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. 

Third, the maximum in a(T) at a T ax a little above Tc increases dramatically as the tolerance factor t is lowered to the O —O phase boundary appearing in the interval 0.952 < t < 0,957, Comparison of Fig, 25 and Fig. 28 shows a pressure dependence dt/dP > 0, which is anomalous according to Eq. (1) of Goodenough, this volume, and indicates an unusually large compressibility of the (Mn—0) bond. From the virial theorem, we can understand such a large compressibility as an indication of a double-well (Mn—O) potential at a cross-over from localized to itinerant electronic behavior. The large compressibility of the (Mn—O) bond length in the two-phase domain has been corroborated by several experiments,... 

An alternative view is to consider pairing of itinerant vibronic states. The periodicity of the travelling CDW to which the electrons are coupled would introduce a pairing of electrons of momentum k -I- q and -(k -I- q) to open a gap at the Fermi surface that has d-wave symmetry with a maximum in the ( 71, 0) and (0, 7r) directions. This approach, which has been considered by Seibold and Varlamov [320] is more consistent with the massive evidence now available for strong electron-lattice interactions in the copper oxides, as in the perovskites, at the cross-over from localized to itinerant electronic behavior. 

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