High-temperature order-disorder transition and polaronic conductivity in PrBaCo2 O5.48

Neutron powder diffraction and transport measurements have been used to investigate the PrBaCo2 O5.48 compound between room temperature and 820 K. A structural phase transition, involving a rearrangement of oxygen vacancies, was found at TOD =776 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 ∼350 K yield high values of ρ, indicating that the compound is rather semiconducting than metallic as usually accepted. A model in terms of thermally activated hole (polaronic) hopping is proposed.