Hexagonal LaF3:Yb3+/Ln3+ and tetragonal LaOF:Yb3+/Ln3+ (Ln = Ho, Tm, Er) have been successfully prepared via a two-step reaction, which includes a facile aqueous ligand free solution method and the following heat treatment of the as-prepared LaF3 precursor. The phase formation evolution from LaF3 to LaOF with different phase structures was characterized by X-ray diffraction (XRD), scanning electron microscopy, Fourier transform infrared, and Raman spectroscopy. At an annealing temperature of 500 °C pure hexagonal LaF3:Yb3+/Ln3+ (Ln = Ho, Tm, Er) nanoparticles with an average size of 32 nm were obtained and they showed a strong visible upconversion and a modest infrared emission upon 976 nm laser excitation. Further, using an annealing temperature of 900 °C, tetragonal LaOF:Yb3+/Ln3+ (Ln = Ho, Tm, Er) nanoparticles with a size of around 44 nm were obtained (obtained from XRD) and an expressive enhancement in the emission of the VIS and near-infrared regions was observed. These results envision applications that require efficient emissions such as fluorescent and thermal images, and LaF3 nanocrystals have recently been widely explored for applications in biological systems.