Synthesis, Structural Characterization, and Physical Properties of the Type‑I Clathrates <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = K, Rb, Cs) and Cs₈Cd₁₈As₂₈

Abstract

The first arsenide clathrates <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = K, Rb, Cs) and Cs₈Cd₁₈As₂₈ have been synthesized in high yields via a two-step route. These compounds adopt the type-I structure and exhibit structural characteristics different from the recently reported antimonide clathrates Cs₈Zn₁₈Sb₂₈ and Cs₈Cd₁₈Sb₂₈. In arsenide clathrates, Zn (or Cd) and As atoms are statistically mixed at the three framework sites: 6<i>c</i>, 16<i>i</i>, and 24<i>k</i>; the alkali metals reside inside the cages at the 2<i>a</i> and 6<i>d</i> sites, with the 2<i>a</i> site being only partially filled. Single-crystal X-ray diffraction studies confirm that the Cd atoms preferably occupy the 6<i>c</i> and 24<i>k</i> sites over the 16<i>i</i> site, with more than 80% of Cd found at the former two positions. A unique structural feature is a framework disorder coupled with the partial occupancy of the cage’s 2<i>a</i> site. Optical absorption measurements and electronic property measurements reveal a semimetallic-like behavior for Cs₈Cd₁₈As₂₈ and semiconductor-like behavior for <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = Rb, Cs)