We have examined the influence of oxygen-isotope on the pressure effects on the metal-insulator (M-I) transition in La0.65Ca0.35MnO3 (A) and on both the M-I and charge ordering (CO) transitions in La0.57Ca0.43MnO3 (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.
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