Promising Bialkali Bismuthides Cs(Na, K)₂Bi for High‐Performance Nanoscale Electromechanical Devices: Prediction of Mechanical and Anisotropic Elastic Properties under Hydrostatic Tension and Compression and Tunable Auxetic Properties (Editor’s Pick)

Using first‐principles calculations, we predict highly stable cubic bialkali bismuthides Cs(Na, K)₂Bi with several technologically important mechanical and anisotropic elastic properties. We investigate the mechanical and anisotropic elastic properties under hydrostatic tension and compression. At zero pressure, CsK₂Bi is characterized by elastic anisotropy with maximum and minimum stiffness along the directions of [111] and [100], respectively. Unlike CsK₂Bi, CsNa₂Bi ex‐ hibits almost isotropic elastic behavior at zero pressure. We found that hydrostatic tension and com‐ pression change the isotropic and anisotropic mechanical responses of these compounds. Moreover, the auxetic nature of the CsK₂Bi compound is tunable under pressure. This compound transforms into a material with a positive Poisson’s ratio under hydrostatic compression, while it holds a large negative Poisson’s ratio of about −0.45 along the [111] direction under hydrostatic tension. An aux‐ etic nature is not observed in CsNa₂Bi, and Poisson’s ratio shows completely isotropic behavior under hydrostatic compression. A directional elastic wave velocity analysis shows that hydrostatic pressure effectively changes the propagation pattern of the elastic waves of both compounds and switches the directions of propagation. Cohesive energy, phonon dispersion, and Born–Huang con‐ ditions show that these compounds are thermodynamically, mechanically, and dynamically stable, confirming the practical feasibility of their synthesis. The identified mechanisms for controlling the auxetic and anisotropic elastic behavior of these compounds offer a vital feature for designing and developing high‐performance nanoscale electromechanical devices.

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Recommended citation: Shahram Yalameha, Zahra Nourbakhsh, Ali Ramazani and Daryoosh Vashaee, Nanomaterials 2021, 11, 2739.