Effect of Bromine Deficiency on the Lattice Dynamics and Dielectric Properties of Alpha-Phase Diisopropyl Ammonium Bromide Molecular Crystals .

Diisopropyl ammonium bromide (DIPAB) molecular ferroelectric crystals exhibiting a large electric polarization (\textasciitilde 23$\mu $C/cm$^{\mathrm{2}})$, a large dielectric constant and a low tangent loss of 0.00068-0.0008 in the $\alpha $-phase. Although XRD shows overall excellent crystallinity, the analysis of vibrational spectra of $\alpha $-DIPAB obtained by FT-IR and Raman spectrometry suggests the presence of disorder in synthesized crystals as indicated by the presence of broad features in Raman spectra. Using vdW $+$ DF2 calculations, we identified majority of vibrational modes present in experimental spectra, specifically analyzing the ones due to Br-disorder. We find that the Br deficiency strongly affects the electric properties of $\alpha $-DIPAB. Particularly, the experimentally measured dielectric constant is large (\textasciitilde 20), while DFT-based calculations of the ideal DIPAB give much smaller values (\textasciitilde 2-3). However, the Br-deficiency leads to a drastic increase of the calculated dielectric constant (\textasciitilde 15-20). Finally, using vdW$+$DF2 method we show that the van der Waals forces have only a slight effect on the structural parameters.