ESR observation of averaged-valence and charge-density-wave states in quasi-one-dimensional halogen-bridged binuclear metal complexes

Electron spin resonance (ESR) measurements have been performed on quasi-one-dimensional iodo-bridged binuclear metal complexes (MMX-chain compounds) A4[Pt2(pop)4I]·nH2O (A=Li, Na, K, Rb, Cs; pop=P2O5H22−). ESR signals show a clear distinction between averaged-valence (AV) and charge-density-wave (CDW) states predicted from optical studies in these complexes. While spin susceptibility χ of the CDW states shows Curie-like behavior, χ of the AV states shows a crossover from Curie to weakly temperature-dependent behavior at high temperatures, characteristic of the Heisenberg antiferromagnetic chains. The Na (n=2) and Cs (n=0) compounds are typical ones exhibiting the characters of the AV and CDW states, respectively. Furthermore, a signal around g=4 is observed, which is more remarkable in the CDW states, suggesting the formation of the state with S=1 or higher spin values as a defect state with a continuous fine-structure parameter distribution.