Transmission spectra of singe crystals of MAPbI$_3$ hybrid perovskites near the edge of fundamental absorption

The paper presents the transmission spectra of hybrid perovskite MAPbI$_3$ single crystals near the fundamental absorption edge in a wide temperature range. The absorption coefficient $\alpha$ of the single crystal samples is estimated at a temperature $T$ = 150 K for the light with a photon energy $E$ = 1.6 eV and at $T$ = 40 K for $E$ = 1.8 eV. The obtained values turned out to be several orders of magnitude smaller than the values of $\alpha$ for thin-film samples known from the literature. A sharp shift of the fundamental absorption edge by $\sim$ 100 meV was observed at a temperature $T_1$ =160 K of the structural phase transition from the tetragonal to the orthorhombic phase. The temperature hysteresis of the shift of the fundamental absorption edge near $T_1$ was recorded, which is characteristic of a first-order phase transition.