Polaron liquid-gas crossover at the orthorhombic-rhombohedral transition of manganites

High-resolution synchrotron x-ray powder diffraction in La(0.7)Ca(0.3)MnO(3) shows in detail a first-order structural phase transition from orthorhombic (space-group Pnma) to rhombohedral (space-group R (3) over barc) crystal structures near T(S)=710 K. Magnetic susceptibility measurements show that the rhombohedral phase strictly obeys the Curie-Weiss law as opposed to the orthorhombic phase. A concomitant change in the electrical resistivity behavior, consistent with an alteration from nonadiabatic to adiabatic small polaron hopping regimes, was also observed at T(S). The simultaneous change in transport and magnetic properties are identified as a transition from a correlated polaron liquid for T<T(S) to an uncorrelated polaron gas for T>T(S), driven by the change in the crystal symmetry.

FAPESP[02/01856-1]

FAPESP[05/53241-9]

FAPESP[07/01039-7]

CNPq[303272/2004-0]

CNPq[306496/1988-7]