Local Structure of Rare Earth Niobates (RE₃NbO₇, RE = Y, Er, Yb, Lu) for Proton Conduction Applications^▴

Abstract

Structural properties of rare earth niobates (RE3NbO7, RE = Y, Er, Yb, Lu) have been studied using selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). SAED patterns show diffuse scattering features along the [111] direction of the pure fluorite (Fm–3m) structure due to a short‐range ordering of anion vacancies in the fluorite structure or the change of coordination number around some cations. The latter is related to the presence of small domains of pyrochlore phase (Fd‐3m), which are observed in the HRTEM images.

Enthalpies of formation of rare earth niobates, RE3NbO7

“…These results reflect the difference in acidity between niobium, zirconium, and tin oxides. From another point of view, the similarity of the values for titanates and niobates reflects both similar acidity of niobia and titania and the existence of the pyrochlore phase microdomains found by SAED and Raman spectroscopy, suggesting some pyrochlore-like local ordering in the niobates (Kovyazina et al 2003;López-Conesa et al 2013).…”

“…Thus finding new stable materials exhibiting high conductivity seems crucial, especially if the materials are ion conductors in the undoped state. Rare-earth niobates, since they are inherently defective materials (López-Conesa et al 2013), are good candidates. From another point of view, Doi et al (2009) found that, depending on the lanthanide, the materials show different magnetic behavior.…”

“…The same group reported that the lanthanide element also influences the symmetry of the compound (Doi et al 2009). Further structural studies were done by high-resolution transmission microscopy (HRTEM) and selected-area electron diffraction (SAED) (López-Conesa et al 2013). They showed that, although niobates containing small rare earth cations exhibit the fluorite Fm3m structure, microdomains of pyrochlore phase (Fd3m) can be found, suggesting considerable ordering at the nanoscale.…”

Potential thermal barrier coating materials: RE3NbO7 (RE=La, Nd, Sm, Eu, Gd, Dy) ceramics

“…The crystal structures of RE 3 NbO 7 derive from the prototype (Fmm) by the following process. Four tetravalent cations (A 4+ ) are displaced by 3 trivalent RE 3+ and one pentavalent Nb 5+ , and one oxygen vacancy is produced in each unit cell to maintain electrical neutrality . The oxygen vacancy concentration is as high as 12.5% (1/8) in RE 3 NbO 7 , which is the strongest phonon scattering center among diverse lattice imperfections.…”

“…Besides RE 2 Zr 2 O 7 , La 2 Ce 2 O 7 and RE 3 TaO 7 , there is another kind of fluorite‐type ceramics, that is, rare‐earth niobates RE 3 NbO 7 . Normally, researchers focus on the crystal structure and dielectric properties of RE 3 NbO 7 , the thermophysical properties such as thermal conductivity and thermal expansion coefficients of these ceramics are restricted …”