Strong Oxygen-Mass Dependence of the Thermal-Expansion Coefficient in the Manganites $({\mathrm{La}}_{1-\mathit{x}}{\mathrm{Ca}}_{\mathit{x}}{)}_{1-\mathit{y}}{\mathrm{Mn}}_{1-\mathit{y}}{O}_3$

Thermal-expansion and magnetic susceptibility measurements were performed on oxygen-isotope substituted manganites $({\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{)}_{1-y}{\mathrm{Mn}}_{1-y}{\mathrm{O}}_3$ with a ${\mathrm{Mn}}^{4+}$ concentration of $\sim 33\%$. The linear thermal-expansion coefficient $\beta \left(T\right)$ exhibits an asymmetric peak at the Curie temperature $T_C$, indicative of a second-order ferromagnetic transition. Upon replacing ${}^{16}\mathrm{O}$ with ${}^{18}\mathrm{O}$, $T_C$ is lowered by about 10 K, while the second-order jump in the thermal-expansion coefficient $\left[\Delta \beta \right(T_C\left)\right]$ is raised by about 20%. Such a “colossal” oxygen-isotope effect on $\Delta \beta \left(T_C\right)$ is very surprising, but can be explained quantitatively on the basis of double exchange and strong polaronic effects.