ON ELECTRONIC PHASE TRANSITIONS IN LOWER OXIDES OF VANADIUM

The phase transitions found in the lower oxides of vanadium are discussed in terms of a model recently proposed by Fröhlich, in which Coulomb correlations play a dominant role: below the temperature of the phase transition, Tt, they suppress the band motion of the d electrons in favour of localization, whilst as T -> Tt they cause the thermal ionization of these electrons (out of their localized states into delocalized band states) to become a co-operative process, through screening effects; at Tt, the population of the d band increases catastrophically leading to a first-order phase transition into the metallic state. Experimental evidence, collected from VO, V2O3 and VO2, which tends to support the mechanisms of the model is presented. The VO 2 phase transition is investigated in detail and the following conclusions reached: the crystallographic transformation which occurs at T t(similar, equals 340°K) can be regarded as a consequence of the a priori electronic transition; for T < Tt, crystallographic data together with the observed magnetic behaviour strongly suggest that the d electrons are trapped in homopolar bands between pairs of vanadium ions - accordingly, the system behaves as a low-mobility semiconductor and exhibits Van Vleck temperature-independent paramagnetism; for T > Tt, all the d electrons move with very low mobility (similar 0·1 cm2 v-1 s-1) in a degenerate t2g sub-band, of width about 1 ev, which is split off from the fivefold degenerate d band by the internal crystal field of the anion sublattice - the system is now metallic and the observed magnetic susceptibility attributed to Kubo temperature-independent paramagnetism and spin paramagnetism of the d conduction electrons, which because of the high effective mass (m* similar, equals 40m) of the latter is temperature dependent. Finally, the thermodynamics of the transition is briefly discussed.