Ученые на станции «Тяньгун» обнаружили ранее неизвестное соединение меди и висмута

Researchers aboard China's Tiangong space station set out to watch an alloy separate into layers. They ended up discovering a compound that — according to the established phase diagram of the copper-bismuth binary system — should not exist. The material, designated Cu₂₀Bi₉, was identified in both spaceflight and Earth-based samples from a ternary alloy experiment and has been characterized in a study set for publication Thursday in the Journal of Applied Physics. A companion summary will appear the same day in AIP Publishing's Scilight. It is a compound no one was looking for — and, critically, one that turns out to be thermodynamically metastable, a nuance that both shapes its future applications and deepens its scientific significance. "The discovery of the Cu₂₀Bi₉ intermetallic compound came as a complete surprise to us," Wenjun Xie, one of the study's two authors, said in the AIP Scilight companion summary. "Our original objective for the space solidification experiment on the Cu₇₅Sn₁₃Bi₁₂ alloy was to reveal the kinetics of the liquid phase separation and the evolution of the solidified microstructures under microgravity in space." What Makes a Copper-Bismuth Compound So Surprising To understand why Cu₂₀Bi₉ stopped the researchers in their tracks, a brief look at what copper and bismuth normally do together is essential. Copper and bismuth are notoriously incompatible. In the liquid state, the two metals have extremely limited mutual solubility — they tend to separate like oil and water, a behavior materials scientists describe as immiscibility. The binary phase diagram for the Cu-Bi system, which maps every thermodynamically stable configuration those two elements can adopt as a function of temperature and composition, shows essentially no stable intermetallic compounds at ambient conditions. Prior synthesis of copper-bismuth intermetallics required extreme high-pressure conditions — a 2017 high-pressure synthesis study identified CuBi and Cu₁₁Bi₇ only at elevated pressures. Under ordinary conditions, the two metals simply do not bond into ordered compounds. What changed the picture was the addition of a third element. The alloy under study — Cu₇₅Sn₁₃Bi₁₂, meaning 75% copper, 13% tin, and 12% bismuth by atomic percent — brought tin into the mix. Tin (Sn) has robust chemical compatibility with both copper and bismuth and forms well-characterized intermetallics with each. In the ternary system, it appears to act as a chemical mediator, enabling phase behavior that neither copper nor bismuth would pursue on its own. The exact mechanism by which tin facilitates the formation of Cu₂₀Bi₉ is one of the questions Qichao Zhong and Xie are now investigating as explicitly identified follow-on research. Metastable — and Why That Word Matters The JAP paper's abstract introduces a term the original coverage of this discovery overlooked: metastable. The paper states explicitly that theoretical calculations of the formation and cohesive energies suggest that Cu₂₀Bi₉ is thermodynamically metastable per the JAP paper, which is further supported by isothermal annealing experiments. Thermodynamic metastability is not a flaw — it is a thermodynamic address. A metastable compound occupies a local energy minimum rather than the global minimum, meaning it does not spontaneously fall apart at room temperature, but given sufficient heat or time, it will eventually transform into lower-energy phases. Diamond is the most famous metastable material: carbon at room temperature and pressure is more stable as graphite, yet diamonds persist essentially forever under ordinary conditions. Metallic glass is another: amorphous metals are metastable but engineered to retain their structure across practical timescales. The distinction between dynamic stability (the material's atoms stay near their equilibrium positions) and thermodynamic stability (the material is the lowest-energy state available) is one the authors navigate carefully. Their density functional theory-based molecular dynamics simulations confirmed that Cu₂₀Bi₉ is dynamically stable — its atoms experience only small displacements from their equilibrium positions. But the formation and cohesive energy calculations confirmed it is thermodynamically metastable. Both findings are true simultaneously; they describe different things. For practical applications, metastability imposes a real constraint. Any processing condition that supplies significant thermal energy — high-temperature soldering, sintering, extended annealing — risks transforming Cu₂₀Bi₉ into something else. Applications researchers will need to identify the phase's thermal stability window before it can be incorporated into any device or material system. A Crystal Structure Described as a "Square Candy Box" Using a combination of selected-area electron diffraction (SAED), atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive X-ray spectroscopy (EDS) elemental mapping, Zhong and Xie resolved the crystal structure of Cu₂₀Bi₉ in detail. The compound adopts a tetragonal crystal structure belonging to space group P-4 (No. 81), with lattice parameters a = b = 17.648 Å and c = 10.534 Å. A tetragonal structure has a square cross-section but different height — imagine a rectangular box with a square base — which gives rise to the visual analogy Xie offered in the published AIP Scilight piece: a single unit cell looks "like a square candy box." Each unit cell packs 80 copper atoms and 36 bismuth atoms — a large and complex arrangement for a binary intermetallic. The structural model was validated computationally using density functional theory-based molecular dynamics simulations, and simulated SAED patterns and HAADF images derived from the model accurately reproduced the experimentally observed diffraction and imaging data, providing strong independent confirmation that the proposed crystal structure is correct. The ratio 80 copper to 36 bismuth reduces to the 20:9 stoichiometry that gives the compound its name, Cu₂₀Bi₉. The intermetallic designation indicates that copper and bismuth are bonded in this fixed ratio to form a distinct crystalline phase — not merely mixed as a solution, but chemically bonded into a new structural identity with properties potentially distinct from either element alone. What Microgravity Did — and Didn't — Do One of the more practically significant findings is that Cu₂₀Bi₉ appeared not just in the spaceflight samples but in parallel ground-based samples processed simultaneously under standard gravity. The compound can, in the AIP Scilight's own words, "be easily created on the ground as well as in space." This rules out the possibility that Cu₂₀Bi₉ is a uniquely microgravity phenomenon — a compound that could only form in the near-weightlessness of orbit. It is instead a phase accessible to ordinary laboratory synthesis, which is the prerequisite for any practical applications development. What the microgravity environment of Tiangong likely contributed was altered solidification dynamics. In orbit, buoyancy-driven convection is dramatically reduced — fluid does not rise and fall the way it does on Earth under gravity's pull. In immiscible systems like Cu₇₅Sn₁₃Bi₁₂, this suppression of convection changes the speed and geometry of phase separation and solidification. The resulting microstructures may be cleaner and more amenable to high-resolution electron microscopy analysis, potentially making it easier to identify phases that are present but difficult to isolate in Earth-based samples. China's Tiangong space station, fully assembled in 2022, had implemented 181 scientific and application projects as of December 2024, transporting nearly two metric tons into orbit of scientific materials and returning nearly 100 types of experimental samples. The broader materials science program running on the station encompasses metallic alloys, ceramics, and other material systems, and the station is expected to conduct more than 1,000 research projects over the next 10 to 15 years. Does It Have a Known Function Beyond Existing? The honest answer, at this stage, is: not yet. The discovery is approximately two months old relative to publication, and the characterization phase — establishing the basic crystal structure, confirming reproducibility, verifying metastability — has been the primary work. Property measurements for specific applications have not been reported. What can be said is that Cu₂₀Bi₉ enters a well-populated and practically important research landscape. Bismuth-based intermetallics are of active interest across several domains: lead-free soldering (bismuth is a lead substitute studied in lead-free solder research); thermoelectric energy conversion (bismuth telluride and its variants are industry-standard thermoelectrics); heavy-metal sensing; and topological insulator research, where bismuth's large spin-orbit coupling makes Bi-containing compounds candidates for surface-state-based electronics. Cu₂₀Bi₉ does not slot immediately into any of these categories — its specific electronic, thermal, and mechanical properties have not been reported — but it expands the space of known copper-bismuth phases from approximately zero (at ambient conditions) to one. Two follow-on research priorities are explicitly identified in the paper. The first is mechanistic: understanding precisely how Cu₂₀Bi₉ forms within the Cu₇₅Sn₁₃Bi₁₂ ternary system, including the specific role of tin as a mediating element. The second is applied: characterizing the full range of the compound's chemical and physical properties with an eye toward potential uses in both terrestrial and space-based environments. What Microgravity Science Keeps Finding by Accident Cu₂₀Bi₉ joins a documented pattern in space-based materials research: significant discoveries made not because the experiment was designed to find them, but because the unique conditions of orbit altered the experiment in unexpected ways. NASA's ISS 25-year scientific retrospective lists accidental discoveries among its most cited contributions — including the discovery of "cool flames" (a combustion phenomenon difficult to reproduce on Earth) and new protein crystal formation behavior. The history of space materials science, from Soviet Soyuz 6's 1969 welding experiments onward, has repeatedly produced findings that fell outside the original experimental scope. That Cu₂₀Bi₉ is reproducible on Earth does not diminish the role orbit played in its identification. It suggests instead that some discoveries require the unique microstructural conditions of microgravity solidification to become visible — and that the thousands of experiments planned for Tiangong over the coming decade may well produce more. Frequently Asked Questions What is Cu₂₀Bi₉, and where was it found? Cu₂₀Bi₉ is a previously unknown intermetallic compound — a crystalline material in which copper and bismuth bond in a fixed 20:9 atomic ratio to form a new phase with a distinct crystal structure. It was identified in samples from a solidification experiment conducted aboard China's Tiangong space station using a copper-tin-bismuth alloy, and also in parallel samples processed on Earth. The study is set for publication August 14 in the Journal of Applied Physics. Why is a copper-bismuth compound surprising when copper and bismuth are both common metals? The copper-bismuth binary system is unusual in that copper and bismuth are largely immiscible — they resist bonding with each other in ordered structures at ambient conditions, similar to how oil and water repel. No stable intermetallic compound between them had been identified under ordinary pressure before this discovery. The 2017 high-pressure study that produced earlier Cu-Bi intermetallics required conditions far outside normal laboratory or processing environments. The presence of tin in the ternary Cu-Sn-Bi alloy appears to mediate the formation of Cu₂₀Bi₉ in a way the binary system alone would not produce. Is Cu₂₀Bi₉ stable enough to use in materials applications? The compound is thermodynamically metastable per the JAP study, meaning it occupies a local but not global energy minimum. It is stable enough to be characterized — its crystal structure has been resolved and validated computationally — but high-temperature processing conditions could cause it to transform into other phases. Practical applications research will need to establish its thermal stability window and identify the specific electronic, thermal, or mechanical properties that make it useful. No applications have been demonstrated yet. Can this compound be made in a regular Earth-based lab, or does it require space? It can be made on Earth. The same Cu₂₀Bi₉ compound appeared in ground-based samples processed alongside the space experiment, confirming that microgravity is not a prerequisite for its formation. The AIP Scilight companion article notes that the compound can be easily created on the ground as well as in space. The space experiment's main contribution appears to have been the altered solidification conditions that made the compound visible and characterizable — not the only environment in which it can form. ⓒ 2026 TECHTIMES.com All rights reserved. Do not reproduce without permission.