Low-frequency internal friction anomaly in crystalline methane
Abstract
The temperature dependence of the low-frequency internal friction (LFIF) and the dynamic shear modulus f2 in polycrystalline methane samples is studied over the temperature interval 5–85 K. A sharp LFIF peak and the variation of shear modulus are observed at temperatures of 60–70 K. It is suggested that these anomalies are due to a transition of methane molecules from retarded rotation at T<60 K to a more unhindered motion at T>60–70 K. It is found that an increase in the N2 impurity shifts this transition to higher temperatures. The internal friction peak and the increase in the LFIF background around 20.48 K are attributed to the onset and completion of the β-α phase transition caused by orientational ordering of methane molecules. It is assumed that an increase in the internal friction background below 20 K and the LFIF peak in the temperature interval 10–15 K are due to quantum relaxation processes associated with spin conversion in CH4. A qualitative explanation based on the variation of the dynamic behavior of CH4 molecules is offered for LFIF anomalies.