OXYGEN ISOTOPE EFFECT ON THE CHARGE-ORDERING TRANSITION OF La1−x CaxMnO3

The CMR compound La1-xCaxMnO3 shows a transition to commensurate charge ordering (Mn3+-Mn4+) for x = 50%. In the range s = 42-50%, the high temperature paramagnetic insulating phase transforms first onto a conducting ferromagnetic phase at the Curie temperature T-c, and at somewhat lower temperatures T-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 ln m(O) approximate to 0.1. In contrast to this, the charge ordering transition showed a large and negative isotope, 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. (C) 1998 Elsevier Science Ltd. All rights reserved.