Phase equilibria, crystal chemistry, electronic structure and physical properties of Ag-Ba-Ge clathrates
- Author(s)
- Isolde Zeiringer, Mingxing Chen, Ingeborg Bednar, Esmaeil Royanian, Ernst Bauer, Raimund Podloucky, Andrij Grytsiv, Peter Franz Rogl, Herta Silvia Effenberger
- Abstract
In the Ag-Ba-Ge system the clathrate type-I solid solution, Ba8AgxGe46_x_yhy, extends at 800 °C from binary Ba8Ge43h3 (h is avacancy) to Ba8Ag5.3Ge40.7. For the clathrate phase (1 6 x 6 5.3) the cubic space group Pm_3n was established by X-ray powder diffraction and confirmed by X-ray single-crystal analyses of the samples Ba8Ag2.3Ge41.9h1.8 and Ba8Ag4.4Ge41.3h0.3. 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 the6d site randomly mixed with Ge and vacancies, which become filled in the compound Ba8Ag4.8Ge41.2 when the Ag content increases. At 600 °C the phase region of the clathrate solution Ba8AgxGe46_x_yhy becomes separated from the Ba-Ge boundary and extends from6.6 to 9.8 at.% Ag. The compound Ba6Ge25 (clathrate type-IX) dissolves at 800 °C a maximum of 1.5 at.% Ag. The homogeneity regions of the two ternary compounds BaAg2_xGe2+x (ThCr2Si2-type, 0.2 6 x 6 0.7) and Ba(Ag1-xGex)2 (AlB2-type, 0.65 6 x 6 0.75) were established at 800 °C. Studies of transport properties for the series of Ba8AgxGe46_x_yhy 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, Ba8Ge43. The proximity of the electronic structure at Fermi energy ofBa8AgxGe46_x_yhy to a gap is also corroborated by density functional theory calculations. This gap near the Fermi energy gives riseto distinct features of the temperature-dependent electrical resistivity and the Seebeck effect is in very good agreement with the experiment findings.
- Organisation(s)
- Department of Physical Chemistry, Department of Mineralogy and Crystallography
- External organisation(s)
- Technische Universität Wien
- Journal
- Acta Materialia
- Volume
- 59
- Pages
- 2368-2384
- No. of pages
- 17
- ISSN
- 1359-6454
- DOI
- https://doi.org/10.1016/j.actamat.2010.12.033
- Publication date
- 2011
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 104017 Physical chemistry, 105113 Crystallography, 103018 Materials physics
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/4bfed949-f7a0-4bc2-bb78-f3daaf5d2b76