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

High-resolution synchrotron x-ray powder diffraction in La0.7Ca0.3MnO3 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.