HIGH-TEMPERATURE DIFFRACTION STUDY OF LNCOO3 PEROVSKITES - A HIGH-ORDER ELECTRONIC PHASE-TRANSITION

A powder X-ray diffraction study of selected orthorhombic LnCoO3 (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 NdCoO3 to HoCoO3, 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 T1 (for Nd T1 less-than-or-equal-to 300 K, which increases gradually with Ln atomic number, and for Ho T1 = 730 K), and then increases to T2 (T2 = 730 K for Nd and greater-than-or-equal-to 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 3d electrons in t2g and e(g) orbitals gradually transform within the temperature range between T1 and T2 to an itinerant state forming the pi*5-sigma*1(3)d electron band with sigma* half-filled.