Site inequivalence inFe1+xNb3−xSe10

Abstract

${\mathrm{Fe}}_{1+x}{\mathrm{Nb}}_{3\ensuremath{-}x}{\mathrm{Se}}_{10}$ has a novel structure made up of trigonal prismatic and octahedral coordinated metal atoms. The Se atoms are of two major types, those bound in Se pairs and single Se atoms, and while only Nb cations are found on the trigonal sites and the Fe (along with the remaining Nb) is found on the octahedral sites. To permit the study of the effects of disorder and site inequivalence on the electronic structure of this compound, the x-ray photoelectron spectra of the core and valence electrons were examined. The spectra show two distinct types of Se. The relative intensities of the two Se spectra and comparisons with the spectra of related compounds identify the Se at greater binding energy with the closely spaced Se pairs in the trigonal chains and those at smaller binding energy with the rest of the Se atoms. The spectra of the Nb atoms in the two different sites were not resolved and differ by not more than 0.2 eV in binding energy. However, the Nb $3d$ spectra showed more broadening than in similar compounds, probably due to the Nb site inequivalence and disorder intrinsic to the octahedral chains. All Nb atoms are consequently chemically similar. Chemical shifts indicate an oxidation state for Nb in ${\mathrm{Fe}}_{1+x}{\mathrm{Nb}}_{3\ensuremath{-}x}{\mathrm{Se}}_{10}$ between these of Nb${\mathrm{Se}}_{2}$ and Nb${\mathrm{Se}}_{3}$. The spectra of the Fe atoms were relatively broad, and showed no resolved structure. These results are consistent with a recent semiempirical tight-binding band calculation which suggests that the conduction-electron density is low and resides mainly on the trigonal prismatic chain.