[en] Cesium lead halide perovskite nanocrystals have been generated a broad range of interest owing to their optical, electronic properties with potential applications in solar cell and other optoelectronic devices. Herein, we report on a facile and solvent-free strategy to synthesize orthorhombic CsPbBr₃ and tetragonal CsPb₂Br₅ nanocrystals by mechanosynthesis of stoichiometric powder mixture of CsBr and PbBr₂ at room temperature. Structure and microstructure characterizations of as prepare powders have been carried out by analyzing the respective XRD patterns employing Rietveld refinement method. Here, we employ water vapor to direct transformation of unstable 3D CsPbBr₃ perovskite structure to air stable 2D CsPb₂Br₅ perovskite-related layered structure under PbBr₂ rich condition. Alternatively, CsPb₂Br₅ was converted to CsPbBr₃ under CsBr rich condition by thermal annealing at 400 °C. Moreover, the as-synthesized CsPbBr₃ and CsPb₂Br₅ nanoparticles exhibit sharp photoluminescence emission at 531nm and 512nm and optical band gap of 2.44eV and 3.05eV respectively as revealed from the UV–V is absorption spectra. In the present study on reversible compositional and structural transitions of cesium lead halide perovskites will further deepen our understanding for their controlled synthesis, post-processing, and decomposition pathway. (author)