On the electronic phase transitions in the lower oxides of vanadium

Abstract

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, T_t, they suppress the band motion of the d electrons in favour of localization, whilst as T -> T_t 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 T_t, 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 VO₂ 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 < T_t, 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 > T_t, all the d electrons move with very low mobility (similar 01 cm² v^-1 s^-1) in a degenerate t_2g 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.