USE OF L-EDGE X-RAY-ABSORPTION SPECTROSCOPY TO CHARACTERIZE MULTIPLE VALENCE STATES OF 3D TRANSITION-METALS - A NEW PROBE FOR MINERALOGICALAND GEOCHEMICAL RESEARCH
G. Cressey et al., USE OF L-EDGE X-RAY-ABSORPTION SPECTROSCOPY TO CHARACTERIZE MULTIPLE VALENCE STATES OF 3D TRANSITION-METALS - A NEW PROBE FOR MINERALOGICALAND GEOCHEMICAL RESEARCH, Physics and chemistry of minerals, 20(2), 1993, pp. 111-119
2p (L2, 3) X-ray absorption spectra are presented for a range of miner
als to demonstrate the usefulness of L-edge spectroscopy as a symmetry
- and valence-selective probe. 2p XAS provides a sensitive fingerprint
of the electronic states of 3d transition metals and can be applied t
o phases containing mixtures of such elements. Calculated spectra for
3 d(n) --> 2p5 3 d(n+1) transitions provide a basis for the interpreta
tion of the measured spectra. Thus, in principle, multiple valence sta
tes of a particular 3 d metal can be precisely characterized from a si
ngle L-edge spectrum. Examples of vanadium L-edge spectra are presente
d for a range of minerals; these complex spectra hold information conc
erning the presence of vanadium in multiple valence states. The Cu L-e
dge spectrum of sulvanite (Cu3 VS4) indicates the presence of both Cu and Cu2+; the V L-edge spectrum of the same sample shows that both V2
+ and V5+ are present. Spectral simulations representing mixtures of F
e d5 and Fe d6 states are used to quantify Fe3+/SIGMAFe in s spinel, a
glass, and an amphibole, all of which contain Fe as a major component
. To illustrate the sensitivity of 2p XAS in a dilute system, the Fe L
-edge spectrum of amethyst (alpha-SiO2 : Fe) has been recorded; this s
pectrum shows that approximately 68% of the Fe in amethyst is Fe2+, an
d approximately 32% is Fe3+. Although previous studies on amethyst usi
ng other spectroscopic methods cite evidence for Fe4+, there is no ind
ication in the L-edge spectrum for Fe4+ in amethyst. Comparison of the
oretical and experimental spectra not only allows the valence states o
f 3 d ions to be recognised, but also provides site-symmetry informati
on and crystal field parameters for each ion site.