Far-infrared studies of the phase transitions in beta -quinol-methanol and methanol-D4 clathrates.

Abstract

Low-temperature far-infrared transmission studies of \ensuremath{\beta}-quinol-methanol and \ensuremath{\beta}-quinol-methanol-D4 clathrates have been performed. Far-infrared transmission spectra in the region of 30 to 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at 4.6 K reveal the presence of lattice lines that are not present in \ensuremath{\beta}-quinol clathrates with small guest inclusions and which shift and broaden as the temperature is increased. In order to further investigate these effects, a cw far-infrared laser has been employed to interrogate the behavior of these lattice lines as a function of both temperature and density of filled cages. Utilizing the coincidence between the 54.25-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ emission from a ${\mathrm{CH}}_{2}$${\mathrm{F}}_{2}$ far-infrared laser and a guest-induced 55.2-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ lattice line, we find the absorption coefficient exhibits an abrupt change in slope at the same temperatures at which heat capacity and low-frequency dielectric absorption anomalies have been reported. The temperature at which the absorption coefficient anomaly occurs increases linearly from 22 to 67 K for methanol densities of 40% to 95%, respectively. We believe these effects can be attributed to cooperative behavior due to long-range strain-mediated interactions.