From Three-Dimensional Clathrates to Two-Dimensional Zintl Phases AMSb 2 (A = Rb, Cs; M = Ga, In) Composed of Pentagonal M-Sb Rings.

Three new antimonide Zintl phases, RbGaSb 2 , CsGaSb 2 , and CsInSb 2 , were discovered during exploration of corresponding A-M-Sb (A = Rb, Cs; M = Ga, In) ternary systems while searching for new clathrates. The AGaSb 2 phases crystallize in the tetragonal space group P 4 2 / nmc (No. 137) in the LiBS 2 structure type, while CsInSb 2 crystallizes in lower symmetry in the orthorhombic space group Cmce (No. 64) in the KGaSb 2 structure type with additional disorder of one of the Cs sites. The crystal structures of all three reported AMSb 2 compounds are composed of two-dimensional [MSb 2 ] - tetrahedral layers separated by Rb + or Cs + cations. [MSb 2 ] - layers are built from fused M-Sb pentagons and hexagons, which are also the main structural units for A 8 M 27 Sb 19 clathrate cages. The semiconductor nature of AMSb 2 was suggested by band structure calculations and confirmed by transport property characterization. CsGaSb 2 is a rare example of an n-type pnictide Zintl phase. All reported compounds exhibit low thermal conductivity typical for complex antimonides of heavy elements.