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Single crystals of the type-I clathrate Ba8Ni3.5Ge42.10.4(space group Pm3n, no. 223, a= 10.798(2) Å, l= 30 mm, ∅ = 8 mm) were grown from the melt using the Bridgman technique. Their composition, determined by microprobe analysis, reveals a distinctly lower Ni content than previously reported for the lower limit (x= 5.4) of the homogeneity range of the clathrate-I phase Ba8NixGe46−x. From single crystal X-ray diffraction data we introduce a crystal structure model that takes point defects (vacancies) in the Ge network into account. It reveals that both Ni and accumulate at a single site (6c) and that, as a consequence, the Ge network distorts considerably. Ba8Ni3.5Ge42.10.4shows metal-like behaviour (dρ/dT> 0) albeit with high resistivity at room temperature (ρ(300 K) 1 mΩ cm). Together with the low charge carrier concentration of 2.3 e−/unit cell at 300 K this is typical of a degenerate semiconductor. The lattice thermal conductivity is distinctly smaller than that of Ba8Ge433, where the vacancies partially order, and smaller than those of Ba–Ni–Ge type-I clathrates without vacancies, suggesting that disordered vacancies efficiently scatter heat-transporting phonons. We provide evidence that the maximum value of the thermoelectric figure of merit reached in Ba8Ni3.5Ge42.10.4, ZT(680 K) ≅ 0.21, can be further improved by adjusting the charge carrier concentration. [ABSTRACT FROM AUTHOR]