| Popis: |
The experimental results on the crystal structure, transport properties, magnetic susceptibility and NMR properties of the mixed oxide V 1− x Nb x O 2 system are discussed in terms of existing theories for VO 2 . At low temperatures and small Nb concentrations, the homopolar bonding picture of Goodenough is found to be a good starting point to account for impurity effects. At high temperatures and small Nb concentrations, the results are not inconsistent with an exchange enhancement picture for the pure metal, or with a narrow band model. At high temperatures and large Nb concentrations, the existence of a magnetic semi-conducting tetragonal phase is suggested to be due to the simultaneous presence of randomness and of electron-electron correlations. It is concluded that both electron-electron interactions and crystal-field effects play a role in explaining the properties of pure VO 2 . |
