Both KCaF3 and RbCaF3 are known to undergo ferroelastic phase transitions, the former at temperatures near 550 K and the latter at 195 and 40 K. The two compounds comprise ends of the completely miscible solid solution K1-xRbxCaF3. Simulations of molecular dynamics have been performed for this solid solution, where Rb+ ions were substituted for K+ ions in KCaF3 in varying concentrations of randomly distributed defects representing constitutional crossover to RbCaF3. First-principles calculations were used to obtain the potentials between ion pairs in these samples; the pair potentials were used to calculate the temperature dependence of the interaxis angles which were used as markers for the polymorphic phase transition. The calculated transition temperature thus obtained decreases in a nearly linear fashion as a function of Rb+ ion concentration. Experimental transition temperatures obtained from differential scanning calorimetry of actual samples agree well with the calculated temperatures for those compositions with transitions above room temperature.