High-temperature diffraction study of LnCoO 3 perovskites: A high-order electronic phase transition

Abstract

A powder X-ray diffraction study of selected orthorhombic LnCoO 3 (Ln = Nd, Gd, Dy, and Ho) perovskites has been performed between 300 and 1098 K. The room-temperature result shows that polyhedral distortion increases with decreasing Ln ionic radius. The distortions for both the Co octahedra and Ln dodecahedra increase from NdCoO 3 to HoCoO 3, and the average Co-O bond length remains unchanged. Although there are no symmetry-related structural changes up to 1098 K, it was observed that, with increasing temperature, the polyhedral distortion first decreases up to T 1, (for Nd T 1 ⩽ 300 K, which increases gradually with Ln atomic number, and for Ho T 1 = 730 K), and then increases to T 2 ( T 2 = 730 K for Nd and ⩾ 1100 K for Ho), after which the distortion decreases again. This change in distortion is also accompanied by a large unit cell volume increase. These observations are attributed to a high-order cobalt electronic phase transition, in which the localized cobalt 3 d electrons in t 2 g and e g orbitals gradually transform within the temperature range between T 1 and T 2 to an itinerant state forming the π ∗5σ ∗13 d electron band with σ ∗ half-filled.