Dynamic hardening of C60:A heat-diffusion central-peak model

Abstract

The elastic constant C of pure solid C 60 , measured at low frequencies with a dynamical mechanical analyzer, exhibits in the vicinity of the order-disorder phase transition at T c ϭ260 K a qualitatively different type of behavior. Besides the expected negative dip, an additional positive peak appears close to T c in the real part of C, resulting in a double anomaly. The experiments show that this hardening is due to a slow relaxational process. Within the macroscopic Landau theory we discuss possible origins of this unusual effect. We present a qualitative description based on the heat-diffusion central-peak model, which describes lowfrequency dynamics near phase transitions. Extending this model with a critically temperature-dependent thermal diffusion time, we can explain the double anomaly by a crossover from isothermal to adiabatic behavior and back. The difference between the two limits arises due to a coupling between the fluctuations in order parameter and in temperature.

Ferroelastic crystals and their nonlinear elastic properties observed in frequency range from gigahertz to milihertz

“…PR Q 5 1, which permitted observation of changes in the sound velocity of the order of 1.4% at T C . The observations of a similar effect were reported in [52].…”

“…where c 0 is the elasticity coefficient at T ) T C and C V is the specific heat at a constant volume [52].…”

“…The analysis of the results of elastic properties' investigation by the low-frequency methods, for example DMA, requires taking into regard the effect of thermal relaxation, which strongly depends on the geometric size of the sample [51][52][53]. As the relative changes in the elastic properties of the RLHS crystal are analysed, it is assumed that the relative change in the storage modulus DE is proportional to the relative change in the elasticity tensor c 0 eff .…”

Pretransitional effect below the ferroelectric-paraelectric phase transition in beta-LiNH4SO4

“…Splitting of phase transition anomalies is sometimes also observed in other systems, and quite often the origin of this behaviour is not clear. For example in fullerene crystals C 60 a double peak behaviour was very recently observed in specific heat [17] and elastic constant [18] measurements. The explanations range from the assumption of an intermediate phase, via the coexistence of two phases over an extended temperature range [17] to the assumption of a slow relaxational process related to heat-diffusion [18].…”

Acoustic phonon dispersion in single-crystal

“…At least two different mechanisms could be responsible for the observed acoustic dispersion: one is connected with the crossover from isothermal to adiabatic elastic behaviour (temperature fluctuations) and the other is due to the order parameter relaxation (the Landau-Khalatnikov mechanism). The corresponding theory was already worked out in [12], and we will here only summarize the main results. To include the effects of heat diffusion we use the following free energy:…”