A series of single crystals of a Ca 1−x Y x F 2+x solid solution with a fluorite structure containing 1-19 mol.% YF 3 (x =0.01-0.19) has been grown. Thermal analyzer STA 449 F3 Jupiter in DSC mode recorded the temperature dependences of the heat capacity C p (T) in the temperature range from the room temperature to 1300 • C. A diffuse phase transition in the solid state for concentrations x =0.01-0.03 is fixed as an anomaly on the C p (T) curves with a maximum at 1150 ± 50 • C. With an increase in the content of YF 3 (x =0.05-0.19), a very wide structured peak is recorded in the range of 650-1100 • C. The heat capacity anomaly is associated with the reversible rearrangement of defect nanoclusters, which affects the change in the anion sublattice. KEYWORDS CaF 2 -YF 3 phase diagram, inorganic fluorides, fluorite, diffuse phase transition, solid solution, heterovalent isomorphism, defect clusters.