Analysis of the susceptibility minimum observed in 0.4Ca(NO3)2–0.6KNO3 by dielectric spectroscopy and light scattering

Abstract

An intriguing feature of the relaxation of glass formers is the broad minimum in the dynamic susceptibility spectrum, lying intermediate between the high frequency vibrational absorption (or Boson peak) and the slower, temperature-dependent structural relaxation and diffusion. As first shown by Lunkenheimer et al. [Phys. Rev. Lett. 77, 318 (1996)], high frequency dielectric spectra can be accounted for by invoking a temperature-dependent, frequency-independent contribution (“constant loss”) to the loss spectrum. Herein, we analyze dielectric relaxation and light scattering data for 0.4Ca(NO3)2–0.6KNO3 (CKN), which exhibit broad minima at GHz to THz frequencies. Over a wide temperature range, the spectra can be accurately described by the additive superposition of a constant loss to the structural relaxation and the Boson peak. Moreover, the temperature dependence of the constant loss term is the same for the two spectroscopies. The behavior of the constant loss inferred from this analysis is minimally affected by the details of the fitting procedure, demonstrating the robustness of the method. We also show that dielectric relaxation data for a silver iodide–silver selenate glass conform to the predictions of mode coupling theory, even though a liquid state theory is not obviously applicable to a glassy ionic conductor. This underscores the value of alternative interpretations of the high frequency dynamics in glass-forming liquid, including contributions from a constant loss such as described herein.