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

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 A₈Sn_46-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.