[en] Crystals of the R_1-ySr_yF_3-y phases (R = La, Ce, Pr, or Nd; 0 ≤ y ≤ 0.16) with tysonite (LaF₃) structure are grown by the Bridgman method. A linear dependence of the crystal density on the SrF₂ concentration has been revealed for each series of solid solutions. A scheme of heterovalent isomorphous substitution in the tysonite structure, R³⁺ + F^1-→ M²⁺ + V_F, with compensation of the difference in the cation charges due to the formation of anion vacancies, is confirmed experimentally. The optical transmission spectra are measured in the wavelength range λ = 2.18-22.22 μm. It is shown that the 50%-transmission edge in the IR range is 10.5 μm for all crystals. The refractive index and the polarizability of crystals decrease with decreasing SrF₂ content according to a linear law. The melting curves of the R_1-ySr_yF_3-y phases studied have a maximum; therefore, these phases can be recommended for use as crystals with a high homogeneity and different optical properties in comparison with the corresponding RF₃ crystals