Abstract
Since the discovery of superconductivity in boron-doped diamond with a critical temperature (TC) near 4 K, great interest has been attracted in hard superconductors such as transition-metal nitrides and carbides. Here we report the new discovery of superconductivity in polycrystalline hexagonal ε-NbN synthesized at high pressure and high temperature. Direct magnetization and electrical resistivity measurements demonstrate that the superconductivity in bulk polycrystalline hexagonal ε-NbN is below ∼11.6 K, which is significantly higher than that for boron-doped diamond. The nature of superconductivity in hexagonal ε-NbN and the physical mechanism for the relatively lower TC have been addressed by the weaker bonding in the Nb-N network, the co-planarity of Nb-N layer as well as its relatively weaker electron-phonon coupling, as compared with the cubic δ-NbN counterpart. Moreover, the newly discovered ε-NbN superconductor remains stable at pressures up to ∼20 GPa and is significantly harder than cubic δ-NbN; it is as hard as sapphire, ultra-incompressible and has a high shear rigidity of 201 GPa to rival hard/superhard material γ-B (∼227 GPa). This exploration opens a new class of highly desirable materials combining the outstanding mechanical/elastic properties with superconductivity, which may be particularly attractive for its technological and engineering applications in extreme environments.
Highlights
Hard superconducting materials have attracted considerable interest in materials science, condensed matter physics and solid-state chemistry since the discovery of the superconductivity in superhard boron-doped diamond with a transition temperature of TC ∼ 4 K1–5
The synthetic specimen was free of visible microcracks with an average grain size of about 1 μm, exhibiting an equilibrated microstructure with homogeneous fine grains. (C) High resolution TEM (HRTEM) of the synthesized specimen; the corresponding observed and simulated selected area electron diffraction (SAED) patterns and the enlarged portion of the high-resolution TEM (HRTEM) image are displayed as insets. (D) Crystal structure of the hexagonal ε-NbN (P63/mmc, No 194)
Polycrystalline hexagonal ε-NbN bulk specimens used for the current magnetization and electrical resistivity measurements were synthesized from niobium nitride starting material (Goodfellow, claimed 99% purity) at 10 GPa and 1100∼ 1200 °C for 1.5 hour in a high-pressure multi-anvil apparatus at Stony Brook University
Summary
Hard superconducting materials have attracted considerable interest in materials science, condensed matter physics and solid-state chemistry since the discovery of the superconductivity in superhard boron-doped diamond with a transition temperature of TC ∼ 4 K1–5. Recent first-principles theoretical calculations of the thermodynamic properties and structural stability in NbN polymorphs[22,23] (e.g. NaCl-, NiAs- and WC-type NbN) predicted that the hexagonal-structured NbN (e.g. WC- and NiAs-type) exhibited higher hardness and lower total energy than the cubic δ-NbN. These results indicated that the hexagonal phases were more stable than the cubic counterpart which appeared to be the most energetically unfavorable structure or metastable phase with the rock-salt structure. We report the discovery of superconductivity in bulk polycrystalline hexagonal ε-NbN, and the findings of its ultra-incompressibility, high shear rigidity and hardness
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