Abstract
High-resolution synchrotron x-ray powder diffraction in ${\text{La}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_{3}$ shows in detail a first-order structural phase transition from orthorhombic (space-group $Pnma$) to rhombohedral (space-group $R\overline{3}c$) crystal structures near ${T}_{S}=710\text{ }\text{K}$. 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.
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