As the only semiconductor material exhibiting ultrahigh thermal conductivity under ambient conditions, cubic boron arsenide (BAs) is currently attracting great interest. Thanks to the development of high-quality BAs single crystal growth techniques, investigation of its basic physical properties has now become possible. Here, the mechanical properties of BAs single crystals are studied by experimental measurements combined with first-principles calculations. A Vickers hardness of 22 GPa suggests that BAs is a hard material, although not among the hardest. The bulk and Young's moduli are measured to be 142 and 388 GPa, respectively. These important mechanical performance parameters, in conjunction with the unusual high thermal conductivity, show great potential for BAs to serve in next-generation semiconductor applications.

Using first-principles calculations, we uncovered that t-B3N4 is metastable at ambient pressure, but becomes stable under high pressure. The metallic conduction of t-B3N4 is interrupted by the insulated boron sheets stacked along the c axis, giving rise to its 2D metallicity.

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As one of the important materials, nanocrystalline Au (n-Au) has gained numerous interests in recent decades owing to its unique properties and promising applications. However, most of the current n-Au thin films are supported on substrates, limiting the study on their mechanical properties and applications. Therefore, it is urgently desired to develop a new strategy to prepare n-Au materials with superior mechanical strength and hardness. Here, a hard n-Au material with an average grain size of ~ 40 nm is prepared by cold-forging of the unique Au nanoribbons (NRBs) with unconventional 4H phase under high pressure. Systematic characterizations reveal the phase transformation from 4H to face-centered cubic (fcc) phase during the cold compression. Impressively, the compressive yield strength and Vickers hardness (H V ) of the prepared n-Au material reach ~ 140.2 MPa and ~ 1.0 GPa, which are 4.2 and 2.2 times of the microcrystalline Au foil, respectively. This work demonstrates that the combination of high-pressure cold-forging and the in-situ 4H-to-fcc phase transformation can effectively inhibit the grain growth in the obtained n-Au materials, leading to the formation of novel hard n-Au materials. Our strategy opens up a new avenue for the preparation of nanocrystalline metals with superior mechanical property.nanocrystalline Au, high hardness, high strength, high-pressure forging, 4H Au nanoribbons