2019, Kriegner, Dominik, Springholz, Gunther, Richter, Carsten, Pilet, Nicolas, Müller, Elisabeth, Capron, Marie, Berger, Helmut, Holý, Václav, Dil, J. Hugo, Krempaský, Juraj

Ferroelectric materials are used in actuators or sensors because of their non-volatile macroscopic electric polarization. GeTe is the simplest known diatomic ferroelectric endowed with exceedingly complex physics related to its crystalline, amorphous, thermoelectric, and—fairly recently discovered—topological properties, making the material potentially interesting for spintronics applications. Typically, ferroelectric materials possess random oriented domains that need poling to achieve macroscopic polarization. By using X-ray absorption fine structure spectroscopy complemented with anomalous diffraction and piezo-response force microscopy, we investigated the bulk ferroelectric structure of GeTe crystals and thin films. Both feature multi-domain structures in the form of oblique domains for films and domain colonies inside crystals. Despite these multi-domain structures which are expected to randomize the polarization direction, our experimental results show that at room temperature there is a preferential ferroelectric order remarkably consistent with theoretical predictions from ideal GeTe crystals. This robust self-poled state has high piezoelectricity and additional poling reveals persistent memory effects. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.

2020, Rabeah, Jabor, Briois, Valérie, Adomeit, Sven, La Fontaine, Camille, Bentrup, Ursula, Breckner, Angelika

Operando EPR, XANES/EXAFS, UV-Vis and ATR-IR spectroscopic methods have been coupled for the first time in the same experimental setup for investigation of unclear mechanistic aspects of selective aerobic oxidation of benzyl alcohol by a Cu/TEMPO catalytic system (TEMPO=2,2,6,6-tetramethylpiperidinyloxyl). By multivariate curve resolution with alternating least-squares fitting (MCR-ALS) of simultaneously recorded XAS and UV-Vis data sets, it was found that an initially formed (bpy)(NMI)CuI- complex (bpy=2,2′-bipyridine, NMI=N-methylimidazole) is converted to two different CuII species, a mononuclear (bpy)(NMI)(CH3CN)CuII-OOH species detectable by EPR and ESI-MS, and an EPR-silent dinuclear (CH3CN)(bpy)(NMI)CuII(μ-OH)2⋅CuII (bpy)(NMI) complex. The latter is cleaved in the further course of reaction into (bpy)(NMI)(HOO)CuII-TEMPO monomers that are also EPR-silent due to dipolar interaction with bound TEMPO. Both Cu monomers and the Cu dimer are catalytically active in the initial phase of the reaction, yet the dimer is definitely not a major active species nor a resting state since it is irreversibly cleaved in the course of the reaction while catalytic activity is maintained. Gradual formation of non-reducible CuII leads to slight deactivation at extended reaction times. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.