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

Due to the simultaneously broken inversion symmetries along multiple directions, ABP₂X₆ (A = Cu or Ag, B = In or Bi, and X = S or Se) monolayers exhibit the coexistence of sizable piezoelectric coefficients d₁₁, d₂₁, and d₃₁, under unidirectional in-plane strains or stresses, as determined from first-principles calculations. Remarkably, the AgBiP₂S₆ and AgBiP₂Se₆ monolayers possess d₁₁ 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 d₃₁ coefficient of AgBiP₂Se₆ is as high as 0.36 pm V⁻¹. The ABP₂X₆ monolayers possessing unique multidirectional piezoelectricity pave novel avenues for the application of piezoelectric devices with multiple functions.