Moessbauer effect studies on modified VO2

Moessbauer experiments are made on modified VO2 of exactly determined iron concentration with precise structural characterization. The following compositions are investigated: Vsub(1-x)Fesub(x)O2 with x up to 0.022, Vsub(1-x)Fesub(x)Osub(2-y) (x = 0.002 and 0.011), and Vsub(0.99-z)Fesub(0.01)Mosub(z)O2 with z up to 0.04. For the M1 phase of stoichiometric Fe-doped samples only a single doublet with low quadrupole splitting is found. The Moessbauer spectra of the T phase confirm its character as a transitional phase between M1 and M2. The behaviour of the Fe-doped VO2 is discussed with the assumption of Fe3+-V4+ bonds on the paired cation sites. The Moessbauer spectra of the oxygen deficient samples and of the Fe-Mo doped VO2 show a single doublet with larger quadrupole splitting compared with the stoichiometric M1 phase. This result can be accounted for by the effect of the electronic defects present in these samples. (author)