The ELEFANT Concept for Extended Low-X-ray-Contrast Electron-Precise Frameworks with Adaptive Noradamantane-Like Topologies

Abstract

A persistent structural challenge in ternary Ge/As compounds is that X-ray crystallography cannot reliably distinguish germanium and arsenic due to their near-identical electron counts, complicating site assignments and composition determination. We developed a systematic approach linking composition, valence electron count, and structural connectivity to arrive at chemically meaningful structure descriptions, demonstrated here by 11 new compounds: AxGe8+xAs16–x (A = Na, K, Rb), Rb1.7Ge2.7As3.3, NaGe3As3, Na2Ge3As3, Na6Ga6Ge6As12, K0.6Ge2.6P3.4, Rb0.7Ge2.7P2.3, NaSi3As3, and Na2Ge7As7. We introduce the ELEFANT concept (Extended Low-X-ray-contrast Electron-precise Frameworks with Adaptive Noradamantane-like Topology) to describe A–(Tr)–Tt–Pn compounds containing noradamantane-like substructures. All structures share a common (5)+1-ring motif, enabling calculation of the valence electrons per 12 framework atoms (VE/12FA) and the bond-to-lone-pair ratio (B/LP). These parameters define structure groups, reveal electronic boundaries, and constrain framework connectivity. DFT calculations predict semiconducting behavior for all compounds, consistent with transport and impedance measurements. By connecting stoichiometry, bonding, and topology, ELEFANT provides a unified framework for understanding, validating, and modifying A–Tt–Pn semiconductors through controlled substitution of chemically similar elements such as Ga, Ge, and As.