Rare-earth nickelates (RNiO3) are an interesting oxide family because of their remarkable and reversible properties related to their structural changes. However, until recently, nickelates were difficult to synthesize without severe or sophisticated conditions. Consequently, a deep understanding of the nucleation and growth process for these versatile perovskites still lacks to date. Here, by correlation of the theory and the experimental data, is presented a clarification of the crystallization mechanism involved for SmNiO3-δ thin films synthesized by a simple route that combines reactive magnetron sputtering and air-annealing. A thermo-kinetic approach to the amorphous-to-crystalline phase transformation is developed after fol- lowing the evolution over time at 475, 500, and 525 °C through in situ high-temperature X-ray diffraction. Then, the kinetic parameters, the optimal temperature, and the necessary activation energy of transformation are determined from the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. Notably, the emergence of contrasting grains created on the free surfaces compared to the nanocrystallites formed from the bulk is revealed from the detailed study at 500 °C by Transmission electron microscopy. As classical nucleation theory outlines, such a growth difference is associated with heterogeneous and homogeneous processes. Furthermore, the progression with the annealing time of the crucial stabilization of the Ni3+ and the electronic structure is analyzed by Electron energy-loss spectroscopy and X-ray photoelectron spectroscopy. Finally, the optical properties measurements demonstrate a metal-insulator transition (MIT) at 125 °C, a thermochromic performance of 32%, and mainly, the crystallization significance to achieve functional nickelates thin films, which pave the way as promising candidates for solar thermal applications