ROENTGENOGRAPHIC, ELECTRICAL AND SPECTROSCOPIC INVESTIGATIONS OF THE PALLADIUM CHALCOGENIDES PDS1-XSEX

Silvery reflecting melting cakes of PdS1-xSex (0.0 less-than-or-equal-to x less-than-or-equal-to 1.0) have been prepared by annealing mixtures of Pd, S and Se at 873 K in evacuated quartz tubes. The PdS-type mixed crystals are stable only in the ranges 0.0 less-than-or-equal-to x less-than-or-equal-to 0.2 and 0.9 less-than-or-equal-to x less-than-or-equal-to 1.0, whereas a new phase appears in the intermediate region. The new phase PdS1-xSex (0.2 < x < 0.9) crystallizes in the space group P4(2)/mbc and Z = 24. Lattice constants are a = 1126.2(7), c = 683.2(7) pm (PdS0.6Se0.4) and a = 1130.8(9), c = 685.4(8) pm (PdS0.4Se0.6). The relationships between the new structure type and the PtS and PdS structure types are discussed. The infrared reflection spectra of polycrystalline samples of PdS were measured between 10 and 300 K in the frequency range from 30 to 400 cm-1. Both phonons and plasmons contribute to the spectra. The temperature dependence of the plasmon contribution shows an unusual behaviour which is explained by the assumption of a phonon assisted hopping conductivity mechanism at low temperatures and a Drude-like transport at high temperatures. The normal mode which may be involved in the hopping process is proposed from a normal coordinate calculation using a three-parameter short range valence force field model. The electrical transport data are in general agreement with the optical ones. The infrared spectra of the mixed crystals PdS1-xSex (0.2 < x less-than-or-equal-to 1) are discussed.