Spontaneous Ion Migration via Mechanochemical Ultrasonication in Mixed Halide Perovskite Phase Formation: Experimental and Theoretical Insights.

We present a simple yet powerful synthesis process to prepare compound-phase perovskite nanoparticles (MAPbX3-nYn; MA = CH3NH3+ and X/Y = I, Br, or Cl). This is achieved by mixing two pure-phase perovskites (MAPbX3 and MAPbY3) by using ultrasonic vibration as a mechanochemical excitation. Unlike conventional methods, this procedure does not require any effort in designing a reaction or choosing any particular precursor. X-ray diffraction and TEM studies confirm compound-phase formation in all possible stoichiometries. The origin behind ultrasonic mixing lies in the generation of mechanical stress and high temperature arising from acoustic cavitation during reaction. Long-term experimental stability of the compound-phase is comprehended theoretically by simulating the temperature-dependent Gibbs free energy. Negative mixing entropy plays a crucial role during the synthesis which leads to better stabilization of the compound-phase perovskite over the pure-phase. The ease of synthesis and remarkable phase stability make this process effective and less cumbersome for perovskite nanoparticle synthesis.