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

In the Ag-Ba-Ge system the clathrate type-Ι solid solution, Ba8AgxGe46-x-y□y, extends at 800 deg. C from binary Ba8Ge43□3 (□ is a vacancy) to Ba8Ag5.3Ge40.7. For the clathrate phase (1 ≤ x ≤ 5.3) the cubic space group Pm3-bar n was established by X-ray powder diffraction and confirmed by X-ray single-crystal analyses of the samples Ba8Ag2.3Ge41.9□1.8 and Ba8Ag4.4Ge41.3□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 Ba8Ag4.8Ge41.2 when the Ag content increases. At 600 oC the phase region of the clathrate solution Ba8AgxGe46-x-y□y becomes separated from the Ba-Ge boundary and extends from 6.6 to 9.8 at.% Ag. The compound Ba6Ge25 (clathrate type-ΙX) dissolves at 800 oC a maximum of 1.5 at.% Ag. The homogeneity regions of the two ternary compounds BaAg2-xGe2+x (ThCr2Si2-type, 0.2 ≤ x ≤ 0.7) and Ba(Ag1-xGex)2 (AlB2-type, 0.65 ≤ x ≤ 0.75) were established at 800 deg. C. Studies of transport properties for the series of Ba8AgxGe46-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, Ba8Ge43. The proximity of the electronic structure at Fermi energy of Ba8AgxGe46-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.