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 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: 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 Cs8Cd18As28 and semiconductor-like behavior for A(8)Zn(18)As(28) (A = Rb, Cs).