High-temperature order-disorder transition and polaronic conductivity in PrBaCo 2 O 5.48 S. Streule, 1, * A. Podlesnyak, 1 D. Sheptyakov, 1 E. Pomjakushina, 2,1 M. Stingaciu, 2,1 K. Conder, 2 M. Medarde, 2,1 M. V. Patrakeev, 3 I. A. Leonidov, 3 V. L. Kozhevnikov, 3 and J. Mesot 1 1 Laboratory for Neutron Scattering, ETH Zürich & Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 2 Laboratory for Developments and Methods, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 3 Institute of Solid State Chemistry RAS, Ekaterinburg 620219, Russia Received 19 January 2006; published 24 March 2006 Neutron powder diffraction and transport measurements have been used to investigate the PrBaCo 2 O 5.48 compound between room temperature and 820 K. A structural phase transition, involving a rearrangement of oxygen vacancies, was found at T OD = 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 , indicating that the compound is rather semiconducting than metallic as usually accepted. A model in terms of thermally activated hole polaronic hopping is proposed. DOI: 10.1103/PhysRevB.73.094203 PACS numbers: 61.12.Ld, 72.80.Ga Transition metal oxides with perovskite structure display a wide variety of fascinating properties. Among the most prominent examples are superconductivity occurring in cu- prates and colossal magnetoresistivity in manganites. These materials, whose crystal structures consist of simple metal- oxygen planes, show intriguing phase diagrams, which in- clude complicated magnetic and structural phase transitions, and charge and orbital ordering. An additional degree of free- dom is found in Fe and Co ions since, depending on the ratio of the crystal field and intra-atomic exchange energies, they can adopt up to three different spin states. The energy differ- ence between them is small and can be tuned, by changing temperature or pressure. 1–3 The layered cobaltites RBaCo 2 O 5+ R =rare-earth, 0    1 have received par- ticular attention 4–9 due to the discovery of giant magnetore- sistivity in the materials with  = 0.4, R = Er, Gd. 5,10 Materials with  = 0.5 show a change in resistivity in the temperature range 300 K  T  350 K depending on the rare earth R. 5 Although this transition is usually associated to a spin-state change of the Co 3+ ion on the octahedral site, 4–6,10–13 the underlying mechanism for the spin-state crossover is still under debate. At high temperatures cobaltites are furthermore interest- ing for their high ionic conductivity, thus showing potential for applications as gas sensors, oxidation catalysts, or mate- rials for fuel cells. Ionic conductivity in cobaltites was first observed in 1991 by Teraoka 14 in La 0.6 Sr 0.4 Co 0.8 B 0.2 O 3- B =Fe, Co, Ni, Cu and it is believed that oxygen order- disorder phenomena play an important role for its occurrence. 15–19 Since no information exists about the high-temperature structures of layered cobaltites, we decided to investigate PrBaCo 2 O 5.48 by means of neutron powder diffraction NPD. This is a powerful technique for structure determina- tion, which provides reliable infomation on the oxygen ion positions and occupancies. Transport measurements were performed to detect possible changes in the electronic prop- erties of the sample in the investigated temperature range. The PrBaCo 2 O 5+ sample was synthesized by the solid state reaction method and its oxygen content adjusted as de- scribed previously. 20 The oxygen content was determined to be 5.48± 0.01 by iodometric titration. 21 We have checked by thermogravimetry in inert atmosphere, that there is no mass change of the sample up to 825 K. This ensures constant oxygen content within the whole temperature range of our NPD measurements. Phase purity was checked with a con- ventional x-ray powder diffractometer Cu K radiation and differential scanning calorimetry DSC was used to detect possible phase transitions. dc resistivity and thermoelectric power were measured in He atmosphere 300–830 K by the four-probe method on a bar-shaped 2  2  15 mm 3 ceramic sample. 22 Coulometric titration 23 was used to determine the equilibrium oxygen content in PrBaCo 2 O 5+ 0.16    0.76 cobaltites at different values of temperature and partial oygen pressure. The neutron powder diffraction measure- ments were carried out on the high resolution diffractometer HRPT Ref. 24 at SINQ PSI, Switzerland in the angular range 7 deg  2 165 deg and temperature range 300 K  T  820 K, using two wavelengths  = 1.494 and 1.889 Å. The sample holder was made out of steel and tightly sealed under helium atmosphere. Our NPD data were refined using the program FULLPROF. 25 We now turn to the description of our data based on NPD measurements using  =1.494 Å, unless otherwise indi- cated. We find that for T  780 K the crystal structure can be refined using the orthorhombic Pmmm space group. 5,6,26 In this model Fig. 1 the simple cubic perovskite cell is doubled along the c axis due to the ordering of Pr and Ba ions and along the b axis due to ordering of oxygen vacan- cies unit cell a p  2a p  2a p , with a p the cubic lattice con- stant. The oxygen site O4 has been found to be almost empty, while the O3 site is nearly fully occupied, see Table I. This results in planes of CoO 6 octahedra and CoO 5 pyramids parallel to the ac planes, which alternate along the b direc- tion. The oxygen occupancy was refined for the positions O3 and O4 Table I. The anisotropic B factor was restrained to be the same for Co1/Co2, as well as for O1/O2/O3/O4 and O5/O6. Refined structural parameters, occupancies,  2 and PHYSICAL REVIEW B 73, 094203 2006 1098-0121/2006/739/0942035/$23.00 ©2006 The American Physical Society 094203-1