XMCD at the Transition Metal K-Edges as a Probe of Small Pressure-Induced Structural Distortions in Prussian Blue Analogues

Prussian blue analogues (PBAs) and derivatives have been extensively studied for the wide variety of their properties (magnetic, optical, electrochemical, thermal, ...), and slight structural distortions have been repeatedly called on in order to explain them. Variable pressure X-ray magnetic circular dichroism (XMCD) at the Ni and Fe K-edges of a series of four NiFe PBAs (CxNi4[Fe(CN)6](8+x)/3·nH2O (x = 0, 1, 2 for C = Cs and x = 2 for C = Rb)) at 10 K over the 0–7 GPa pressure range was used here to get new insights into these slight structural distortions. XMCD signals have been found to exhibit a high sensitivity to slight structural distortions and to offer promising perspectives for their quantifications. Furthermore, XMCD signals at the transition metal K-edges are not yet well understood and PBAs appear as particularly well-suited model compounds in order to disentangle the components at the origin of these XMCD signals. The pressure dependence of the XMCD signals can be qualitatively explained in terms of 3d–4p and 4p–4p orbital overlapping changes and symmetry change of the transition metal sites. Therefore, this work also shed new light on exchange interactions between 3d localized and 4p delocalized electrons as well as on the interpretation of XMCD signals at the TM K-edges.