Tl-doped type-II clathrate Cs8Na16-xTlxGe136 was synthesized from its elements. The structure was characterised by single crystal and powder diffraction. Cs8Na16-xTlxGe136 crystallized in the space group Fd3[combining macron]m. The melting point decreased with increasing Tl content at 925.0 °C, 922.8 °C and 915.7 °C for Cs8Na14.76(3)Tl1.24(2)Ge136, Cs8Na9.94(2)Tl6.06(2)Ge136 and Cs8Na8.36(2)Tl7.64(2)Ge136 in Ar atmosphere, respectively. Cs8Na9.94(2)Tl6.06(2)Ge136 exhibited diamagnetism with χ = -3.11 × 10(-3) emu mol(-1) at 1.8 K. The heat capacity of Cs8Na9.94(2)Tl6.06(2)Ge136 presented an Einstein peak at 10 K. The fitted parameters for Cs8Na9.94(2)Tl6.06(2)Ge136 were λ = 0.115(4) J mol(-1) K(-2), β = 0.0289(2) J mol(-1) K(-4), NE = 7.94(4), θE = 56.9(1) K, ND = 152.06(4) and θD = 217(1) K using the equation Cp = Ce + CD + CE from 1.8 K to 10 K. The thermoelectric measurement showed metallic resistivity and a negative Seebeck coefficient, indicating electron-type charge carriers. The electronic structure calculation for Cs8Na10Tl6Ge136 confirmed the nature of the metallic transport behavior with conducting electrons, mainly from the Ge p orbitals.
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