Synthesis of Two-Dimensional CsPb2X5 (X = Br and I) with a Stable Structure and Tunable Bandgap by CsPbX3 Phase Separation.

ABSTRACT

Perovskite-related materials with various dimensionalities have attracted sustained attention owing to their extraordinary electronic and optoelectronic properties, but it is still challenging in the synthesis of compounds with desired compositions and structures. Herein, a two-dimensional (2D) CsPb2I5 perovskite has been synthesized by the conversion of CsPbI3 at high-pressure and high-temperature (high P-T) conditions, which is quenchable at ambient conditions. In situ synchrotron X-ray diffraction shows that high-pressure monoclinic CsPbI3 converts into tetragonal CsPb2I5 and cubic CsI at 8.7 GPa upon heating from 644 to 666 K. Keeping the tetragonal structure stable, CsPb2I5 exhibits tunable optical properties with the bandgap changing from ∼2.4 eV at ambient pressure to ∼1.4 eV at 36.9 GPa. Further experiments demonstrate similar structural evolution in the typical three-dimensional CsPbBr3 perovskite into 2D CsPb2Br5 at high P-T conditions, indicating that the conversion of CsPbX3 (X = Br and I) into CsPb2X5 is ubiquitous.