Representative Conducting Oxides

In this chapter, various representative oxides will be discussed in detail to present useful ideas on their electronic transport phenomena. They are representative by virtue of the following characteristic features: ReO3 (Sect. 4.1): The structure is simple cubic and it shows the highest conductivity in the normal oxides. The conduction band is a simple de — O2p type. SnO2 and TiO2 (Sect. 4.2): SnO2 is sometimes called a transparent metal and it is a broad s — p band semiconductor. TiO2 has the same lattice structure but its electron-phonon interaction is large and it is often disputed whether the electrons form large polarons or localized small polarons. LiTi2O4 and LiV2O4 (Sect. 4.3): LiTi2O4 may be considered as a heavily doped TiO2. When the polarons condence in such a substance with a strong electron-phonon interaction, superconductivity appears, and until the discovery of Cu-oxides, its critical temperature of 13.7 K was the highest among the oxides. In metallic LiV2O4, a localized moment appears, in contrast to LiTi2O4. WO3 and M x W03 (Sect.4.4): The carriers may be large polarons in WO3. They are heavily doped in M x WO3 where the M ions distribute randomly and there a metal-insulator transition occurs at certain carrier concentrations. Percolation theory will be useful here. M x V2O5and MMMoO3 (Sect. 4.5): These are low dimensional substances. The former is quasi-one dimensional and the carriers may be small polarons. A bipolaron state has been reported. Mo-bronzes form various low dimensional lattices and charge density waves, CDWs, have been observed. NiO (Sect. 4.6): NiO is an insulator while the simple Hartree—Fock mean field theory predicts that it should be metallic. In this material, the localized nature of the electrons is strong due to the strong electron correlation and many investigations have been carried out to elucidate a “hopping” conduction. However, the nature of the electrons is not yet clear. V2O3 (Sect. 4.7): This shows two metal-insulator transitions. The higher temperature one may be the Mott transition with the metallic phase at the lower temperature side. Below the lower transition temperature, the crystal becomes antiferromagnetic and insulating, accompanied by lattice distortion. Fe3O4 (Sect. 4.8): This is ferrimagnetic below 860 K and shows a transition at 123 K with a jump in the electrical conductivity, which was ascribed to the order-disorder transition of Fe2+ and Fe3+. Many results have been accumulated on the nature of correlated polarons in this fluctuating-valence-material. EuO (Sect. 4.9): This is a ferromagnetic NaC1 type oxide. The reduced material shows the metallic conductivity below the Curie temperature and the conductivity jump there is order 1013. The MIT is due to the magnetic interaction between the localized 4 f magnetic moments and the propagating electrons. High T c Cu-Oxides (Sect. 4.10): These are d γ conductors whereas most of the metallic oxides are de conductors. CuO2 planes constitute a multilayer structure and the superconducting transition temperature increases with the layer number at least up to four and is higher than 120 K.