title: Simultaneous First-Order Valence and Oxygen Vacancy Order/Disorder Transitions in (Pr0.85Y0.15)0.7Ca0.3CoO3-δ via Analytical Transmission Electron Microscopy.
abstract: Perovskite cobaltites have been studied for years as some of the few solids to exhibit thermally driven spin-state crossovers. The unanticipated first-order spin and electronic transitions recently discovered in Pr-based cobaltites are notably different from these conventional crossovers, and are understood in terms of a unique valence transition. In essence, the Pr valence is thought to spontaneously shift from 3+ toward 4+ on cooling, driving subsequent transitions in Co valence and electronic/magnetic properties. Here, we apply temperature-dependent transmission electron microscopy and spectroscopy to study this phenomenon, for the first time with atomic spatial resolution, in the prototypical (Pr0.85Y0.15)0.70 Ca0.30CoO3-δ. In addition to the direct spectroscopic observation of charge transfer between Pr and Co at the 165 K transition (on both the Pr and O edges), we also find a simultaneous order/disorder transition associated with O vacancies. Remarkably, the first-order valence change drives a transition between ordered and random O vacancies, at constant O vacancy density, demonstrating reversible crystallization of such vacancies even at cryogenic temperatures.
authors.affiliation: Department of Physics, University of Illinois at Chicago , Chicago, Illinois 60607, United States.
authors.forename: Ahmet
authors.initials: A
authors.lastname: Gulec
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota , Minneapolis, Minnesota 55455, United States.|Materials Science Division, Argonne National Laboratory , Argonne, Illinois 60439, United States.
authors.forename: Daniel
authors.initials: D
authors.lastname: Phelan
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Chris
authors.initials: C
authors.lastname: Leighton
authors.affiliation: Department of Physics, University of Illinois at Chicago , Chicago, Illinois 60607, United States.
authors.forename: Robert F
authors.initials: RF
authors.lastname: Klie
keywords: electron energy loss spectroscopy; oxygen vacancy ordering; perovskite cobaltites; spin-state transitions; transmission electron microscopy