A Computational Study on Type-I Sn Clathrates with Inorganic and Organic Guests

Abstract

Semiconducting clathrates are a class of inclusion compounds where monatomic cations or anions are encapsulated in a framework made mainly of Si, Ge, Sn, and Sb atoms. In this study, we use density functional theory (DFT) computations to investigate a series of type-I clathrates consisting of a Sn framework that hosts a range of inorganic or organic cations (Rb+, Cs+, NH4+, CH3NH3+, (CH3)2NH2+, (CH3)3NH+, (CH3)4N+, (NH2)2CH+, and (CH3)3S+), anions (F–, Cl–, Br–, I–, HCOO–, NO3–, BF4–, SCN–, and CF3SO3–), a neutral molecule (NH3), and a salt (CsI). Among these guest species, only Rb+, Cs+, Cl–, Br–, and I– ions are so far experimentally confirmed in Sn-based, type-I clathrates. The DFT analysis of the structural models includes geometry optimization and calculation of formation enthalpies. The results demonstrate that both stereochemical and electron counting limitations apply in A8Sn46–x clathrates for the guest–host interactions, and ca. 1/4 of the initially proposed compounds are either thermodynamically unstable or the clathrate framework collapses. The presence of framework defects (x = 2) is favored only for A = Rb+, Cs+, NH4+, and NO3–. Electronic density of states calculations predict metallic behavior for defect-free clathrates (x = 0) with cationic guests and most anionic guests. For x = 2, narrow band gap semiconducting behavior is found for the smaller cations A = Rb+, Cs+, NH4+, CH3NH3+, (CH3)2NH2+, and (NH2)2CH+, as well as for A = HCOO– and SCN–. Larger band gaps of ∼1 eV are found for clathrates with neutral guests, such as (NH3)8Sn46. Further screening of the thermodynamically most stable compounds is performed through Hirshfeld surface and Bader charge analysis to elucidate the host–guest interactions. Overall, this work suggests new pathways in the development of sustainable thermoelectric or inclusion materials by exploring the potential of Sn-based clathrates.