DETERMINING THE ELECTRON-PHONON COUPLING WITH RESONANT INELASTIC SOFT X-RAY SCATTERING
The recent availability of high-resolution resonant inelastic soft X-ray scattering (RIXS) offers great potential to directly probe the electron-phonon coupling (EPC). However, the impact of RIXS in phonon physics was limited because its use was not optimised. Here, a new method to determine the EPC with RIXS promises exciting progress.
ELECTRONIC STRUCTURE, MAGNETISM AND DYNAMICS
A novel meta-stable pentavalent plutonium solid phase on the pathway from aqueous Pu(VI) to PuO2 nanoparticles, K.O. Kvashnina (a,b), A.Y. Romanchuk (c), I. Pidchenko (a,b), L. Amidani (a,b), E. Gerber (a,b,c), A. Trigub (d), A. Rossberg (a,b), S. Weiss (b), K. Popa (e), O. Walter (e), R. Caciuffo (e), A. C. Scheinost (a,b), S.M. Butorin (f) and
S.N. Kalmykov (c), Angew. Chem. Int. Ed. 58, 17558-17562 (2019); https://doi. org/10.1002/anie.201911637. (a) ESRF (b) Institute of Resource Ecology, Helmholtz Zentrum Dresden-Rossendorf (HZDR), Dresden (Germany) (c) Department of Chemistry, Lomonosov Moscow State University (Russia)
(d) National Research Centre Kurchatov Institute , Moscow (Russia) (e) Directorate for Nuclear Safety and Security, European Commission, Joint Research Centre, Karlsruhe (Germany) (f) Department of Physics and Astronomy, Molecular and Condensed Matter Physics, Uppsala University (Sweden)
 A.B. Kersting et al., Nature 397, 56-59 (1999).  A.P. Novikov et al., Science 314, 638-641 (2006).  V. Neck et al., C. R. Chim. 10, 959-977 (2007).  E. Gerber et al., Nanoscale 12, 18039-18048 (2020)  A.Y. Romanchuk et al., Dalton Trans. 47, 11239-11244 (2018).
PRINCIPAL PUBLICATION AND AUTHORS
The intermediate Pu(V) phase was characterised for the first time using HERFD at the Pu M4 edge and model calculations in the framework of the Anderson Impurity Model. The Pu M4 HERFD method allows for the unambiguous identification of the Pu oxidation state, demonstrates the existence of Pu(V) and provides quantitative estimates for varying Pu chemical states. The local structure of the intermediate Pu(V) phase, similar to NH4PuO2CO3, was identified by a
combination of the Pu L3 HERFD experiment and ab-initio calculations, and was found to be stable over a period of several months. The redox reactions behind aqueous Pu(VI) PuO2 NPs and formation of Pu(V) affect the substantial increase of the solubility. This finding provides a significant step towards a better understanding of Pu chemistry and emphasises the value of the HERFD technique for studies of PuO2 NPs formation under different solution conditions.
The EPC plays a crucial role in a variety of phenomena such as transport, optical constants and thermal properties. Thus, the measurement of the EPC with energy and momentum resolution is one of the central problems in condensed matter physics. High-energy-resolution RIXS may change the perspectives in phonon physics as the intensity of the lattice excitation reflects the EPC. Figure 90 illustrates the genesis of the phonon signal in the RIXS process. During the lifetime of the intermediate state, the photo- excited electron and core hole modify the charge density and bring the lattice out of equilibrium [1,2]. When the core hole decays, one or more phonons are left behind. Thus, the intensities of the phonon I1 and its overtone (biphonon) I2 increase with the EPC, although I2 with smaller values.
The simplest way to recover the EPC, proposed in , is to consider the intensity ratio I2/I1. This method has been applied to favourable cases, such as molecules  and 1D systems ,
Fig. 90: Schematics of phonon generation during the RIXS process.
The figure gives an artist view of the quadrupolar component of the charge distribution created in the