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 particle swarm optimization method in conjunction with density functional calculations is used to search the lower energy structures for the cationic water clusters (H2O)5(+). Geometry optimization, vibrational analysis, and infrared spectrum calculation are performed for the most interesting clusters at the MP2/aug-cc-pVDZ level. The relationships between their structural arrangements and their energies are discussed. According to their relative Gibbs free energies, their energy order is determined and four lowest energy isomers are found to have a relative population surpassing 1% below 350 K. Studies reveal that, among these four isomers, one new cluster found here also contributes a lot to the experimental infrared spectrum. Based on topological analysis and reduced density gradient analysis, some meaningful points are found by studying the structural characteristics and the bonding strengths of these cationic water clusters: in the first solvation shell, the central H3O(+) motifs may have a stronger interaction with the OH radical than with the water molecules. The interaction in the second solvation shell may also be stronger than that in the first solvation shell, which is opposite to our intuition.

Na3OBr is expected to be a promising superionic conductor for use as solid state electrolyte in large scale energy storage systems, but its lattice dynamic and thermal conductivity properties have not been well studied till now. In this work, we performed a detailed study on these properties as well as its electronic and elastic properties using first principles method. The results indicate that Na3OBr is a direct band gap crystal. It is mechanically stable but elastically anisotropic. Its phonons at the Brillouin zone center were classified using group theory analysis and its Born effective charges, LO–TO splitting, and dielectric constants were calculated and discussed. Its phonon dispersion curves were obtained and their origins were revealed. Based on the phonon dispersion curves, its thermal conductivity as a function of temperature was predicted, which give a value of 7.30 Wm−1 K−1 at 300 K. Further study reveals that, for Na3OBr, the simple Slack's model can give almost the same thermal conductivity curve as that obtained from the phonon dispersion curves.

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