Molecular reorientation in ortho-carborane studied by dielectric spectroscopy

Abstract

The dielectric properties of ortho-carborane have been investigated in a broad frequency range of 20 Hz≤ν≤1 GHz and at temperatures 10 K≤T≤380 K. At T≳275 K the ortho-carborane molecules undergo fast and isotropic reorientations with frequencies larger than 1 GHz. Below a phase transition at Tc≊275 K the reorientation is partly restricted and the dynamics of the remaining reorientational motion is reduced. At 160 K, the dielectric results reveal a small anomaly which we assign to a second phase transition. At temperatures 150 K<T<275 K the dipolar active orientational degrees of freedom freeze in on the time scale of the measuring frequencies. The corresponding peaks in the frequency dependence of the dielectric loss can well be described using the Fourier transform of the Kohlrausch–Williams–Watts (KWW) function with a stretching parameter β≊0.8. However, for frequencies well above the peak maximum we find significant deviations from KWW behavior. In the temperature and frequency range investigated, the dynamics of the dipolar relaxation process behaves thermally activated with an activation energy of 0.52 eV. In the classification scheme of Angell et al. the fragility of ortho-carborane can be estimated as m≊20 characterizing it as a strong glass former. Ortho-carborane fits well into the correlation scheme of Böhmer et al. where polydispersivity and fragility are related to each other. At high temperatures and low frequencies conductivity processes (hopping) dominate the dielectric response of ortho-carborane.