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Transformation of hexagonal Lu to cubic LuH single-crystalline films
With the recent report of near ambient superconductivity at room temperature
in the N-doped lutetium hydride (Lu-H-N) system, the understanding of cubic
Lu-H compounds has attracted worldwide attention. Generally, compared to
polycrystal structures with non-negligible impurities, the single-crystalline
form of materials with high purity can provide an opportunity to show their
hidden properties. However, the experimental synthesis of single-crystalline
cubic Lu-H compounds has not been reported thus far. Here, we developed an easy
way to synthesize highly pure LuH single-crystalline films by the
post-annealing of Lu single-crystalline films (purity of 99.99%) in the H
atmosphere. The crystal and electronic structures of films were characterized
by X-ray diffraction, Raman spectroscopy, and electrical transport.
Interestingly, Lu films are silver-white and metallic, whereas their
transformed LuH films become purple-red and insulating, indicating the
formation of an unreported electronic state of Lu-H compounds possibly. Our
work provides a novel route to synthesize and explore more single-crystalline
Lu-H compounds
Lu-H-N phase diagram from first-principles calculations
Using a comprehensive structure search and high-throughput first-principles
calculations of 1483 compounds, this study presents the phase diagram of
Lu-H-N. The formation energy landscape of Lu-H-N was derived and utilized to
assess the thermodynamic stability of compounds. Results indicate that there
are no stable Lu-H-N ternary structures in this system, but metastable ternary
structures, such as Lu20H2N17 (C2/m), Lu2H2N (P3-m1), were observed with small
Ehull (< 100 meV/atom). Moreover, applying hydrostatic pressure up to 10 GPa
causes the energy convex hull of the Lu-H-N to shift its shape and stabilizes
binary phases such as LuN9 and Lu10H21. Additionally, interstitial empty sites
in LuH2 were noted, which may explain the formation of Lu10H21 and LuH3-xNy. To
provide a basis for comparison, X-ray diffraction patterns and electronic
structures of some compounds are also presented
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