Germanium Vacancies and Charge Transport Properties in Ba₈Zn_xGe_46−x−y◻_y

The Ba 8 Zn x Ge 46− 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 − 0.54 x 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 Ba 8 Zn x Ge 46− 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 Ba 7.95 Zn 7.83 Ge 38.17 which also displays the largest power factor (PF = 3.75 μW·K −2 ·cm −1 at 300 K) in this series.