The phase separation due toA-site-cation size mismatch in La0.5Ca0.5-xBaxMnO3

Abstract

The relationship between electronic and magnetic properties and thelattice effect due to replacement of A-site cations with sizemismatch has been studied in the La0.5Ca0.5-xBaxMnO3system with the amount of Mn4+ fixed at 0.48. Three sorts ofcompound (x = 0, 0.1 and 0.2) have been prepared, and resistivity,magnetic susceptibility, thermopower and x-ray photoelectronspectroscopy (XPS) measurements were carried out. Every compoundexhibits the metal-insulator transition at TMI and aferromagnetic-paramagnetic phase transition at Tc, which isdifferent from TMI. Decrease of Tc and increase ofthe magnetic susceptibility below Tc with increasing x, andcomplex behaviour of the resistivity are observed. It is possible toexplain these results on the basis of the assumption of a phaseseparation into ferromagnetic metal and insulator phases. The XPSmeasurement result indicates that in the x = 0.2 compound, theinsulator phase exists at the temperature at which ferromagnetism isobserved. I find that a good fit of the thermopower data for thex = 0.2 compound is obtained by taking into account contributions ofboth ferromagnetic metal and insulator phases at T<Tc. It issuggested that the lattice effect with large values of not only tbut also σ2 promotes the phase separation into ferromagneticmetal and Jahn-Teller insulator phases at T<Tc.