Halogen-bridged Mixed-valence Transition-metal Complexes Research Articles

Neutral-Type One-Dimensional Mixed-Valence Halogen-Bridged Platinum Chain Complexes with Large Charge-Transfer Band Gaps.

One-dimensional (1D) electronic systems have attracted significant attention for a long time because of their various physical properties. Among 1D electronic systems, 1D halogen-bridged mixed-valence transition-metal complexes (the so-called MX chains) have been thoroughly studied owing to designable structures and electronic states. Here, we report the syntheses, structures, and electronic properties of three kinds of novel neutral MX-chain complexes. The crystal structures consist of 1D chains of Pt-X repeating units with (1R,2R)-(-)-diaminocychlohexane and CN(-) in-plane ligands. Because of the absence of a counteranion, the neutral MX chains have short interchain distances, so that strong interchain electronic interaction is expected. Resonance Raman spectra and diffuse-reflectance UV-vis spectra indicate that their electronic states are mixed-valence states (charge-density-wave state: Pt(2+)···X-Pt(4+)-X···Pt(2+)···X-Pt(4+)-X···). In addition, the relationship between the intervalence charge-transfer (IVCT) band gap and the degree of distortion of the 1D chain shows that the neutral MX chains have a larger IVCT band gap than that of cationic MX-chain complexes. These results provide new insight into the physical and electronic properties of 1D chain compounds.

Stability of CDW and SDW in halogen-bridged mixed-valence transition-metal compounds

Charge-density-wave (CDW) and spin-density-wave (SDW) are the main ground states of halogen-bridged mixed-valence transition-metal complexes. Their stability was studied in the framework of the tight-binding approximation. It was found that the missing bond-charge repulsion in the extended Hubbard model tends to suppress the CDW. A transition CDW-to-SDW would take place in the case of strong electron–electron (e–e) interactions. In addition, the effect of the dimensions on the CDW and SDW was studied by including the interchain interactions. The calculation showed that interchain interactions intend to suppress the formation of CDW and SDW.

Effect of quantum lattice fluctuations on the optical-absorption spectra of halogen-bridged mixed-valence transition-metal complexes

The effect of quantum lattice fluctuations on the optical-absorption spectra in the ground state of halogen-bridged mixed-valence transition-metal linear-chain complexes is studied by using a one-dimensional extended Peierls–Hubbard model. The nonadiabatic effects due to finite phonon frequency ω π >0 are treated through an energy-dependent electron-phonon scattering function δ( k′, k) introduced by means of an unitary transformation. The calculated optical-absorption spectra do not have the inverse-square-root singularity, but they have a peak above the gap edge and there exists a significant tail below the peak, which are consistent with the optical-absorption coefficient or the optical conductivity measurements of this material.

Control of phonon properties in electron-phonon-proton coupled systems

In halogen-bridged mixed-valence transition-metal complexes, a new theoretical viewpoint is to control the phonon properties by taking into account the cooperative electron-proton coupling. Developing a cluster theory, proton-induced ordering is predicted to occur among one-dimensional charge-density-wave (CDW) chains (in-phase or out-of-phase). It is discussed that the intra-chain CDW state derived from the competition between electron-phonon and electron-electron interactions can be changed by the proton motion on the interchain hydrogen bonds.

Model of electron-proton correlation in quasi-one-dimensional halogen-bridged mixed-valence complexes: Role of proton motion.

The role of interchain hydrogen (H) bonds in one-dimensional electron systems is studied in halogen-bridged mixed-valence transition-metal complexes. A cluster theory is developed to predict the proton-induced ordering among charge-density-wave (CDW) chains in terms of pseudospin formalism for proton motion on H bonds. It is pointed out that the electron-proton coupling can control the CDW state which is produced from the competition between electron-electron and electron-phonon coupling.

Competition between CDW and superconductivity in low-dimensional electron-proton coupled systems

From a new point of view, the competition between CDW and superconductivityis discussed in halogen-bridged mixed-valence transition-metal complexes. The light proton motion on the hydrogen bond between a ligand and a counter anion is introduced in a strongly coupled form with the one-dimensional electron system. It is concluded that protons modulate the electron-electron interaction, and the electron-proton coupling induces a strong attraction to lead superconductivity surpassing the CDW state.

Localized excitations in competing bond-order-wave, charge-density-wave, and spin-density-wave systems

The characteristics of localized excitations in quasi-one-dimensional systems are rather sensitive to the interplay between the electron-phonon (e-ph) and electron-electron (e-e) interactions giving rise to competition and possible coexistence of various symmetry-broken ground states such as the bond-order wave (BOW), the charge-density wave (CDW), and the spin-density wave (SDW). Such effects are observable in halogen-bridged mixed-valence transition-metal complexes, and can be elucidated within the Bogoliubov--de Gennes formalism using an extended Peierls-Hubbard model. The coexistence of local BOW, CDW, and SDW distortions is demonstrated in this paper for polarons and self-trapped excitons (STE) in different symmetry-broken ground states. An extensive study of localized excitations over a wide range of the on-site e-ph coupling ${\ensuremath{\lambda}}_{2}$ and the Hubbard interaction U leads to the following observations. (a) As ${\ensuremath{\lambda}}_{2}$ increases at fixed U, the number of bound states inside the gap changes from two to four for the STE case and from two to three for the polaron case. (b) Upon its further increase, one type of STE with a certain pattern of SDW distortion and charge transfer is transforming into another type of STE with a different pattern. (c) A nonmonotonic dependence of the lattice relaxation energy on ${\ensuremath{\lambda}}_{2}$ is predicted within the lattice relaxation approach developed by Su and Yu earlier, and is attributed to a crossover from the weak-coupling to strong-coupling behavior showing up as the emergence of new bound states inside the gap. Moreover, the nonradiative transition rate of STE is also calculated and is used to tentatively interpret the very short lifetime of STE in PtCl complexes. Such nonmonotonic dependence of the relaxation rate on the coupling constant may also be observed in other quasi-one-dimensional systems as well.

Two-band model for halogen-bridged mixed-valence transition-metal complexes. II. Electron-electron correlations and quantum phonons.

We study the effects of electron-electron interactions in halogen-bridged mixed-valence transition-metal linear-chain complexes ({ital MX} chains) applying a simple 3/4-filled two-band discrete tight-binding (extended) Peierls-Hubbard model with both on-site and intersite electron-phonon couplings. We employ a variety of methods: perturbation theory, Hartree-Fock approximation, and exact diagonalization in the limit of classical adiabatic phonons, and a variational approach allowing for a finite phonon frequency, i.e., quantum phonons and isotope effect. This variety of methods has proved necessary to obtain a complete picture, due to the structural richness of this model. We investigate the competition between the electron-electron and electron-phonon interactions in a wide range of parameter regimes for both ground and excited states. We focus on values relevant to the {ital MX} chains, probing the experimental variation as {ital X} and {ital M} are varied among {ital X}=Cl, Br, and I and {ital M}=Pt, Pd, and Ni (spanning electron-phonon-interaction-dominated to electron-electron-interaction-dominated materials).

Two-band model for halogen-bridged mixed-valence transition-metal complexes. I. Ground state and excitation spectrum.

We consider a 3/4-filled, two-band discrete tight-binding Peierls-Hubbard model for an isolated chain of a halogen-bridged, mixed-valence, transition-metal linear-chain complex (HMMC or {ital MX} chain). We have employed the adiabatic approximation in which the quantum fluctuations associated with phonons are implicitly treated as an external field for the electrons, and treat electron-electron effects in the Hartree-Fock approximation. We investigate ground states as functions of the model parameters and doping-induced and photoinduced excitations---kinks, polarons, bipolarons, and excitons. Results for several experimental observables, including the lattice distortion, the excess charge and spin densities of defects, and the optical absorption, are compiled. For the ground state, we find that the bond-order-wave (BOW) portion of the one-band phase diagram is eliminated from the two-band phase diagram, in agreement with the lack of real materials in the pure BOW phase. The extent of electron-hole asymmetry and of spatial localization or delocalization of defects is explored. Two separate solitons or polarons are compared with corresponding bipolarons. We demonstrate explicitly the need to employ the two-band model for a realistic modeling of the {ital MX} systems, focusing on three specific systems: (a) highly distorted, valence-localized (strongly charge-disproportionated) PtCl, (b) moderately distorted PtBr, and (c) weakly distorted, valence-delocalizedmore » (weak charge-density wave) PtI. The compilation of results reported here constitutes a reference resource against which the rapidly expanding experimental data can be compared.« less