INTRINSIC CHARGE-TRANSFER IN THE MIXED HALIDE PTCLXI1-X LINEAR-CHAIN SOLIDS

In this paper, we report theoretical results on an intrinsic charge transfer effect which possibly occurs in the mixed halide linear chain compound: PtClxI1-x. PtCl and PtI are two of a family of mixed-valence-transition-metal linear chain compounds: [M2(en)4X2]Y4 (M = Pt, Pd, Ni, X = Cl, Br, I, en denotes the ethylenediamine ligand, and Y is a counter-ion (such as ClO4-) to neutralize the chain). The respective pure chains are identified to have charge-density-wave (CDW) ground states, with strong and weak CDW strengths for PtCl and PtI, respectively. Our modeling suggests that when PtI and PtCl are mixed, electrons spontaneously transfer from the PtI segment to the PtCl segment. This charge transfer results in the formation of localized electron bi-polarons in the PtCl segment and of extended hole bi-polarons in the PtI segment. As in classical p - n junctions, the driving force for this phenomenon arises from the fact that the Pt valence band of PtI is higher in energy than the Pt conduction band of PtCl. The energy gain through the charge transfer is large enough to overcome the elastic energy cost from the change of CDW dimerization pattern in both segments. Preliminary experimental result on the crystal structure of PtClxI1-x is consistent with the present theory.