Impressive Structural Diversity and Polymorphism in the Modular Compounds ABi₃Q₅ (A = Rb, Cs; Q = S, Se, Te)

An outstanding example of structural diversity and complexity is found in the compounds with the general formula ABi₃Q₅ (A = alkali metal; Q = chalcogen). γ-RbBi₃S₅ (I), α-RbBi₃Se₅ (II), β-RbBi₃Se ₅ (III), γ-RbBi₃Se₅ (IV), CsBi ₃Se₅ (V), RbBi₃Se₄Te (VI), and RbBi₃Se₃Te₂ (VII) were synthesized from A ₂Q (A = Rb, Cs; Q = S, Se) and Bi₂Q₃ (Q = S, Se or Te) at temperatures above 650 °C using appropriate reaction protocols. γ-RbBi₃S₅ and α-RbBi₃Se ₅ have three-dimensional tunnel structures while the rest of the compounds have lamellar structures. γ-RbBi₃S₅, γ-RbBi₃S₅, and its isostructural analogues RbBi ₃Se₄Te and RbBi₃Se₃Te₂ crystallize in the orthorhombic space group Pnma with a = 11.744(2) Å, b = 4.0519(5) Å, c = 21.081(3) Å, R1 = 2.9%, wR2 = 6.3% for (I), a = 21.956(7) Å, b = 4.136(2) Å, c = 12.357(4), Å, R1 = 6.2%, wR2 = 13.5% for (IV), and a = 22.018(3), Å, b = 4.2217(6), Å, c = 12.614(2) Å, R1 = 6.2%, wR2 = 10.3% for (VI). γ-RbBi ₃S₅ has a three-dimensional tunnel structure that differs from the Se analogues, α-RbBi₃Se₅ crystallizes in the monoclinic space group C2/m with a = 36.779(4) Å, b = 4.1480(5), Å, c = 25.363(3), Å, β = 120.403(2)o, R1 = 4.9%, wR2 = 9.9%. β-RbBi₃Se₅ and isostructural CsBi₃Se ₅ adopt the space group P2₁/m with a = 13.537(2), Å, b = 4.1431(6), Å, c = 21.545(3) Å, β = 91.297(3)°, R1 = 4.9%, wR2 = 11.0% for (III) and a = 13.603(3), Å, b = 4.1502(8) Å, c = 21.639(4), Å, β = 91.435(3)°, R1 = 6.1%, wR2 = 13.4% for (V). α-RbBi₃Se₅ is also three-dimensional, whereas β-RbBi₃Se₅ and CsBi₃Se₅ have stepped layers with alkali metal ions found disordered in several trigonal prismatic sites between the layers. In γ-RbBi₃Se₅ and RbBi₃Se₄Te, the layers consist of Bi ₂Te₃-type fragments, which are connected in a stepwise manner. In the mixed Se/Te analogue, the Te occupies the chalcogen sites that are on the "surface" of the layers. All compounds are narrow band-gap semiconductors with optical band gaps ranging 0.4-1.0 eV. The thermal stability of all phases was studied, and it was determined that γ-RbBi ₃Se₅ is more stable than the and α- and β-forms. Electronic band calculations at the density functional theory (DFT) level performed on α-, β-, and γ-RbBi₃Se ₅ support the presence of indirect band gaps and were used to assess their relative thermodynamic stability.