Rare-earth nickelates (RNiO₃) are functional oxides with a vast landscape of properties, with the metal-insulator transition (MIT) being the most attractive one. This latter can be modified by varying the rare-earth atomic composition (R’_1-xR_xNiO₃) and the consequent tilt of the crystalline structure. However, obtaining solid solutions of this perovskite family is even more complex due to the specific composition control and the bottleneck that has been their synthesis over the years. Even so, this work aims to customize the thermochromic behavior of the layers to enhance infrared thermal regulation in solar collectors between 60 and 80 °C. Therefore, Nd_1-xSm_xNiO₃ thin films were synthesized using reactive magnetron sputtering and air-annealing. Comprehensive XRD and TEM analysis confirms the efficient synthesis of various crystallized nickelates within the SmNiO₃ to NdNiO₃ range. This methodology facilitates the tunability of MIT temperatures between -80 and 120 °C as Sm atoms progressively replace Nd atoms. Optical performance, assessed through FTIR spectroscopy, aligns with literature-reported MIT temperatures. Additionally, a detailed examination of the structural, electrical, and electronic properties of the Nd_0.2Sm_0.8NiO₃ combination is presented. Lastly, ellipsometry measurements reveal a metal-like to dielectric-like phase change in the imaginary part of the dielectric function with photon energy, while the real part indicates oxidation in the upper film volume. These findings advance the understanding of Nd_1-xSm_xNiO₃ nickelate thin films and their potential applications in thermochromic devices for solar energy utilization.