Synthesis, Structural Characterization, and Physical Properties of the Type-I Clathrates A₈Zn₁₈As₂₈ (A = K, Rb, Cs) and Cs₈Cd₁₈As₂₈

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