Journal
JOURNAL OF MATERIALS CHEMISTRY C
Volume 7, Issue 10, Pages 3003-3014Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c8tc06458j
Keywords
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Funding
- Wroclaw Centre for Networking and Supercomputing [415]
- Polish Ministry of Science and Higher Education [3807/ZIBJ DUBNA/2017/0]
- International Program Joint Institute for Nuclear Research Dubna [04-4-1121-2017/2020]
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The extraordinary thermal stability of [NH2CHNH2](3)Bi2I9 (FA(3)Bi(2)I(9)) crystals, up to around 600 K, has been enhanced by using the evaporating method. DSC measurements have revealed reversible structural phase transitions: at 203 K (Phase I Phase II), 173 K (II III), 134.7 K (III IV), 131.4 K (IV V), and 120 K (V VI). The crystal structures of FA(3)Bi(2)I(9) adopt the polar space groups P6(3)mc at 220 K (Phase I) and Cmc2(1) at 190 K (Phase II). The pyroelectric current (I-pyr) measured in the temperature range covering all the phases of FA(3)Bi(2)I(9) confirmed their polar nature. The reversibility of the spontaneous polarization in each phase has been confirmed by the observed hysteresis loops. All the phase transitions are dielectrically active. The dielectric response (epsilon*(,T) close to 134.7 and 131.4 K) is characteristic of ferroelectrics with a critical slowing-down process. Optical bandgaps estimated from the UV-vis measurements and calculated using the DFT method are equal to 1.85 and 1.89 eV, respectively. On the basis of the structural, dielectric, and spectroscopic results, the molecular mechanisms of the phase transitions have been proposed.
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