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HL: Fachverband Halbleiterphysik
HL 37: Oxide Semiconductors
HL 37.1: Vortrag
Donnerstag, 15. März 2018, 09:30–09:45, EW 203
Polaronic entropy stabilizes mixed-valence compound K4O6 — Patrick Merz1, Claudia Felser1, Martin Jansen1,2, •Christoph Freysoldt3, and Jörg Neugebauer3 — 1Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden — 2Max-Planck-Institut für Festkörperforschung,Heisenbergstr. 1, 70569 Stuttgart — 3Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40273 Düsseldorf
Alkali sesquioxides, M4O6, contain peroxide O22− and superoxide O2− ions and are prime examples of correlated open-shell p-electron systems. They show a rich phase diagram arising from the interplay of magnetic, orbital, and charge ordering depending on temperature and the cation size (chemical pressure). We report on the reversible endothermic formation of K4O6 from the separate peroxide and superoxide phases at ≈350∘ C. Rapid cooling leads to a metastable, charge-ordered phase, that decomposes when heated to 100∘ C. In order to better understand thermodynamic stability by means of density-functional theory (DFT) calculations, we have developed a scheme to include Hubbard-U corrections to account for correlations between the molecular sites. The calculations suggest that the stability at elevated temperatures is driven by the polaronic entropy associated with charge hopping between the O2 molecular units. The ionic displacements associated with polaron hopping are complex and large, sometimes >1 Å in distance. Our findings highlight that the commonly used separation between electronic and phononic excitations in finite-temperature materials modelling may miss crucial stabilization mechanisms.