Effect of Phase Transition on the Optical Absorption Edge and Electronic States in Ferroelectric Materials of (CH_<b>3</b>NH_<b>3</b>)_<b>3</b>M_<b>2</b>Br_<b>9</b>(M: Sb, Bi)

Abstract

We have measured the temperature dependence of the absorption spectra in the band-edge region and the reflection spectra of the (CH 3 NH 3 ) 3 M 2 Br 9 (M:Sb,Bi) single crystals exhibiting serveral successive phase transitions. The dispersion-like reflection structure due to the band-edge exciton transition is observed at 2.92 eV and 2.95 eV for (CH 3 NH 3 ) 3 Bi 2 Br 9 and (CH 3 NH 3 ) 3 Sb 2 Br 9 , respectively. The reflection structure remains even at room temperature reflecting a large binding energy. The fundamental absorption edge in both crystals possesses an exponential shape following the empirical Urbach rule. The steepness coefficients characterizing a broadening of the absorption edge exhibit a distinct jump at the phase transition temperature. The results indicate that the ordering of orientational motion of methylammonium cations changes not only the crystal structure and dielectric properties but also the Urbach tail in both crystals.