Electronic absorption spectroscopy of natural Fe

Authors: Michail N Taran Monika KochMüller Klaus Langer

Publish Date: 2005/05/20

Volume: 32, Issue: 3, Pages: 175-188

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Abstract

Natural Fe2+ Fe3+bearing spinel solid solutions from the spinel sshercynite and gahnitehercynite series were analyzed and studied by electronic absorption spectroscopy in the spectral range 30000–3500 cm−1 in the temperature and pressure ranges 77 ≤ TK ≤ 600 and 10−4 ≤ PGPa ≤110 Two crystals were lightviolet in color type I and six green or bluishgreen type II The spectra of both types of spinels are dominated by an UVabsorption edge near 28000 to 24000 cm−1 depending on the iron contents and a very intense band system in the NIR centered around 5000 cm−1 which is caused by spinallowed ddtransition of tetrahedral Fe2+ derived from 5 E→5 T2 The strong band is in all spinels studied split into four subbands which can only be observed in very thin platelets Between the UVedge and the highenergy wing of the NIRband there occur a number of very weak bands in type I spinels while the green type II spinels show some of these with significantly enhanced intensity The intensity of the very weak bands is nearly independent from temperature Such bands are attributed to spinforbidden electronic transitions of IVFe2+ Temperature and pressure dependence of the intensity enhanced bands of spinels type II indicate that they are caused by IVFe2+ and VIFe3+ They are attributed to spinforbidden transitions 6 A 1g → 4 A 1g 4 E g → 4 T 2g and → 4 T 1g of VIFe3+ the two latter being strongly intensified by exchangecoupling interaction with adjacent IVFe2+ The pressure dependence of IVFe2+ ddband system in the NIR caused by spinallowed 5 E→5 T2 transition noticeably differs from that of octahedral Fe2+ an effect which is attributed to a dynamic JahnTeller effect of IVFe2+ in the spinel structureThe German Science Foundation BonnBad Gobesberg generously supported this work through traveling grants to the senior author Two referees A Della Giusta and an anonymous referee gave very reasonable remarks and suggestions which allowed significant improving the paper

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