Features of Ca$_{1-x}$Y$_x$F$_{2+x}$ solid solution heat capacity behavior: diffuse phase transition

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 Cp(T) in the temperature range from the room temperature to 1300$^\circ$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$\pm$50$^\circ$C. With an increase in the content of YF$_3$ ($x$ = 0.05–0.19), a very wide structured peak is recorded in the range of 650–1100$^\circ$C. The heat capacity anomaly is associated with the reversible rearrangement of defect nanoclusters, which affects the change in the anion sublattice.