Synthesis of Na-Cu-Ge ternary clathrates with type I and II structures in film form

Abstract

Na-Cu-Ge ternary clathrate films with about 0.5 μm in thickness were prepared on sapphire substrates by vacuum evaporation of Na on Cu–Ge composite films followed by lamp heating under vacuum. XRD measurements confirmed the syntheses of type II (Na_x (Cu_zGe_1-z)₁₃₆) and type I (Na₈(Cu_zGe_1-z)₄₆) clathrates. The former was obtained as a single phase from the Cu–Ge film with Cu inclusion of 4∼5 %, whereas the latter was earned as the phase of mixture of type II clathrate and Cu_0.85Ge_0.15 alloy from the films with Cu inclusion of 14 % or higher. Rietveld analyses of XRD data and Raman measurements indicated expansions of lattice constants by Cu inclusions in type I and II clathrate structures. The complete opacity observed in optical transmission indicates that Na_x (Cu_zGe_1-z)₁₃₆ exhibit metallic behavior.