Prediction of mechanical and anisotropic elastic properties of Cs(Na, K)2Bi compounds under hydrostatic tension and compression and tunable auxetic properties
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Cs(Na, K)2Bi compounds are introduced with exotic mechanical properties for application in nanoscale electromechanical devices. The mechanical properties of the compounds are investigated under hydrostatic tension and compression using first-principles calculations. We show that hydrostatic tension and compression change the isotropic and anisotropic mechanical responses of these materials. Furthermore, our results show that the auxetic nature of CsK2Bi 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 under hydrostatic tension. The auxetic nature is not observed in CsNa2Bi; however, an interesting phenomenon occurs in the Poisson’s ratio under hydrostatic compression, in which it exhibits completely isotropic behavior. An elastic wave velocity analysis also indicates that hydrostatic pressure effectively changes the propagation pattern of the elastic waves and switches propagation directions. Therefore, the mechanisms identified in this work to control the auxetic and anisotropic elastic behavior of these compounds provide a critical feature for the design and development of high-performance nanoscale electromechanical devices.