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Combined Fast Reversible Liquid-like Elastic Deformation with Topological Phase Transition in Na$_3$Bi

By means of first-principles calculations, we identified the structural phase transition of Na$_3$Bi from hexagonal ground state to cubic $cF$16 phase above 0.8 GPa, in agreement with the experimental findings. Upon the releasing of pressure, \emph{cF}16 phase of Na$_3$Bi is mechanically stable at ambient condition. The calculations revealed that the $cF$16 phase is topological semimetal, in similarity to well-known HgTe and it even exhibits an unusually low $C^\prime$ modulus (only about 1.9 GPa) and a huge anisotropy, $A^u$ of as high as 11, the third highest value among all known cubic crystals in their elastic behaviors. These facts render \emph{cF}16-type Na$_3$Bi very soft with a liquid-like elastic deformation in the (110)$<$1$\overline{1}$0$>$ slip system. Importantly, as accompanied with this deformation, Na$_3$Bi shows a topological phase transition from a topological semimetal state at its strain-free cubic phase to a topological insulating state at its distorted phase. Because the $C^\prime$ elastic deformation almost costs no energy in a reversible and liquid-like soft manner, \emph{cF}16-type Na$_3$Bi would potentially provide a fast on/off switching way between topological insulator and topological semimetal, which would be beneficial to the quantum electronic devices for practical applications.