Solid phase synthesis of metal-free perovskite crystalline materials

Highlights: • A simple and efficient solid phase method was successfully applied to synthesize metal-free perovskite materials. • Increasing annealing temperature improves the quality for the growth of metal-free perovskite crystalline materials. • Syntheses of two different metal-free perovskite materials were successfully developed using the ball milling method. Metal-free perovskites, a new class of perovskite materials, show attractive piezoelectric, ferroelectric and pyroelectric properties because of their intrinsic structural properties. The introduction of organic components also leads to desirable properties such as structural adjustability, high mechanical flexibility, environmentally friendly processing conditions and low treatment temperatures. However, explorations of new metal-free perovskite materials suffer from the drawbacks of their previous liquid phase synthesis method, such as high time cost and poor experimental reproducibility. Here, we reported the first solid-phase synthesis of metal-free perovskite crystalline materials. Using the optimized ball milling method, we found that the starting materials were fully converted to pure metal-free perovskite DABCO-NH₄Br₃ (DABCO=N,N′-diazabicyclo [2.2.2] octonium) in the size of 5–10μm. This solid phase synthesis strategy was successfully applied to the synthesis of MDABCO-NH₄I₃ (MDABCO=N-methyl-N′-diazabicyclo [2.2.2] octonium), demonstrating the great potential for the preparation and exploration of metal-free perovskite materials via solid phase synthesis.