Symmetry Violations in Partially Oxidized One-Dimensional (1D)Transition Metal Polymers. Metal-Ligand-Metal(M-L-M) Bridged Systems

Abstract

The band structure of the metal-ligand-metal (M-L-M) bridged quasi one-dimensional (ID) cyclopentadienylmanganese polymer, MnCp 1, has been studied in the unoxidized state and in a partly oxidized modification with one electron removed from each second MnCp fragment. The tight-binding approach is based on a semiempirical self-consistent-field (SCF) Hartree-Fock (HF) crystal orbital (CO) model of the INDO-type (intermediate neglect of differential overlap) combined with a statistical averaging procedure which has its origin in the grand canonical ensemble. The latter approximation allows for an efficient investigation of violations of the trans lation symmetries in the oxidized ID material. The oxidation process in 1 is both ligand-and metal-centered (Mn 3d.2 states). The mean-field minimum corresponds to a charge density wave (CDW) solution with inequivalent Mn sites within the employed repeat-units. The symmetry adapted solution with electronically identical 3d centers is a maximum in the variational space. The coupling of this electronic instability to geometrical deformations is also analyzed. The ligand amplitudes encountered in the hole-state wave function prevent extremely large charge separations between the 3d centers which are found in ID systems without bridging moieties (e.g. Ni(CN)g~ chain). The symmetry reduction in oxidized 1 is compared with violations of spatial symmetries in finite transition metal derivatives and simple solids. The stabilization of the valence bond-type (VB) solution is physically rationalized (i.e. left-right correlations between the 3d centers). The computational results derived for 1 are generalized to oxidized transition metal chains with band occupancies that are simple fractions of the number of stacking units and to ID systems that deviate from this relation. The entropy-influence for temperatures 7* + 0 is shortly discussed (stabilization of domain or cluster structures). S y m m e try V io la tio n s in P a r t i a l l y O x id iz e d O n e -D im e n s io n a l ( I D ) T r a n s itio n M e ta l P o ly m e r s . M e ta l-L ig a n d -M e ta l ( M -L -M ) B rid g e d S y s te m s IntroductionThe structural, optical, magnetic, chemical and charge carrier properties of low-dimensional mate rials with organometallic building blocks have been studied extensively in the past decade [1 -5]. A large number of systems show semiconducting or highly conducting properties in solids that crystal lize in form of segregated stacks (i.e. adjacent donor and acceptor strands). This arrangement often leads to an incomplete charge transfer between the two one-dimensional columns. Different types of stack ing patterns are accessible in such systems with 3d, 4d or 5d atoms as central units and a broad spec trum of organic ligands. The transfer channels and the transport mecha nisms of these materials can be modified by changes in the molecular skeletons or counterions. It is pos sible to discriminate two different carrier paths even in the simplest, highly idealized description: a) ID stacks ...

On the Role of the Nonlocal Hartree-Fock (HF) Exchange in Narrow-Band Materials

“…For reasons of computational simplicity we have restricted the tight-binding analysis to non-paramagnetic (unoxidized) states of the complex ID materials; the HF-bands are thus either completely filled or empty. The severe numerical problems already encountered in partially oxidized chains with smaller stacking units have been mentioned in [27,60,61]. An accelerated Hartree damping algorithm of the charge-density-bond-order matrices has been used throughout the iterative SCF steps in order to prevent shell (band) swapping between the occupied and virtual one-particle levels [62].…”