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Recently, gold nanoribbons (Au NRBs) with hexagonal (4H) phase have emerged as a new kind of ultrathin Au nanostructure with unusual metastable crystal phase, which could possess unique physiochemical properties and various promising applications. However, its geometric structure and phase stability under realistic service conditions, like Joule heating or external thermal loading, have not been well studied. Here, we systematically investigated the thermal responses in geometries and crystalline phases of ultrathin 4H Au NRBs under controlled in situ heating. It is found that upon moderate electron beam irradiation heating below 400 K, "Rayleigh instability" can be gradually observed, showing an obvious geometric effect, but the 4H phase remains stable. However, phase transition from 4H to face-centered cubic phases occurs when heating temperature approaches 800 K by using a dedicated …