Temperature dependence of infrared spectra of [Pd(dddt)2]2AuBr2 and [Pd(dddt)2]2SbF6 salts

Polarised reflectance and transmittance spectra of two isostructural charge-transfer salts [Pd(dddt)2]2AuBr2 and [Pd(dddt)2]2SbF6 (where dddt=5,6-dihydro-1,4-dithiin-2,3-dithiolate ligand) have been measured over the frequency range 650–6500 cm−1 as a function of temperature (90–300 K). Additionally, powder absorption spectra of both compounds have been studied in the region 400–40000 cm−1 at room temperature. Plasma-edge-like dispersion observed in the reflectance spectra is analysed in terms of a Drude–Lorentz model. A metal–insulator phase transition in [Pd(dddt)2]2SbF6 at T≈200 K is observed in the spectra and discussed. Temperature dependence of vibrational bands, especially those related to the coupling of electrons with the totally symmetric vibrations of Pd(dddt)2, is investigated.