Synthesis, structural characterization, and physical properties of the type-I clathrates A ₈Zn ₁₈As ₂₈ (A = K, Rb, Cs) and Cs ₈Cd ₁₈As ₂₈

The first arsenide clathrates A ₈Zn ₁₈As ₂₈ (A = K, Rb, Cs) and Cs ₈Cd ₁₈As ₂₈ 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 Cs ₈Zn ₁₈Sb ₂₈ and Cs ₈Cd ₁₈Sb ₂₈. In arsenide clathrates, Zn (or Cd) and As atoms are statistically mixed at the three framework sites: 6c, 16i, and 24k; the alkali metals reside inside the cages at the 2a and 6d sites, with the 2a site being only partially filled. Single-crystal X-ray diffraction studies confirm that the Cd atoms preferably occupy the 6c and 24k sites over the 16i 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 2a site. Optical absorption measurements and electronic property measurements reveal a semimetallic-like behavior for Cs ₈Cd ₁₈As ₂₈ and semiconductor-like behavior for A ₈Zn ₁₈As ₂₈ (A = Rb, Cs).