Inelastic neutron scattering study of the PTI chain

Summary form only given. [Pt(en)2] [Pt(en)2I2] (ClO4), where en=ethylenediamine (C/sub 2/H/sub 8/N/sub 2/), that crystallizes in the C2/m monoclinic space group (b unique axis), is characterized by a linear sublattice -Pt-I-Pt-I-, parallel to the b crystallographic axis. One dimensional dynamical models have been proposed to describe the longitudinal vibrations of the chain, in particular to help in the analysis of the Resonance Raman spectra where lines, presumably associated with valence defects of platinum, are observed. Up to now, these models were adjusted with only a few vibrational frequencies measured at the Brillouin zone center (Raman and Infrared data), while the whole phonon spectrum w(q) was calculated. Measurement of the low frequency phonon spectrum (ca.0-3THz) by inelastic neutron scattering has been undertaken in order to check for the validity and the limits of the one dimensional description. The experiments have been preformed on a sample composed of three single crystals with total volume 50 mm /sup 3/. The longitudinal acoustic branch along the chain has been identified in spite of the large number of phonon branches in this frequency range. The experimental results, in fairly good agreement with the values calculated in the one-dimensional approximation, will be presented together with a two-dimensional model (taking into account the interaction between the chains and the neighboring ions) that explains the peculiar shape of several other dispersion branches. This experimental study also revealed the existence of forbidden diffraction lines that could indicate the existence of a charge disproportionation (Pt/sup 3+/spl delta/ and (Pt/sub3-/spl delta).