Electronic and magnetic properties of iron hydride under pressure: An experimental and computational study using x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Fe K edge

Abstract

The application of a 3.5 GPa pressure on Fe in a H 2 environment leads to the formation of iron hydride FeH. Using a combination of high pressure x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Fe K edge, we have investigated the modification of electronic and magnetic properties induced (i) by the transition from bcc-Fe to dhcp (double hexagonal)-FeH under pressure and (ii) by the compression of FeH up to 28 GPa. XAS and XMCD spectra under pressure have been computed in bcc-Fe and dhcp-FeH within a monoelectronic framework. Our approach is based on a semirelativistic density-functional theory (DFT) calculation of the electron density in the presence of a core hole using plane waves and pseudopotentials. Our method has been successful to reproduce the experimental spectra and to interpret the magnetic and electronic structure of FeH. In addition, we have identified a transition around 28 GPa, which is a purely magnetic transition from a ferromagnetic state to a paramagnetic state.

Novel Fe hydrides and magnetism in the Fe-H system at high pressures

“…If the magnetic moments of the h-sites arrange in an antiferromagnetic order after 30 GPa, their influence on the c-sites would be canceled so that c-sites become paramagnetic. The ferromagnetic-antiferromagnetic transition would explain the absence of the XMCD signal in dhcp-FeH above 30 GPa 16,23 , while the residual magnetic signature will be still consistent with the XES 30 , earlier SMS 20 , and our data ( Fig. 2c, Fig.…”

“…The magnetic transition in dhcp-FeH appears to be more complicated than an ordinary continuous ferromagnetic-paramagnetic transition. On one hand, x-ray magnetic circular dichroism (XMCD) measurements show a decrease of the integrated XMCD intensity and the collapse of the ferromagnetic state above around 30 GPa 16,23 , consistent with Nuclear Forward Scattering (NFS) investigations at room temperature 22,30 . On the other hand, the total local spin properties related to the local magnetic moments probed by Fe K β x-ray emission spectroscopy (XES) gradually decrease with pressure and show an anomaly in relative weights of the ordered magnetic moments only above 43 GPa 30 .…”

“…1), does not show long-range magnetic ordering 21 . Moreover, the data on pressure-induced magnetic transition in dhcp-FeH is currently controversial with a transition pressure ranging from 22 GPa to 65 GPa depending on the experimental technique 16,20,22,23 .…”

A cookbook for the investigation of coordination polymers by transition metal K-edge XMCD

“…All these results were obtained thanks to worldwideavailable beamlines allowing for TM K-edge XMCD measurements, using either a scanning setup (Rogalev & Wilhelm, 2015;Haskel et al, 2007;Nakamura & Suzuki, 2013;Poldi et al, 2020) 1 or a dispersive one, for instance on ODE (Baudelet et al, 2011(Baudelet et al, , 2016 at SOLEIL (France), ID24 ISSN 1600-5775 (Mathon et al, 2004;Pascarelli et al, 2016) at ESRF, EDXAS (Cezar et al, 2010) at LNLS (Brazil), or BL-08 (Patra et al, 2019) at Indus-2 (India). In parallel to these experimental works, several attempts have been made during the last 30 years to understand and disentangle the physical effects originating TM K-edge XMCD (Schü tz et al, 1987;Ebert et al, 1988;Brouder & Hikam, 1991;Guo, 1998;Bouldi et al, 2017Bouldi et al, , 2018 but with little success, especially for coordination polymers such as PBAs.…”

Evolution of electronic and magnetic properties of nominal magnetite nanoparticles at high pressure probed by x-ray absorption and emission techniques

“…As discussed before, if NPs interacted with silicone oil at ambient and high-pressure conditions, 70% of the volume of the NP would be affected, and spectroscopic features in XANES and XMCD spectra are expected to be significantly affected. We compared the evolution of spectral features in XANES spectra under pressure to see if such evolution could indicate formation of Fe 2 SiO 4 [46], FeH [47], goethite (FeO(OH)) [48], hematite(alpha-Fe 2 O 3 ) [49] and wurtzite (FeO) [49] as a result of reaction with silicon oil. We see no evidence for a sizable contribution from any of these compounds to the XANES spectra.…”