Pressure dependences of the metal-insulator transition temperature of La0.7Ca0.3Mn1-x(Fe/Ge)xO3 perovskites

The temperature dependences of the resistivity of La0.7Ca0.3Mn1-xFexO3 and La0.7Ca0.3Mn1-xGexO3 (x = 0, 0.02, 0.04) mixed crystals were studied under hydrostatic pressures up to 15 kbar. The substitution of Fe for Mn results in an increase of resistivity and a decrease of the metal-insulator transition temperature T-m, while the substitution of Ge for Mn leads to a more complicated behaviour: at first, the metal-insulator transition temperature T-m decreases in the range 0 < x(Ge) < 0.02, increases for x(Ge) > 0.02 and decreases again for x(Ge) > 0.04. The maximum of the rho(T) curves T-m shifts to the higher temperatures with a rate of dT(m)/dP = (1.6-29) R/kbar dependent on the T-m value. From the results obtained, a correlation between T-m and its pressure derivative dT(m)/dP could be established: higher dT(m)/dP values correspond to lower T-m's. This result is in accordance with the general trend of dT(m)/dP versus T-m as derived for other manganites and is discussed in terms of a competition between super exchange and double exchange. (C) 2000 Elsevier Science B.V. All rights reserved.