Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect

TL;DR

This study investigates how electron doping via Nd substitution affects the valence states and site disorder in double perovskite (Sr,Nd)2FeMoO6, revealing that doping mainly influences antisite Mo atoms rather than Fe valence.

Contribution

It provides new insights into the valence state changes and disorder mechanisms in electron-doped double perovskites using Mossbauer spectroscopy.

Findings

Nd doping increases antisite disorder in the structure.

Fe valence remains mostly unchanged with slight shifts towards divalency.

Electron doping primarily affects antisite Mo atoms, favoring lower valence states.

Abstract

Double perovskite, (Sr1-xNdx)2FeMoO6, was doped with electrons through partial substitution of divalent Sr by trivalent Nd (0 < x < 0.2). The Fe valence and the degree of B-site order were probed by 57Fe Mossbauer spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms occupying wrong sites, i.e. antisite disorder. It had very little effect on the Fe valence: a small but visible increase in the isomer shift was seen for the mixed-valent FeII/III atoms occupying the right site indicating a slight movement towards divalency of these atoms, which was more than counterbalanced by the increase in the fraction of antisite Fe atoms with III valence state. It is therefore argued that the bulk of the electron doping is received by antisite Mo atoms, which - being surrounded by six MoV/VI atoms - prefer the lower IV/V valence state. Thus under Nd substitution, the charge-neutrality requirement inflicts a lattice disorder such that low-valent MoIV/V can exist.