Electronic and Structural Properties of the Isolated Chains Confined in the Pores of Mordenite.

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We have studied the electronic and structural properties of one-dimensional chalcogenides confined in the channels of the synthetic mordenite with about 6.7 <ang> diameter. The samples were prepared by exposing Se or Te vapour at 300-500゜C on the dehydrated mordenite powder. EXAFS measurements for the Se in mordenite were performed in National Laboratory for High-Energy Physics. In the channels of mordenite the chalcogen atoms form helical chains and the nearest neighbour distance is shorter than that in the crystalline state. The measurements of photoacoustic spectroscopy (PAS) were carried out to obtain the optical absorption spectra in the temperature range down to the liquid herium temperature. The absorption edge for Se confined in mordenite lies at higher energy than that for the crystalline Se. The above results suggest that the covalent bonds along the Se chain becomes stronger when the coupling between adjucent Se chains is reduced. At low temperatures two types of absorption bands appear around 1 eV and 1.9 eV in the mid-gap on illumination of light with band-gap energy. The ESR data reveal that the 1.9 eV absorption band can be assigned to defect states associated with bond-breaking by the illumination. The optical gap seems to be filled when the <Na^+> ions in the pores of mordenite are exchanged by <Co^(2+)> or <Ni^(2+)> ions. This may suggest that the one-dimensional semiconducting selenium is transformed to a metallic state.