High pressure study on maganites-the double exchange and polaron effects

We have examined the influence of oxygen-isotope on the pressure effects on the metal-insulator (M-I) transition in La 0.65Ca 0.35MnO 3 (A) and on both the M-I and charge ordering (CO) transitions in La 0.57Ca 0.43MnO 3 (B) under hydrostatic pressures. The pressure was found to enhance Tc at different rates, depending on the O-isotopic mass of the sample. The greater dlnTc/dP observed in the O-18 sample could be understood qualitatively in terms of the double exchange and the band reduction effect of polarons. In the B-samples, the situation is much more complex. Pressure enhances Tc linearly at rates independent of the O-isotopic mass but varies Tco nonlinearly in a complex way. The thermal hysteresis of the CO transition diminishes with increase in pressure suggesting the suppression of the first order CO transition under pressure in both isotopes.