Transition from localized to itinerant electronic behavior

The transition from localized to itinerant electronic behavior occurs where the interatomic interactions become greater than the intraatomic interactions. A measure of the strength of the interatomic interactions is the bandwidth W and of the strength of the intraatomic interactions is the energy C/eff that separates successive redox energies. The transition from localized to itinerant electronic behavior occurs where... 

Of particular interest for the present volume are perovskite-related oxides in which M is a 3d-block transition-metal atom and the A sites are occupied by a lanthanide, yttrium, and/or an alkaline earth. Alkali ions can also be accommodated in the larger A sites, which are coordinated by twelve oxygen near neighbors. With a transition-metal atom M, the thermal expansion of the (A—0) bond is greater than that of the (M—O) bond, so t increases with temperature. Normally the (A—O) bond is also more compressible than the (M—0) bond, which makes t decrease with increasing pressure [4]. However, at a transition from localized to itinerant electronic behavior, an unusually high compressibility of the (M—O) bond results in a dt/dP > 0 (see Eq. (20) below) [5]. 

The ability to adjust to a t < 1 allows for extensive cation substitutions on both the A and M sites the structure is also tolerant of large concentrations of both oxygen and cation vacancies. The perovskites considered in this volume are stoichiometric with MO3 arrays containing a single transition-metal atom M. Emphasis is given to the peculiar physical properties that occur at the transition from localized to itinerant electronic behavior and from Curie-Weiss to Pauli paramagnetism at a Mott-Hubbard transition on the MO3 array. The transition from localized to itinerant electronic behavior can be approached from either the itinerant-electron side or the localized-electron side in single-valent MO3 arrays by isovalent substitutions on the A sites that vary the tolerance factor t. It can also be crossed in mixed-valent... 

A reduced covalent mixing at V(III) relative to V(IV) makes W,j < U in the LnVOa perovskites. However, the t configuration at the V(III) ions of LaV03 appears to be near the transition from localized to itinerant electronic behavior ... 

A transition from localized to itinerant electronic behavior occurs with increasing x in the VO3 array whereas in the TiOs array there is only a crossing of the Mott-Hubbard transition from strongly to weakly correlated electrons. 

A fundamental question is whether the transition between localized and itinerant electronic behavior is continuous or discontinuous. Mott (1949) was the first to point out that an on-site electrostatic energy Ua > Wr, is needed to account for the fact that NiO is an antiferromagnetic insulator rather than a metal. Hubbard (1963) subsequently introduced U formally as a parameter into the Hamiltonian for band electrons his model predicted a smooth transition from a Pauli paramagnetic metal to an antiferromagnetic insulator as the ratio W/U decreased to below a critical value of order unity. This metal-insulator transition is known as the Mott-Hubbard transition. 

LaMni xScx03,1 draw attention to evidence from high-pressure studies that the er-bonding e electrons of LaMnC>3 approach the transition from localized to itinerant behavior. 

---