Multidirectional Intrinsic Piezoelectricity of 2D Metal Chalcogen–Diphosphate ABP₂X₆ Monolayers

Due to the simultaneously broken inversion symmetries along multiple directions, ABP 2 X 6 (A = Cu or Ag, B = In or Bi, and X = S or Se) monolayers exhibit the coexistence of sizable piezoelectric coefficients d 11 , d 21 , and d 31 , under unidirectional in‐plane strains or stresses, as determined from first‐principles calculations. Remarkably, the AgBiP 2 S 6 and AgBiP 2 Se 6 monolayers possess d 11 coefficients of 17.33 and 17.25 pm V −1 , respectively; these are much higher than those of extensively studied 2D piezoelectric materials. Meanwhile, the out‐of‐plane d 31 coefficient of AgBiP 2 Se 6 is as high as 0.36 pm V −1 . The ABP 2 X 6 monolayers possessing unique multidirectional piezoelectricity pave novel avenues for the application of piezoelectric devices with multiple functions.