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

Polycrystalline samples of perovskite-structured BaCeO3 and SrCeO3 wer e studied by Raman scattering spectroscopy in the temperature range 77 -983 K. Changes in the spectra of BaCeO3 with temperature are explaine d in terms of a second-order phase transition (D2h --> D4h) at T0 = 42 7 K. Analysis of the temperature dependence of soft modes suggests a p artial displacive character. A dramatic decrease in band intensity abo ve 1000 K indicates a transition to the cubic (O(h)) structure, for wh ich no Raman bands are expected. The Raman spectra of SrCeO3 indicate that no phase transitions occur in the temperature range 77-983 K. Roo m temperature spectra of BaCeO3 doped with various rare earth ions (Nd 3+, Gd3+, Yb3+) are compared. Doping with 5 and 10 mol.% Nd stabilizes the high-temperature tetragonal and cubic phases, respectively. The c rystal structure is a sensitive function of Nd dopant concentration.