High-temperature order-disorder transition and polaronic conductivity inPrBaCo2O5.48

Abstract

Neutron powder diffraction and transport measurements have been used to investigate the ${\mathrm{PrBaCo}}_2{\mathrm{O}}_{5.48}$ compound between room temperature and $820\mathrm{K}$. A structural phase transition, involving a rearrangement of oxygen vacancies, was found at $T_{OD}=776\mathrm{K}$. Across the transition the perovskite structure loses its vacancy ordering, and the crystal symmetry changes from orthorhombic $Pmmm$ to tetragonal $P4∕mmm$. The resisitivity measurements for temperatures above $\sim 350\mathrm{K}$ yield high values of $\rho$, indicating that the compound is rather semiconducting than metallic as usually accepted. A model in terms of thermally activated hole (polaronic) hopping is proposed.