Photoinduced infrared absorption of quasi-one-dimensional halogen-bridged binuclear transition-metal complexes

Abstract

We investigate the optical conductivity of photogenerated solitons in quasi-one-dimensional halogen-bridged binuclear transition-metal (MMX) complexes with particular emphasis on a comparison among the three distinct groups: A 4 [Pt 2 (P 2 O 5 H 2 ) 4 X]$nH 2 O (XZCl, Br, I; AZ Na, K, NH 4 ,.), Pt 2 (RCS 2 ) 4 I (RZC n H 2nC1 ,) and Ni 2 (CH 3 CS 2 ) 4 I which exhibit a mixed-valent ground state with the X sublattice dimerized, that with the M 2 sublattice dimerized and a Mott-Hubbard magnetic ground state, respectively. Soliton-induced absorption spectra for A 4 [Pt 2 (P 2 O 5 H 2 ) 4 X]$nH 2 O should split into two bands, while that for Pt 2 (RCS 2 ) 4 I and Ni 2 (CH 3 CS 2 ) 4 I should consist of a single band. The excitonic effect is significant in Ni 2 (CH 3 CS 2 ) 4 I. q MX chains, that is, a family of quasi-one-dimensional halogen (X)-bridged transition-metal (M) complexes, provide an exciting stage [1-4] performed by electronelectron correlation, electron-lattice interaction, low dimensionality and d-p orbital hybridization. In recent years, binuclear metal analogs which are referred to as MMX chains have stimulated renewed interest [5,6] in this system. The existent MMX compounds consist of three groups: A 4 [Pt 2 (pop) 4 X]$nH 2 O (XZCl, Br, I; AZLi, Cs,.; popZ diphosphonateZP 2 O 5 H 2K 2 ) [7,8], Pt 2 (RCS 2 ) 4 I (RZalkyl chainZC n H 2nCl ) [9,10] and Ni 2 (dta) 4 I (dtaZdithioacetate ZCH 3 CS K 2 ) [11]. A 4 [Pt 2 (pop) 4 X]$nH 2 O resembles conventional MX compounds and generally exhibits a similar ground state of mixed valence with halogen-sublattice dimerization [12,13], which is referred to as the chargedensity-wave (CDW) state. Pt 2 (RCS 2 ) 4 I exhibits a distinct ground state with metal-sublattice dimerization [14], which is referred to as the alternate charge-polarization (ACP) state. Ni 2 (dta) 4 I exhibits the averaged-valence state without any lattice distortion [15] and is regarded as an insulator of the Mott-Hubbard (MH) type. These ground states can be tuned by pressure [16][17][18][19] as well as replacing the bridging halogens [13,20], counter ions [18,21] and ligand molecules [22].In such circumstances, topological excitations [23] such as solitons and polarons have been found for an MMX Hamiltonian of the Su-Schrieffer-Heeger type [24] and an analogy between MMX chains and trans-polyacetylene has been pointed out. The direct Mðd z 2 ÞK Mðd z 2 Þ overlap effectively reduces the on-site Coulomb repulsion and therefore electrons can be more itinerant in MMX chains. In fact MMX chains exhibit much higher room-temperature conductivity than MX chains [14]. Solitons generally have lower formation energies and smaller effective masses than polarons in MMX chains [25]. Then we take more and more interest in MMX solitons as charge or spin carriers. While MMX uniform absorption spectra have recently been investigated [26], but photoinduced ones, which serve as prominent probes for nonlinear excitations, have neither measured nor calculated yet. Thus motivated, we study optical conductivity fo...

Competing Ground States of Metal–Halide Ladders

“…The present AFM phase boundaries should be refined beyond the Hartree-Fock scheme when NiX ladders are synthesized, where quantum correlations are supposed to be significant. 45 Any AFM phase might be stabilized with weak but nonnegligible interactions between neighboring ladders. Figures 3(A) and 3(B) demonstrate the competition between the most likely phases IP-M -CDW and OP-M -CDW on condition that the inter-metal Coulomb interactions are slightly larger than the inter-halogen ones, which is understandable within the naivest consideration of d-electron correlations.…”

“…The present AFM phase boundaries should be refined beyond the Hartree-Fock scheme when NiX ladders are synthesized, where quantum correlations are supposed to be significant. 45 Any AFM phase might be stabilized with weak but nonnegligible interactions between neighboring ladders.…”

Rationalization of the behavior of M2(CH3CS2)4I (M = Ni, Pt) chains at room temperature from periodic density functional theory and ab initio cluster calculations

“…Most of the existing theoretical work on MMX chains deals with the description of the metal‐semiconductor transition, excitation mechanisms, and structural distortions. [53–61] Both the Pt and Ni MMX chains undergo phase transitions with associated changes of conductivity. These changes are strongly dependent on the nature of the bridging ligands that form the M 2 units.…”

“…The electronic state of the chain was assigned basically an AV state. [41–63] However, the electronic structure of the homologous Ni chain seems to be less established. X‐ray photoelectron spectra for the Ni complexes suggest valence delocalization on a regular chain structure.…”

Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds

“…This is not the case with strongly correlated nickel complexes. 48 The following variational calculation is carried out so as to minimize E 1 with respect to δl, ξ, and δn at each d. When we take particular interest in the lth energy surface, E l may be minimized instead. However, the whole energy scheme is not sensitive to the variational target and remains unchanged visually.…”