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Abstract Highly sensitive stimuli‐responsive luminescent materials are crucial for applications in optical sensing, security, and anticounterfeiting. Here, we report two zero‐dimensional (0D) copper(I) halides, (TEP)₂Cu₂Br₄, (TEP)₂Cu₄Br₆, and 1D (TEP)₃Ag₆Br₉, which are comprised of isolated [Cu₂Br₄]²⁻, [Cu₄Br₆]²⁻, and [Ag₆Br₉]³⁻polyanions, respectively, separated by TEP⁺(tetraethylphosphonium [TEP]) cations. (TEP)₂Cu₂Br₄and (TEP)₂Cu₄Br₆demonstrate greenish‐white and orange‐red emissions, respectively, with near unity photoluminescence quantum yields, while (TEP)₃Ag₆Br₉is a poor light emitter. Optical spectroscopy measurements and density‐functional theory calculations reveal that photoemissions of these compounds originate from self‐trapped excitons due to the excited‐state distortions in the copper(I) halide units. Crystals of Cu(I) halides are radioluminescence active at room temperature under both X‐ and γ‐rays exposure. The light yields up to 15,800 ph/MeV under 662 keV γ‐rays of¹³⁷Cs suggesting their potential for scintillation applications. Remarkably, (TEP)₂Cu₂Br₄and (TEP)₂Cu₄Br₆are interconvertible through chemical stimuli or reverse crystallization. In addition, both compounds demonstrate luminescence on‐off switching upon thermal stimuli. The sensitivity of (TEP)₂Cu₂Br₄and (TEP)₂Cu₄Br₆to the chemical and thermal stimuli coupled with their ultrabright emission allows their consideration for applications such as solid‐state lighting, sensing, information storage, and anticounterfeiting. 

Abstract Graphite anodes offer low volumetric capacity in lithium‐ion batteries. By contrast, tellurene is expected to alloy with alkali metals with high volumetric capacity (≈2620 mAh cm⁻³), but to date there is no detailed study on its alloying behavior. In this work, the alloying response of a range of alkali metals (A = Li, Na, or K) with few‐layer Te is investigated. In situ transmission electron microscopy and density functional theory both indicate that Te alloys with alkali metals forming A₂Te. However, the crystalline order of alloyed products varies significantly from single‐crystal (for Li₂Te) to polycrystalline (for Na₂Te and K₂Te). Typical alloying materials lose their crystallinity when reacted with Li—the ability of Te to retain its crystallinity is therefore surprising. Simulations reveal that compared to Na or K, the migration of Li is highly “isotropic” in Te, enabling its crystallinity to be preserved. Such isotropic Li transport is made possible by Te's peculiar structure comprising chiral‐chains bound by van der Waals forces. While alloying with Na and K show poor performance, with Li, Te exhibits a stable volumetric capacity of ≈700 mAh cm⁻³, which is about twice the practical capacity of commercial graphite.