Oxygen-isotope effect on the charge-ordering transition ofLa1−xCaxMnO3

The colossal-magnetoresistance compound ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ shows a transition to commensurate charge ordering $\left({\mathrm{Mn}}^{3+}{–\mathrm{Mn}\mathrm{}}^{4+}\right)$ for $x=50\%.\mathrm{}$ In the range $x=42\mathrm{}–50\%,$ the high-temperature paramagnetic insulating phase transforms first into a conducting ferromagnetic phase at the Curie temperature $T_C,$ and at somewhat lower temperature $T_{\mathrm{CO}}$ into the charge-ordered phase. The oxygen isotope effect on $T_C$ is small in this range, ${\alpha }_O=-d\ln T_C/d{\mathrm{lnm}}_O\approx \mathrm{0.14.}$ In contrast to this, the charge-ordering transition shows a large and negative isotope effect, i.e., the heavier mass ${(}^{18}\mathrm{O})$ favors charge ordering. At the border of the existence of the charge-ordered phase, ${}^{18}\mathrm{O}$ substitution can induce this transition in an ${}^{16}\mathrm{O}$ sample that does not show this transition. The effect can be reversed by back exchange. These results show the extreme mass dependence of the charge-ordering transition in this system.