Transition from localized to itinerant electronic

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... 

Our canvas here is to provide a qualitative description of current models for the NM-M transition, developed for both metal-ammonia and metal-methylamine solutions. For this purpose we also draw upon interpretations from other systems in which the transition from localized to itinerant electron regimes is well recognized (78). 

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. 

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