AbstractCesium lead halide perovskite (CsPbX₃, X=Cl, Br, I) nanocrystals embedded in Cs₄PbX₆ or CsPb₂X₅ matrices have received interests due to their excellent optical properties. However, their precise endotaxial structures are not known, and the origin of photoluminescence remains controversial. Here we report a sonochemistry technique that allowed us to synthesize high-quality CsPbBr₃-based microcrystals in all ternary phases, simply by adjusting precursor concentrations in a polar aprotic solvent, N,N-dimethylformamide. The microcrystals with diverse morphologies enabled us to visualize the lattice alignments in the dual-phase composites and confirm CsPbBr₃ nanocrystals being the photoluminescent sites. We demonstrate high solid-state quantum yield of >40% in Cs₄PbBr₆/CsPbBr₃ and lasing of CsPbBr₃ microcrystals as small as 2 µm in size. Real-time optical analysis of the reaction solutions provides insights into the formation and phase transformation of different CsPbBr₃-based microcrystals.