Large piezoelectric response in a family of metal-free perovskite ferroelectric compounds from first-principles calculations

Abstract

Metal-free organic perovskite ferroelectric materials have been shown recently to have a number of attractive properties, including high spontaneous polarization and piezoelectric coefficients. In particular, slow evaporation of solutions containing organic amines, inorganic ammoniums, and dilute hydrohalogen acid has been shown to produce several attractive materials in the MDABCO-NH₄-I₃ family (MDABCO is N-methyl-N’-diazabicyclo[2,2,2] octonium). In the present work, we study by first-principles calculations the origin of polarizaiton, electronic density of state, piezoelectric response, and elastic properties of MDABCO-NH₄-X₃ (X = Cl, Br, I). We find that the dipole moments of the MDABCO and NH₄ groups are negligible, and the large spontaneous polarization of MDABCO-NH₄-I₃ mainly results from MDABCO and NH₄ being off-center relative to I ions. Although the piezoelectric response of organic materials is usually very weak, we observe large piezoelectric strain components, d_x4 and d_x5; the calculated d_x5 is 119 pC/N for MDABCO-NH₄-Cl₃, 248 pC/N for MDABCO-NH₄-Br₃ and 178 pC/N for MDABCO-NH₄-I₃. The large value of d_x5 is found to be closely related with the large value of elastic compliance tensor, s₄₄. These results show that MDABCO-NH₄-X₃ metal-free organic perovskites have large piezoelectric response with soft elastic properties.