Phase transitions in BaCeO3: neutron diffraction and Raman studies

Polycrystalline samples and small single crystals of the perovskite BaCeO3 were studied by neutron diffraction and Raman spectrometry between 300 and 1200 K. The controversy about the phase transitions originally deduced from our previous Raman study and those observed since by neutron diffraction b y Knight has stimulated this work. Pretransitional effects which are detect ed by Raman much before long-range ordering takes place can partly explain the above disagreement. A continuous monitoring of the structural changes b y neutron diffraction and by Raman spectroscopy including polarization anal ysis has allowed discussion of the transition mechanisms: The first transit ion Pnma-Imma takes place at 573 K and is of second order. Although some mo des soften when the temperature is raised as in many of these perovskite co mpounds the transition is likely partly displacive partly order-disorder. T he Raman modes which disappear transform in modes at the X point of the Bri llouin zone of the Imma phase. The second transition Imma-R (3) over bar c takes place at 673 K and is first order. The last transition R (3) over bar c-Pm3m occurs above 1200 K and the transition temperature which can be ded uced by extrapolation to zero Raman intensity is in good agreement with neu tron results. This second order transition is progressive and begins at abo ut 400 K, the intermediate R (3) over bar c structure appearing as an attem pt for slowing down the structural evolution toward the cubic perovskite fo rm. (C) 1999 Elsevier Science Ltd. All rights reserved.