Phase Evolution and Electrochemical Properties of Nanometric Samarium Oxide for Stable Protonic Ceramic Fuel Cells

Electrochemical properties of metal oxide have a strong correlation with the crystalline structures. In this work, the effect of calcination temperature on the phase evolution and electrochemical properties of Sm₂ O₃ was systematically evaluated. The results demonstrate that the sample calcinated at 700 °C (SM-700) is composed of a pure cubic phase while it begins to convert into a monoclinic phase at a temperature above 800 °C and fully converts into a monoclinic phase at 1100 °C. Moreover, the evolution process causes atomic redistribution, and more oxygen vacancies are formed in cubic phase Sm₂ O₃ , contributing to the improved ionic conductivity. The ionic conductivity of 0.138 S cm^-1 and maximum power density of 895 mW cm^-2 at 520 °C are achieved using SM-700 as electrolyte for protonic ceramic fuel cell (PCFC). The cubic structure remains stable in the durability testing process and the SM-700 based fuel cell delivers enhanced stability of 140 mW cm^-2 for 100 h. This research develops a calcination evolution process to improve the ionic conductivity and fuel cell performance of the Sm₂ O₃ electrolyte for stable PCFC.

Structural phase transition, equation of state and phase diagram of functional rare earth sesquioxide ceramics (Eu1-xLax)2O3

The intriguing functional nature of ceramics containing rare earth sesquioxide (RES) is associated with the type of polymorphic structure they crystallize into. They prefer to be in the cubic, monoclinic or hexagonal structure in the increasing order of cation size, R_RE. Since the functional properties of these ceramics varies with R_RE, temperature and pressure, a systematic investigation delineating the cation size effect is indispensable. In the present work we report the structural stability and compressibility behaviour of the RES ceramics, (Eu_1-xLa_x)₂O₃, of RESs with dissimilar structure and significant difference in cationic radii. The selected compositions of (Eu_1-xLa_x)₂O₃ have been studied using the in-situ high pressure synchrotron X-ray diffraction and the structural parameters obtained through Rietveld refinement. The cubic structure, which is stable for 0.95 Å [Formula: see text] R_RE [Formula: see text] Å at ambient temperature and pressure (ATP), prefers a cubic to hexagonal transition at high pressures. The biphasic region of cubic and monoclinic structure, which is stable for 0.98 Å [Formula: see text] R_RE [Formula: see text] Å at ATP, prefers a cubic/monoclinic to hexagonal transition at high pressures. Further, in the biphasic region of monoclinic and hexagonal structure, observed for 1.025 Å [Formula: see text]R_RE [Formula: see text] Å, the monoclinic phase is found to be progressing towards the hexagonal phase with increasing pressure. The pure hexagonal phase obtained for 1.055 Å [Formula: see text] R_RE [Formula: see text] 1.10 Å is found to be structurally stable at high pressures. The bulk moduli are obtained from the Birch-Murnaghan equation of state fit to the compressibility data and its dependance on the cation size is discussed. The microstrain induced by the difference in cation size causes an internal pressure in the crystal structure leading to a reduction in the bulk modulus of [Formula: see text] and 0.6. A pressure-concentration (P-x) phase diagram upto a pressure of 25 GPa is constructed for (Eu_1-xLa_x)₂O₃. This would provide an insight to the fundamental and technological aspects of these materials and the RESs in general.

Microstrain-assisted polymorphic phase transitions in (Eu1−x La x )2O3

Solid solutions, (Eu1−x La x )2O3 (0 ≤ x ≤ 1), of the rare earth sesquioxides Eu2O3 and La2O3 have been prepared by a simple soft chemistry approach. The composition and morphology of the as-synthesized oxides have been characterized using energy-dispersive spectroscopy and scanning electron microscopy. The particles are of irregular shape and submicrometre size. In order to understand the structural evolution as a function of composition, angle-dispersive X-ray diffraction measurements have been carried out and the structural parameters have been obtained through Rietveld refinement. A structural phase transition from the cubic (C-type) to the monoclinic (B-type) structure and subsequently to the hexagonal (A-type) structure was observed with an increasing substitution of La. A detailed analysis of the transition boundaries in terms of the average cationic radius, R RE, shows that the onset of the C → B transition is at R RE = 0.980 Å, whereas the B → A transition occurs at R RE = 1.025 Å. A biphasic region of cubic and monoclinic structures is observed for 0.2 ≤ x ≤ 0.4 and one of monoclinic and hexagonal structures is observed for 0.5 ≤ x ≤ 0.6. The microstrain induced by the difference in size of the rare earth cations introduces a substitutional disorder in the crystal structure, which is a plausible cause of the observed phase transitions in these oxides.

Emerging La2O2CN2 matrix with controllable 3D morphology for photoluminescence applications

Shape-controlled La₂O₂CN₂ microstructures are synthesized and are used as hosts for doping with various RE³⁺ to realize different colored emissions.