FAR-INFRARED SPECTROSCOPY OF HALOGEN-BRIDGED MIXED-VALENCE PLATINUM-CHAIN SOLIDS - ISOTOPE-SUBSTITUTION STUDIES

Far-infrared transmittance measurements are performed on a series of i sotopically labeled samples of the quasi-one-dimensional solids [Pt(en )2][Pt(en2X2][ClO4]4, (en) = ethylenediamine and X = Cl, Br, or 1. Pro nounced shifts upon deuteration of the (en) ligands reveal that severa l absorption features previously attributed to localized vibrational m odes of chain defects are in fact (en) ligand modes. Recognition of th ese ligand modes resolves the previous ambiguity over the assignment o f the infrared-active chain phonons, permitting the unambiguous assign ment of the 238.7-cm-1 feature in the bromide material to the asymmetr ic stretch (v2) chain phonon, and strongly suggesting the 184.2-cm-1 f eature in the iodide is the V2 phonon of that material. A high-resolut ion examination of the chloride, prepared with both natural Cl isotopi c abundance and with nearly pure Cl-35 , reveals a Cl isotopic fine st ructure that allows conclusive identification of the 359.1-cm-1 featur e as the V2 chain phonon. Lattice-dynamics calculations using a harmon ic-linear-chain model with randomly distributed Cl isotopes yield good agreement with experiment and reveal that isotopic disorder leads to pronounced vibrational localization in PtCl, with the observed fine st ructure arising from modes residing on a few distinct sequences of iso topes occurring with high probability. The radical difference between the infrared fine structure of PtCl and that previously reported for t he Raman-active chain mode is found to result from differences in the dispersion curves for the two phonon branches, and allows indirect det ermination of these dispersion curves.