solution to increase linearly from a value of a = 1.0656 ( x = 0, y = 3) to a = 1.0842 ( x = 4.8, y = 0) nm. Site preference determination using X-ray refinement reveals that Ag atoms preferentially occupy the 6d site randomly mixed with Ge and vacancies, which become filled in the compound Ba 8Ag 4.8Ge 41.2 when the Ag content increases. At 600 °C the phase region of the clathrate solution Ba 8Ag x Ge 46− x − y □ y becomes separated from the Ba–Ge boundary and extends from 6.6 to 9.8 at.% Ag. The compound Ba 6Ge 25 (clathrate type-ΙX) dissolves at 800 °C a maximum of 1.5 at.% Ag. The homogeneity regions of the two ternary compounds BaAg 2− x Ge 2+ x (ThCr 2Si 2-type, 0.2 ⩽ x ⩽ 0.7) and Ba(Ag 1 -x Ge x ) 2 (AlB 2-type, 0.65 ⩽ x ⩽ 0.75) were established at 800 °C. Studies of transport properties for the series of Ba 8Ag x Ge 46− x − y □ y compounds evidenced that electrons are the predominant charge carriers with the Fermi energy close to a gap. Its position can be fine-tuned by the substitution of Ge by Ag atoms and by mechanical processing of the starting material, Ba 8Ge 43. The proximity of the electronic structure at Fermi energy of Ba 8Ag x Ge 46− x − y □ y to a gap is also corroborated by density functional theory calculations. This gap near the Fermi energy gives rise to distinct features of the temperature-dependent electrical resistivity and the Seebeck effect is in very good agreement with the experiment findings.