Structural transitions and conductivity of BaPrO3 and BaPr0.9Y0.1O3-d

Artikel i vetenskaplig tidskrift, 2009

The temperature dependence of the crystal structures of the perovskites BaPrO3 and BaPr0.9Y0.1O3-d has been determined between room temperature and 900 °C using time-of-flight neutron powder diffraction data. The orthorhombic room temperature structures evolve to a primitive cubic cell via three changes of symmetry Pnma -> Imma -> R-3c -> Pm-3m which occur at T~240, ~375, and ~860 °C for BaPrO3. The stability fields of the Pnma and Pm-3m structures are lowered markedly by the presence of yttrium on the B-site. The first and last structural transitions are assigned as continuous events, and analysis of the octahedral tilt angle order parameter suggests the latter is tricritical in nature. The Imma -> R-3c transition is expected to be discontinuous first order and hysteresis effects in dilatometry data support this assignment although no discontinuity is found in the refined structural parameters. Conductivity data collected under dry, oxidising conditions reveal significantly higher conductivity levels in 10% acceptor (Y or Gd) substituted samples compared to undoped BaPrO3. For the acceptor substituted samples electron holes are the dominant charge carriers and the conductivity is weakly influenced by the changing crystal structure. In the undoped BaPrO3, a crossover, probably between extrinsic (impurity-derived) and intrinsic electronic conductivity, coincides with the high temperature rhombohedral to cubic transition.