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

Shahram Yalameha; Zahra Nourbakhsh; Ali Ramazani; Daryoosh Vashaee

doi:10.3390/nano11102739

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

Nanomaterials (Basel). 2021 Oct 16;11(10):2739. doi: 10.3390/nano11102739.

Authors

Shahram Yalameha¹, Zahra Nourbakhsh¹, Ali Ramazani², Daryoosh Vashaee^{3

4}

Affiliations

¹ Faculty of Physics, University of Isfahan, Isfahan 81746-73441, Iran.
² Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA.
³ Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27606, USA.
⁴ Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27606, USA.

Abstract

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 exhibits almost isotropic elastic behavior at zero pressure. We found that hydrostatic tension and compression 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 auxetic 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 conditions 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.

Keywords: auxetic material; elastic anisotropy; mechanical properties; negative Poisson’s ratio.

Abstract

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