153HIGHLIGHTS 2020

Recoil-Induced Asymmetry of the Nondipole Molecular Frame Photoelectron Angular Distributions in the Hard X-ray Regime, M. Kircher (a), J. Rist (a), F. Trinter (b,c), S. Grundmann (a), M. Waitz (a), N. Melzer (a), I. Vela-Perez (a), T. Mletzko (a), A. Pier (a), N. Strenger (a), J. Siebert (a), R. Janssen (a),

L.P. Schmidt (a), A. N. Artemyev (d), M. S. Schöffler (a), T. Jahnke (a), R. Dörner (a) and P.V. Demekhin (d), Phys. Rev. Lett. 123, 243201 (2019); https://doi.org/10.1103/ PhysRevLett.123.243201. (a) Institut für Kernphysik, J. W. Goethe Universität Frankfurt am Main (Germany)

(b) FS-PETRA-S, Deutsches Elektronen- Synchrotron (DESY), Hamburg (Germany) (c) Molecular Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin (Germany) (d) Institut für Physik und CINSaT, Universität Kassel, Kassel (Germany)

SIMILAR STRUCTURAL TRANSITIONS MAY RESULT IN DRASTICALLY DIFFERENT ELECTROCHEMICAL SIGNATURES

The sodium deintercalation mechanism in two cathode materials for Na-ion batteries with a close composition (Na3+xMnxV2-x(PO4)3, x = 0.8 and x = 1.0) was studied using operando synchrotron X-ray powder diffraction (SXRPD). The study revealed how subtle variations in phase transformation behaviour result in drastically different electrochemical patterns of battery materials.

PRINCIPAL PUBLICATION AND AUTHORS

The middle row of Figure 134 shows a different set of calculations. Here, the combined impact of the recoil by the photoelectron and the high- energy photon onto the molecular system was considered. These calculations assume the following underlying kinematic process: The N2 molecule absorbs the photon and a 1s photoelectron is locally emitted at one of the nuclear centres with an angular distribution given by the calculations of Figures 134a-c. The photoelectron and photon recoil are locally imparted onto one nuclear centre. Now, the intermediate excited N2+*(1s)-state decays, emitting an Auger electron. During the decay lifetime of several femtoseconds, and subsequent fragmentation by a Coulomb explosion, the molecule rotates. As a consequence, the measured relative momentum differs from the initial molecular orientation. The induced rotations have a maximum effect for cases where the molecule is oriented perpendicularly to the recoil momentum, while no additional rotation is induced if the molecule is aligned along the recoil direction. This yields a depletion of the

observed break-up directions perpendicular to the recoil momentum.

This interpretation is supported by the data in Figure 135. Here, the upper row shows the data subset of Figure 134h for different kinetic energies (KER) of the N+ ions. As is seen, for higher kinetic energies of the N+ ions, the up-down asymmetry becomes smaller, since the kick becomes less significant in the total momentum configuration. The lower row shows the angular distribution of prel for a fixed electron emission angle. Without recoil, these distributions would be isotropic. However, a clear alignment along a combined recoil by the photoelectron and photon is observed. Furthermore, the alignment becomes more prevalent for smaller energy of the N+ ions. Thus, to accurately describe these emission distributions, one has to take into account the kinetic energy of the ions, the Auger decay time, and the orientation of the combined recoil momentum of the photon and the electron with respect to the molecular axis.

Na-ion batteries have recently regained great interest due to several cost, safety, and maintenance advantages as compared to widely used Li-ion and lead-acid batteries. Among various cathode materials, NASICON- type Na3V2(PO4)3 exhibits extremely long cyclic stability and an outstanding ability to operate at high (dis)charge rates. To improve the energy density of Na3V2(PO4)3, vanadium substitution by cheaper manganese was implemented. A remarkable diversity of redox couples attained

by vanadium (V3+/V4+, V4+/V5+) and manganese (Mn2+/Mn3+, Mn3+/Mn4+) at high operating potentials in NASICON-type compounds makes it feasible to utilise both transition metals in one material to improve its cost-effectiveness.

It is essential that manganese substitution incrementally changes both electrochemical signatures and the phase transformation behaviour of Na3+xMnxV2-x(PO4)3 materials. To link electrochemical features with the phase