Effect of bromine deficiency on the lattice dynamics and dielectric properties of alpha-phase diisopropylammonium bromide molecular crystals. (February 2018)

• Record Type: Journal Article
• Authors: Alsaad, Ahmad; Marin, Chris M.; Alaqtash, Nabil; Chao, Hsien-Wen; Chang, Tsun-Hsu; Cheung, Chin Li; Ahmad, A.; Qattan, I.A.; Sabirianov, Renat F.
• Is Part Of: Journal of physics and chemistry of solids. Volume 113(2018)
• Page Start: 82
• Page End: 85
• Publication Date: 2018-02
• Subjects: Bromine deficiency -- Lattice dynamics -- Dielectric properties -- Alpha-phase diisopropylammonium bromide -- FT-IR and Raman vibrational spectra; Solids -- Periodicals; Solides -- Périodiques; Solids; Periodicals; 530.41
• DOI: 10.1016/j.jpcs.2017.10.004
• Languages: English
• ISSNs: 0022-3697

Abstract

Abstract: Diisopropylammonium bromide (DIPAB) molecular ferroelectric crystals were synthesized and examined to exhibit a large electric polarization (∼23μC/cm 2 ), a large dielectric constant in the α-phase. Although the PXRD pattern indicates that the α-DIPAB sample has an overall excellent crystallinity, our analysis of its FT-IR and Raman vibrational spectra suggests the presence of disorder in the synthesized crystals as indicated by the presence of broad features in the Raman spectrum. Using vdW+DF2 calculations, we identified the majority of vibrational modes in the experimental spectra and analyzed the ones due to Br-disorder. We found that the bromine (Br) deficiency strongly affects the electric properties of α-DIPAB. Particularly, the experimentally measured dielectric constant of α-DIPAB is large (∼20), whereas the DFT-based calculations of the ideal DIPAB give much smaller values (∼2-3). We find that Br-deficiency is responsible for large dielectric constant of the DIPAB crystal with calculated value of ∼15-20. Furthermore, we showed that the van der Waals forces have a slight effect on the structural parameters, only causing a small shift in the vibrational frequencies. The main vibrational features of the DIPAB crystal in the Raman spectrum were shown to be driven by covalent bonding in the DIPA molecules and hydrogen bonds between the molecules with Br. Highlights: Diisopropylammonium bromide (α-DIPAB) molecular ferroelectric crystals were synthesized and examined to exhibit a large electric polarization (~23 μC/cm 2 ). FT-IR and Raman vibrational spectra suggest the presence of disorder in the synthesized crystals as indicated by the presence of broad features in the Raman spectrum. The bromine (Br) deficiency strongly affects the electric properties of α-DIPAB. Our DFT-based calculations show that Br vacancies drastically alter the strength and nature of crystal bonding, as well as vibrational spectra and dielectric constant of the material. Our data provide a deeper insight into structural, dielectric and lattice dynamical properties of α-DIPAB molecular crystal.