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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.

The adsorption of Au (n = 1-4) clusters on perfect and defective MoS monolayers is studied using density functional theory. For the pristine MoS monolayer, our results show that the electrons are transferred from the support to the adsorbed Au clusters, thus a p-doping effect is achieved in the pristine MoS monolayer by the Au cluster adsorption, which is in good agreement with the experimental findings. The adsorption of Au clusters can introduce mid-gap states, which modify the electronic and magnetic properties of the systems. The adsorbates containing an odd number of Au atoms can introduce a spin magnetic moment of 1 μ into the perfect MoS monolayer, while those systems containing an even number of Au atoms are spin-unpolarized. Two categories of defects, i.e., a single S vacancy and Mo antisite defect with one Mo atom replacing one S atom, are considered for the defective monolayer MoS. Compared with the pristine MoS monolayer, the adsorption energies for Au clusters are significantly increased for the MoS monolayer with a single S vacancy, and there are more electrons transferred from the MoS monolayer with an S vacancy to the Au clusters. The mid-gap states and odd-even oscillation magnetic behavior can also be observed when Au clusters are adsorbed on the MoS monolayer with an S vacancy. For those systems of Au clusters on MoS monolayers with Mo antisite defects, the adsorption energies as well as the magnitude and the direction of transferred charge are similar to those for the MoS monolayer with an S vacancy. The spin-polarizations appear in all systems with Mo antisite defects. Our investigations suggest that the electronic and magnetic properties of MoS nanosheets can be effectively modulated by the adsorption of Au clusters.

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