Trigonal Selenium is an Inorganic Macromolecular Crystal Formed by Linear Long but Finite Polymer Chains of Se

As the thermodynamically stable phase of selenium, trigonal selenium (t-Se) has long been studied as an inorganic solid, with previous studies focusing on its lattice and phase behavior while overlooking its chain-like nature. Herein, t-Se was investigated, for the first time, as a polymer material. Single-molecule force spectroscopy (SMFS) and complementary techniques reveal that t-Se consists of finite linear poly-Se chains with an average contour length of 148 nm (∼63 kDa) rather than the infinite structures traditionally assumed. The single-chain inherent elasticity of poly-Se was experimentally quantified and theoretically calculated, providing the quantitative evidence for its covalently linked Se–Se backbone and flexible mechanical behavior comparable to that of organic polymers with carbon–carbon backbones (e.g., polyethylene). These findings demonstrate t-Se as an inorganic crystal with distinct macromolecular characteristics. Unlike conventional inorganic solids, which are either fully crystalline or amorphous, t-Se shows a degree of crystallinity exceeding 98%, surpassing almost all synthetic polymers including high-density polyethylene (HDPE). This reflects the highly ordered packing of its finite-length poly-Se chains, placing t-Se among the polymers with the highest degree of crystallinity known to date. This bridges the gap between inorganic chemistry and polymer science, opening new avenues for exploring t-Se as a macromolecular material.