Oxygen-isotope effect on the charge-ordering transition of La1-xCaxMnO3

The colossal-magnetoresistance compound La1-xCaxMnO3 shows a transition to commensurate charge ordering (Mn3+-Mn4+) for x = 50%. In the range x = 42-50 %. the high-temperature paramagnetic insulating phase transforms fiat into a conducting ferromagnetic phase at the Curie temperature T-c, and at somewhat lower temperature T-co into the charge-ordered phase. The oxygen isotope effect on T-c is small in this range, alpha(0) = -dlnT(c)/dlnm(0) approximate to 0.14. In contrast to this, the charge-ordering transition shows a large and negative isotope effect, i.e., the heavier mass (O-18) favors charge ordering. At the border of the existence of the charge-ordered phase, O-18 substitution can induce this transition in an O-16 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.