From Three-Dimensional Clathrates to Two-Dimensional Zintl Phases AMSb2 (A = Rb, Cs; M = Ga, In) Composed of Pentagonal M–Sb Rings

Abstract

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 A8M27Sb19 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.