| Research Abstract |
Single crystals of leadhalide perovskite-type compounds (3D : CHィイD23ィエD2NHィイD23ィエD2Pb(ClィイD21-XィエD2BrィイD2XィエD2)ィイD23ィエD2, 2D : (CィイD2mィエD2HィイD22m+1ィエD2NHィイD23ィエD2)ィイD22ィエD2(CHィイD23ィエD2NHィイD23ィエD2)ィイD2n-1ィエD2PbィイD2nィエD2XィイD23n+1ィエD2 (m=4, 6, 8, n=1, 2, X : Cl, I), 1D : CィイD25ィエD2HィイD210ィエD2NHィイD22ィエD2PbXィイD23ィエD2 (X : I, Br), 0D : (CHィイD23ィエD2NHィイD23ィエD2)ィイD24ィエD2PbIィイD26ィエD22HィイD22ィエD2O), in which the connectivity of [PbXィイD26ィエD2]ィイD14-ィエD1 octahedra (X : Cl, I, Br) was varied from 0 to 3 dimension, were prepared by a solution method. The structural dimension dependence of the phase transition was studied systematically (1) through vibrational properties of intramolecular modes, phonon modes, and quasi-elastic modes by Raman scattering and IR measurements, (2) through electronic properties by optical absorption, photoconductivity and photoluminescence measurements, (3) through thermal properties by differential scanning calorimetry measurements. We have found that the successive stru
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ctural phase transitions are governed, in common in these compounds except for the 0D sample, by the rotational and/or orientational ordering of the organic molecules, which induces a change in the electronic state of the [PbXィイD26ィエD2]ィイD14-ィエD1 sublattice. The results suggest that the structural distortion of the [PbXィイD26ィエD2]ィイD14-ィエD1 octahedra sublattice due to the rotational and orientational order/disorder transformation of the organic molecules plays an important role on the change in the electronic states at the structural phase transitions. We have clarified the dynamics and the kinetics of the rotational/orientational transformation of the organic molecules by the analyses of the linewidths of intramolecular modes and quasi-elastic modes. For the 1D sample, we discovered for the first time the successive phase transitions at 248, 253, and 284.5 K and the pressure-induced phase transition at 0.4 GPa due to the rotational/orientational ordering of CィイD25ィエD2HィイD210ィエD2NHィイD22ィエD2. For the 2D sample, we measured the optical absorption and photoluminescence spectra up to 25 GPa, and found the disappearance of the excitonic band around 10 GPa, indicating an electronic transition from a direct to an indirect band structure. Less
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