Structure and properties of type-II clathrate Cs8Na16−xTlxGe136

Abstract

Tl-doped type-II clathrate Cs₈Na_16−xTl_xGe₁₃₆ was synthesized from its elements. The structure was characterised by single crystal and powder diffraction. Cs₈Na_16−xTl_xGe₁₃₆ crystallized in the space group Fdm. The melting point decreased with increasing Tl content at 925.0 °C, 922.8 °C and 915.7 °C for Cs₈Na_14.76(3)Tl_1.24(2)Ge₁₃₆, Cs₈Na_9.94(2)Tl_6.06(2)Ge₁₃₆ and Cs₈Na_8.36(2)Tl_7.64(2)Ge₁₃₆ in Ar atmosphere, respectively. Cs₈Na_9.94(2)Tl_6.06(2)Ge₁₃₆ exhibited diamagnetism with χ = −3.11 × 10⁻³ emu mol⁻¹ at 1.8 K. The heat capacity of Cs₈Na_9.94(2)Tl_6.06(2)Ge₁₃₆ presented an Einstein peak at 10 K. The fitted parameters for Cs₈Na_9.94(2)Tl_6.06(2)Ge₁₃₆ were λ = 0.115(4) J mol⁻¹ K⁻², β = 0.0289(2) J mol⁻¹ K⁻⁴, N_E = 7.94(4), θ_E = 56.9(1) K, N_D = 152.06(4) and θ_D = 217(1) K using the equation C_p = C_e + C_D + C_E 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.