Analysis of Phase Transition Behavior in BaCeO3-System with X-ray Diffraction and Thermal Analyses

BaCe1-xMxO3-δ (M : trivalent ion such as Y, Yb, Nd and so on) has been studied as a high proton conducting oxide operated at 400~600 oC, which is expected as an electrolyte of solid oxide fuel cells, H2 gas sensor and so on. Crystal structure and structural phase transition between room temperature and high temperature are important information for practical application. By using neutron diffraction measurements at high temperatures, Knight reported that crystal structure of BaCeO3 at room temperature was orthorhombic distorted perovskite with space group of Pnma (No. 62) [1]. He also reported three types of structural phase transitions: from primitive orthorhombic perovskite to body-centered one with space group of Imma (No. 74), from the Imma to rhombohedrally distorted one with space group of R c (No. 167) and from the R c to cubic one with space group of Pm m (No. 227) at 290 oC, 400 oC and 900 oC, respectively. Knight also investigated structural phase transition of BaCe1-xMxO3-δ by neutron diffraction measurements. However, variation of the structural phase transition behavior on x has not been clear since his measurements were not carried out systematically, because of limited number of specimens. Probable reasons of the unsystematic measurements are large quantity of the specimens required for the neutron diffraction measurements, preparation of which is highly laborious and limited experimental period for neutron diffraction measurements. Thermal analyses such as DSC, dilatometry and so on, are regarded as more suitable for systematic investigation of the phase transition behavior since they can be generally carried out with far lower cost and require smaller amount of specimens. However, no reliable work has been reported on thermal analyses on BaCeO3 system. Yamanaka and coworkers reported the phase transitions of BaCeO3 by using DSC and dilatometry [2, 3]. They reported anomalies in DSC curve at 267 oC and 327 oC, ascribed to the phase transition. However, their result is not convincing enough since the observed signals were so broad and no anomaly was observed in their measured thermal expansion behavior. Melekh and coworkers reported two kinds of endothermic peaks at 260 oC and 357 oC in DSC curve of BaCeO3 and attributed them to the structural phase transition [4]. However, it is probable that composition of their sample was not accurate since their preparation method involved melting of the specimen. In this study, DSC curve of BaCeO3 with enough accuracy and resolution for measurement of thermodynamic function of the phase transition has been obtained. Correspondence with the results of X-ray diffraction and dilatometric measurements has also been evaluated. BaCeO3 was prepared by solid state reaction method. Nominal amount of BaCO3 powder (99.9 %, Furuuchi Chemistry Corp.) and CeO2 powder (99.9 %, Furuuchi Chemistry Corp.) was mixed in ethanol with Al2O3 mortar, pressed into cylindrical pellets with diameter and height of 20 mm and 1~3 mm, respectively, and sintered at 1300 oC for 10 h in static air.X-ray diffraction measurements (CuKα: 50 kV, 250 mA, RINT2500, Rigaku Co., Ltd.) revealed that the obtained specimen was single phase of BaCeO3 with primitive orthorhombic distorted perovskite. Fig. 1 shows DSC curves of BaCeO3 in static air measured using DSC8270 (Rigaku Co., Ltd.). In the heating procedure, baseline-shift, endothermic peak and another baseline-shift were observed at 260 oC, 385 oC and 895 oC, respectively. The baseline-shift, exothermic peak and another baseline-shift were detected at 260 oC, 370 oC and 895 oC, respectively, at the cooling process. The baseline-shift with no hysteresis indicates existence of the second-order phase transition and the latent heat around 370~385 oC with hysteresis shows existence of the first-order structural phase transition. By using high temperature XRD measurement, it was revealed that the crystal structure of BaCeO3 changed from primitive orthorhombic perovskite through body-centered one to rhombohedrally distorted one around 280 oC and 400 oC. Around 900 oC, the rhombohedral distorted perovskite changed to cubic one. This shows correspondence with the results of neutron diffraction measurements [1]. In addition, dependence of lattice constants and molar volume on temperature measured with high temperature XRD shows agreement with thermal expansion behavior observed with dilatometry.