Assisted Mechanisms for Oxygen Ionic conduction in non-Stoichiometric oxides – AMOXIS

The project aims to study the impact of lattice dynamics and oxygen ordering on the amplification of oxygen mobility, at low temperature, in K2NiF4 type oxides. In particular it will allow to strengthen and to extend a new phonon-assisted diffusion mechanism, we recently proposed to understand, at the atomic level, low temperature oxygen mobility in solid oxides. If confirmed, that low temperature oxygen mobility is essentially triggered by lattice instabilities combined with low energy phonon modes, this will have an important impact not only for a fundamental understanding of the diffusion mechanism, but also for the concept to design and to optimize new oxygen ion conductors. It is evident that this will strongly enhance their potential for a variety of technological applications such as oxygen membranes, sensors or catalysts, especially in the low temperature regime.
Combining neutron diffraction, inelastic neutron scattering and ab initio lattice dynamical calculations, we have recently evidenced the importance of soft phonon modes triggering low temperature oxygen mobility in Brownmillerite type (Ca/Sr)FeO2.5 and electrochemically oxidized La2CuO4.07 with K2NiF4 type structure. For both oxide families we equally found complex oxygen ordering during oxygen intercalation reactions. Electrochemically oxidized Pr2NiO4.25 presents in this regard a special showcase, as we were able to characterize its enormous superstructure on high quality single crystals on ID29@ESRF with giant unit cell dimensions yielding a volume of up to 3.000.000Å3. Our motivation here is to identify oxygen ordering and intimately associated formation of soft phonon modes to be a general prerequisite for low temperature oxygen diffusion mechanisms in solid oxides. This new concept will in fine allow to extend and to upgrade the classical Arrhenius Ansatz, yielding a classical description of ion mobility and associated activation energies at sufficiently high temperatures, to become highly anisotropic. This is based on the fact that the low temperature diffusion takes place in the direction of the generally large amplitudes of low energy phonon modes, allowing to establish shallow diffusion pathways.
This project essentially focusses on two non-stoichiometric oxides, i.e. (Nd/Pr)2NiO4+d (under investigations in many EU projects in view of their application at in SOFCs devices), and on the growth of high-quality single crystals for caracterisation by inelastic (neutron and X-ray) scattering.
It will combine in situ studies on single crystals during oxygen intercalation/release in especially designed home made cells to characterize with X-ray (classic an synchrotron) as well as neutron diffraction the structural evolution as a function of the charge transfer. These studies are completed by TEM, lattice dynamical studies by inelastic neutron scattering (INS), inelastic X-ray scattering (IXS), as well as Raman spectroscopy and 17O solid state NMR. This experimental part will be supported by a strong computational work, in which molecular dynamics and first-principles electronic structure calculations, within Density Functional Theory, will be used to investigate the role of oxygen stoichiometry, lattice parameters and ordering patterns on phonons modes, diffusion mechanisms and pathways.
Special attention will be paid to the (Pr/Nd)2-xSrxNiO4+d series, with the aim to understand the role of Sr doping on the strengthening of soft modes. This project should lead to a sharp understanding of the respective (or coupled) contributions of interstitial and apical oxygen atoms to ionic conduction in these promising oxide series
One strong aspect of this project is that it brings together research groups (ICGM, ICMCB and ILL), with a long tradition in nonstoichiometric oxides, covering competencies from the synthesis (incl. single crystal growth) to the characterization of complex crystal structures, lattice dynamics and phase diagrams.