RAMAN-SCATTERING STUDY OF PHASE-TRANSITIONS IN UNDOPED AND RARE-EARTH ION-DOPED BACEO3 AND SRCEO3

Polycrystalline samples of perovskite-structured BaCeO3 and SrCeO3 were studied by Raman scattering spectroscopy in the temperature range 77-983 K. Changes in the spectra of BaCeO3 with temperature are explained in terms of a second-order phase transition (D2h --> D4h) at T0 = 427 K. Analysis of the temperature dependence of soft modes suggests a partial displacive character. A dramatic decrease in band intensity above 1000 K indicates a transition to the cubic (O(h)) structure, for which no Raman bands are expected. The Raman spectra of SrCeO3 indicate that no phase transitions occur in the temperature range 77-983 K. Room temperature spectra of BaCeO3 doped with various rare earth ions (Nd3+, Gd3+, Yb3+) are compared. Doping with 5 and 10 mol.% Nd stabilizes the high-temperature tetragonal and cubic phases, respectively. The crystal structure is a sensitive function of Nd dopant concentration.