K_21–δNa_2+δIn₃₉ (δ = 2.8): A Cluster‐Replacement Clathrate‐II Structure with an Alkali Metal M₁₃₆‐Network

Abstract

Abstract K 21–δ Na 2+δ In 39 with δ = 2.82 was synthesized (melted at 973 K, annealed at 623 K) from the elements in sealed niobium ampoules. The compound forms prismatic crystals with silver metallic lustre and is unstable in air and moisture. The crystal structure of K 21–δ Na 2+δ In 39 (orthorhombic; space group Pnma , No. 62; a = 17.844(5) Å, b = 17.192(3) Å, c = 25.078(7) Å; Z = 4; Pearson code oP 248; δ = 2.82, obtained from the structure refinement) contains eight empty In icosahedra of two types, A (12 exo‐bonds) and B (7 exobonds), and four open In 15 clusters (15 exo‐bonds). The latter are centered by K atoms and belong to C units, which are defined as [K(Na 2 M 3 In 15 )] heteroatomic clusters (M = K + Na). The spatial distribution of the In icosahedra A, B and heteroatomic clusters C is that of the atoms in the cubic Laves phase MgCu 2 : MgCu 2 ≙ [K(Na 2 M 3 In 15 )][In ] 2 . All the In n clusters are interconnected by exo‐bonds forming a covalent three‐dimensional framework (d(InIn) = 2.832 to 3.301 Å). The remaining alkali metal atoms build up a three‐dimensional M 136 network of the clathrate‐II type with (16 + 8) cages, which envelopes the In icosahedra and [K(Na 2 M 3 In 15 )] clusters. This structure can be described as a cluster‐replacement derivative of the clathrate‐II structure: (H 2 S) 16 (CCl 4 ) 8 · (H 2 O) 136 ≙ [In ] 16 [K(M 5 In 15 )] 8 M 136 , and is one member of a novel hierarchical structure family, based upon cluster‐replacement. The bonding as well as the structural relationships to other phases are discussed.