OXYGEN-ISOTOPE EFFECT ON THE CHARGE-ORDERING TRANSITION OF LA1-XCAXMNO3

The colossal-magnetoresistance compound La1-xCaxMnO3 shows a transitio n to commensurate charge ordering (Mn3+-Mn4+) for x = 50%. In the rang e x = 42-50 %. the high-temperature paramagnetic insulating phase tran sforms fiat into a conducting ferromagnetic phase at the Curie tempera ture T-c, and at somewhat lower temperature T-co into the charge-order ed phase. The oxygen isotope effect on T-c is small in this range, alp ha(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 o f the existence of the charge-ordered phase, O-18 substitution can ind uce this transition in an O-16 sample that does not show this transiti on. The effect can be reversed by back exchange. These results show th e extreme mass dependence of the charge-ordering transition in this sy stem.