Ferroelectricity and piezoelectricity in elemental α-X6 structure

Two-dimensional (2D) elemental ferroelectricity has recently been confirmed and arouses great interest in exploring new elemental asymmetric structures and their symmetry-breaking related properties. In this study, our density functional theory (DFT) calculations reveal the existence of coupled in-plane and out-of-plane electric polarizations in the group $\mathrm{V}$ elements $\alpha \text{−}X6$ ($\mathit{X}=\mathrm{P}$, As). These ferroelectric phases are stabilized through charge transfer between $p_{\mathrm{x}}$ orbitals. The introduction of compressive strain facilitates the transformation from blue phosphorene into $\alpha$-P6 phase with low reaction energy owing to their structural similarity. In addition, strain engineering or atom substitution can effectively reduce the polarization switching barrier. Furthermore, $\alpha \text{−}X6$ structure exhibits large longitudinal piezoelectric strain coefficients $d_{11}$, benefiting from their moderate $e_{11}$ and flexible character. This discovery not only enriches the family of elemental ferroelectrics but also deepens the understanding of the origin of elemental polarization, offering potential candidates for ferroelectric and piezoelectric applications.