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-NH 4 -I 3 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-NH 4 -X 3 (X = Cl, Br, I). We find that the dipole moments of the MDABCO and NH 4 groups are negligible, and the large spontaneous polarization of MDABCO-NH 4 -I 3 mainly results from MDABCO and NH 4 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-NH 4 -Cl 3 , 248 pC/N for MDABCO-NH 4 -Br 3 and 178 pC/N for MDABCO-NH 4 -I 3 . The large value of d x5 is found to be closely related with the large value of elastic compliance tensor, s 44 . These results show that MDABCO-NH 4 -X 3 metal-free organic perovskites have large piezoelectric response with soft elastic properties.npj Computational Materials (2019) 5:17 ; https://doi.

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