METAL-INSULATOR-TRANSITION INDUCED BY OXYGEN-ISOTOPE EXCHANGE IN THE MAGNETORESISTIVE PEROVSKITE MANGANITES

Perovskite manganites derived from LaMnO3 have recently become the sub ject of intensive study following the discovery of 'colossal' magnetor esistance (a magnetically induced change in electrical resistance of u p to several orders of magnitude) in several members of this family of compounds(1). The manganites exhibit a broad range of electronic and magnetic phases, ranging from low-resistance ferromagnetic metals to h igh-resistance insulators, which are extremely sensitive to variation of composition(2), temperature and pressure(3). A recent study showed that such sensitivity also extends to oxygen isotope exchange(4): repl acing O-16 with O-18 in La0.8Ca0.2MnO3 produces an unusually large cha nge in the magnetic properties (a ZI-kelvin decrease in the Curie temp erature). The magnitude of this isotope shift is evidence for the esse ntial role played by electron-phonon coupling(5) in determining the tr ansport properties of these materials, Here we show that this sensitiv ity to oxygen isotope exchange can be even more extreme. In its normal state, the compound La0.175Pr0.525Ca0.3MnO3 undergoes an insulator-to -metal transition as it is cooled below similar to 95 K, But we find t hat, after substituting O-18 for O-16, the compound remains an insulat or down to 4.2 K, so providing a vivid demonstration of the importance of lattice vibrations in these materials.