Structural, electrical, magnetic, and optical properties of bis-benzene-1,2-dithiolato-Au(IV) crystals.

Abstract

Analysis of x-ray diffraction from a single crystal of bis-benzene-1,2-dithiolato-Au(IV) [Au${(\mathrm{bdt})}_{2}$] at 125 K reveals a superstructure along the stacks of Au${(\mathrm{bdt})}_{2}$ molecules corresponding to a dimerization of the molecules along the stack. Within a dimer, intermolecular sulfur-sulfur contacts are shortened from 3.7 to 3.6 \AA{} whereas a lengthening to about 3.8 \AA{} is found between dimers. Electrical resistivity measured by a four-probe method between 230 and 450 K uncovers an activated resistivity with a characteristic energy of 0.30 eV. The room-temperature conductivity at zero applied pressure is 0.11 ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{m}}^{\ensuremath{-}1}$ rising smoothly to 0.67 ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{m}}^{\ensuremath{-}1}$ at 0.55 GPa isotropic pressure. The magnetic susceptibility $\ensuremath{\chi}$ is low compared to the spin susceptibility of a system with one free spin per molecule. An activated behavior of $\ensuremath{\chi}$ is observed, which gives rise to a monotonic increase in $\ensuremath{\chi}$ at between 275 and 420 K. Reflectivity measurements along the $b$ axis (stacking direction) of a single crystal of Au${(\mathrm{bdt})}_{2}$ shows a transition around 5000 ${\mathrm{cm}}^{\ensuremath{-}1}$ (0.6 eV) possessing an oscillator strength \ensuremath{\approx}0.5 electron/molecule. Along the $c$ axis an absorption centered around 8000 ${\mathrm{cm}}^{\ensuremath{-}1}$ is observed. The first transition (5000 ${\mathrm{cm}}^{\ensuremath{-}1}$) is attributed to an intermolecular charge-transfer process while the latter transition (8000 ${\mathrm{cm}}^{\ensuremath{-}1}$) most likely corresponds to an intramolecular excitation. The physical data presented are discussed in the context of a soft Mott insulator.