D. Havlik

Thermal expansion measurements of C60C70 mixed crystals

Abstract The results of thermal expansion and room temperature powder X-ray diffraction measurements on (C60)1−x(C70)x fullerene alloys are presented. Samples have been prepared by controlled condensation from the vapor phase. Strong anomalies in thermal expansion occur at the fcc to sc phase transition in C60-rich, and hcp to distorted hcp one in C70-rich crystals, respectively. The phase transition temperature diminishes with rising mixture ratio both on the C60-rich ( ∼ −4K 1% C 70 ) and on the C70-rich ( ∼ −20K 1% C 60 ) side of the phase diagram. This suggests the existence of an orientational glass phase on the C70-rich side of the phase diagram (in agreement with thermal expansion measurements on sample with 70% C70, which show no sign of the phase transitions at temperatures above 80 K).

Acoustic phonon dispersion in single-crystal

The longitudinal elastic constants of single crystals were measured in the and directions near by an ultrasonic CW resonance technique in the MHz region. The comparison with previous low-frequency (f = 1 Hz) elastic measurements yields a huge acoustic phonon dispersion near . We show that the dispersion is due to the crossover from isothermal to adiabatic behaviour.

CENTRAL PEAK ANOMALIES IN MACROSCOPIC AND IN SCATTERING EXPERIMENTS DUE TO ENTROPY FLUCTUATIONS

The study of dynamical properties of crystals near structural phase transitions is still of great importance. Especially the interest on ultralow-frequency dynamics, i.e. the so called “Central-Peak question” was renewed nowadays through some recent work on the observation of Entropy Fluctuations in macroscopic susceptibilities. This so called Heat-Diffusion Central Peak was previously found in scattering experiments, and the corresponding theory was originally formulated for scattering experiments. In this work we present an overview on previous light scattering measurements of Entropy Fluctuations and very recent results concerning the observation of a Heat-Diffusion Central Peak in macroscopic properties at ultralow frequencies, which provide a new insight into the “old” Central Peak problem. The crossover functions from isothermal to adiabatic behaviour are calculated for the macroscopic susceptibilities.

Acoustic dispersion near structural phase transitions

Abstract We show on various examples, that elastic constant measurements provide a valuable tool for studying the dynamics of solids near phase transitions. Many experimental methods used in the investigation of phase transitions (NMR, neutron scattering, etc.) are able to give information on the dynamics, but in different regions of q- and ω-space, thereby probing different dynamical processes. A comparison of the dynamics obtained from the elastic measurements with other techniques (NMR) demonstrates this very clearly for the cases of KSCN and C60.

Ultraslow dynamics near structural phase transitions

Abstract The subject of ultralow-frequency dynamics, i.e. the so called “Central-Peak” (CP)-phenomena were intensively investigated 20 years ago. It is well established that this CP includes a whole class of phenomena caused by crystal defects (strains, dislocations, impurities, etc.) or by intrinsic dynamics (precursor cluster, domains, entropy fluctuations, etc.). Here we show on some examples, that one can contribute to this question using low frequency elastic measurements. We present new results on very slow (τ ≈ 0.1s) elastic relaxations near the order-disorder phase transitions of KSCN and C60. The data can be well explained by a model which is based on the assumption of temperature (entropy) fluctuations.

Non-exponential relaxation in macroscopic susceptibilities

Abstract The study of dynamical properties of crystals near structural phase transitions is a very challenging subject. The dynamics may have various origins of intrinsic (moving phase transition clusters, domain walls, molecular reorientations, glass freezing, …) or extrinsic (impurities, dislocations, …) nature. Some of these processes lead to a poly-dispersive behaviour of macroscopic susceptibilities. Frequently the obtained data are fitted by empirical functions, e.g. Cole-Cole-, Cole-Davidson-, Havriliak-Negami-, etc. functions, thus leaving the origin for the observed relaxation unclear. We present two examples of non-exponential relaxations whose origins can be described by simple models: The influence of order parameter fluctuations to the elastic susceptibility yielding a dispersion which resembles a Cole-Davidson-type behaviour and the influence of heat diffusion to macroscopic susceptibilities near phase transitions, respectively.

Heat-diffusion Central Peak in homogeneous and inhomogeneous fields

Abstract Recently performed inhomogeneous stress experiments in KSCN showed for the first time the appearance of a Heat-Diffusion Central Peak in a macroscopic elastic susceptibility. In the meantime we have observed the presence of entropy fluctuations also in other crystals. In all these inhomogeneous (q ≠ 0) measurements the corresponding relaxation function is a Debye one. In contrast, in a usual macroscopic homogeneous field (q = 0) experiment the crossover between the isothermal and the adiabatic limit differs significantly from the Debye behaviour. Here we present Three Point Bending (Hz-region, q = 0) and ultrasonic (MHz-region) measurements, performed recently on EAMC-layered crystals, showing that with elastic constant measurements the dynamics from order parameter fluctuations and entropy fluctuations can be well separated. We outline a phenomenological theory which describes a Heat-Diffusion Central Peak in macroscopic homogeneous (q = 0) and inhomogeneous (q ≠ 0) field experiments near ferroi...

Low frequency elastic behaviour of the ferroelastic phase transition of benzil

Abstract Benzil (C6H5—CO)2 exhibits an order - disorder phase transition at Tc = 83.5 K, which leads the crystal from trigonal (P3121, Z = 3) high temperature phase to a monoclinic (P21, Z = 6) low temperature phase. We present the detailed low frequency (1 Hz) elastic measurements and thermal expansion data from 77K to 90K The thermal expansivity shows only a small jump at Tc in z-direction, whereas it strongly increases perpendicular to the z-axes. Longitudinal elastic compliances in z-direction (S33) and perpendicular to z (S⊥ = S11 for T > Tc) show distinct (and different) anomalies around Tc.