Germanium Vacancies and Charge Transport Properties in Ba8ZnxGe46−x−y◻y

Abstract

The Ba8ZnxGe46−x−y◻y (4 ≤ x ≤ 8, ◻ = Ge vacancy) solid solution crystallizes in the clathrate type I structure as indicated by X-ray powder diffraction and transmission electron microscopy. The fraction of vacancies (y) derived from electron probe microanalysis varies like y = 3.3 − 0.54x for 0 ≤ x ≤ 6 and y = 0 for 6 < x ≤ 8. A simplified band picture can explain how the ratio (2−)/(4−) of formal oxidation numbers of zinc and of vacancy leads to the ∼1/2 slope of the y = f(x) relation. Hall effect measurements indicate that the Ba8ZnxGe46−x−y◻y definitely deviate from the Zintl rule when 0 ≤ x < 4, weakly deviate from the the Zintl rule when 4 ≤ x ≤ 6, and fulfill the Zintl rule when 6 < x ≤ 8. Resistivity versus temperature data for 4 ≤ x ≤ 8 are in line with a “bad metal” behavior (x ∼ 4) evolving toward a degenerate semiconducting behavior (x ∼ 8). The Seebeck coefficient increases with x in agreement with the charge carrier concentration variation and reaches −67.1 μV/K in Ba7.95Zn7.83Ge38.17 which also displays the largest power factor (PF = 3.75 μW·K−2·cm−1 at 300 K) in this series.