Charge-Density-Wave to Mott−Hubbard Phase Transition in Quasi-One-Dimensional Bromo-Bridged Pd Compounds

A −PdIII−Br−PdIII− Mott−Hubbard state was observed in a quasi-one-dimensional bromo-bridged Pd compound [Pd(en)2Br](C5−Y)2·H2O (en = ethylenediamine, C5−Y = dipentylsulfosuccinate) for the first time. The phase transition between Mott−Hubbard and charge-density-wave states occurred at 206 ± 2 K and was confirmed by using X-ray, ESR, Raman and electronic spectroscopies, electrical resistivity, and heat capacity. From X-ray powder diffraction patterns and Raman spectra of a series of Pd−Br compounds, [Pd(en)2Br](Cn−Y)2·H2O (n = 4, 5, 6, 7, 8, 9, and 12), chemical pressure from the alkyl chains of the counterions caused the phase transition.