[en] In this work elastic measurements on (NH₄)₄LiH₃(SO₄)₄ (ALHS) which were carried out in the low-frequency range between 1 and 50 Hz are presented. The temperature dependence of the inverse elastic compliance S₁₁^-1 has been determined between 90 K and 420 K. Distinct anomalies have been found in the temperature dependence of S₁₁^-1, which are connected to the motion of domain walls in the ferroelastic phase below T_c=232.5 K. Around T_f=162 K a (partial) ferroelastic 'domain freezing' phenomenon has been observed. To the knowledge of the authors this is the first time that pure ferroelastic domain freezing has been reported. However, below T_f the domain walls seem to retain a certain vibrational degree of freedom which could be responsible for an additional anomaly of the loss modulus which was observed. The elastic behaviour of a crystal of ALHS is dependent on the 'history' of the given sample. During temperature cycling S₁₁^-1 shows differences between the first run of heating and cooling and later runs. Finally, some basic insights concerning the domain wall motion were obtained; it was found that the domain wall mobility decreases by three orders of magnitude in the temperature region 170-230 K. (author)

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[en] Detailed birefringence measurements of hydrogenated and deuterated β-LiNH₄SO₄ in the close vicinity of the high-temperature phase transition (T₁ = 462 K for hydrogenated, T₁ = 465 K for deuterated) are reported. The measurements of the sample were accompanied by simultaneous observation in the polarizing microscope. The temperature dependence of the birefringence data displays an additional anomaly at T₁' approx. 459 K for hydrogenated and T₁' approx. 462 K for deuterated crystals. In the temperature interval between T'₁ and T₁ a regular striped structure was observed in the polarizing microscope. Among several possible interpretations of this phenomenon, the most appealing is the assumption of lamellar coexistence of the paraelectric and ferroelectric phase over a finite temperature interval. (author)

[en] The ferroelectric order and magnetic field induced effects observed in the spiral phase of MnWO₄ are described theoretically. It is demonstrated explicitly that the Dzyaloshinskii-Moriya antisymmetric interactions contribute to the correlation between spins and electric dipoles in the incommensurate and commensurate ferroelectric phases of magnetic multiferroics. However, other single-site symmetric interactions are shown to be involved in the magnetoelectric process, suggesting the possible existence of an electric polarization originating from purely symmetric effects.

[en] The complex elastic constant of sublimation grown C₆₀ single crystals is measured between 100K and 300K in the frequency range 0.6 Hz - 50 Hz. The order disorder phase transition at T_I = 260K shows up in a negative dipanomaly and a peak maximum in the real and imaginary parts of the complex elastic constant, respectively. A very slow relaxation (τ = 0.2s) was measured near T_I. In addition the elastic behaviour around the frequency dependent freezing transition at T_II is measured and described by a single activated relaxation time τ. From our data and the data of previous measurements we find, that the frequency shift of T_II from 180 K to 80 K can be described by a simple Arrhenius behaviour over a frequency range of 11 decades. (orig.)

[en] The properties of materials near structural phase transitions are often successfully described in the framework of Landau theory. While the focus is usually on phase transitions, which are induced by temperature changes approaching a critical temperature T_c, here we will discuss structural phase transformations driven by high hydrostatic pressure, as they are of major importance for understanding processes in the interior of the earth. Since at very high pressures the deformations of a material are generally very large, one needs to apply a fully nonlinear description taking physical as well as geometrical nonlinearities (finite strains) into account. In particular it is necessary to retune conventional Landau theory to describe such phase transitions. In Troester et al (2002 Phys. Rev. Lett. 88 55503) we constructed a Landau-type free energy based on an order parameter part, an order parameter-(finite)strain coupling and a nonlinear elastic term. This model provides an excellent and efficient framework for the systematic study of phase transformations for a wide range of materials up to ultrahigh pressures. We illustrate the model with the specific example of BaCr(Si₄O₁₀), showing that it fully accounts for the elastic softening which is observed near the pressure induced phase transformation

[en] The avalanche statistics in porous materials and ferroelastic domain wall systems has been studied for slowly increasing compressive uniaxial stress with stress rates between 0.2 and 17 kPa s⁻¹. Velocity peaks $v_m=\text{d}h/\text{d}t$ are calculated from the measured strain drops and used to determine the corresponding Energy distributions $N(E\equiv <!--\; ???\; -->v_m^2)$. Power law distributions $N\left(v_m^2\right)\propto <!--\; ???\; -->\left(v_m^2\right){}^{-<!--\; ???\; -->\mathrm{\epsilon <!--\; ??\; -->}}$ have been obtained over 4–6 decades. For most of the porous materials and domain wall systems an exponent $\mathrm{\epsilon <!--\; ??\; -->}=1.5\pm <!--\; ??\; -->0.1$ was obtained in good agreement with mean-field theory of the interface pinning transition. For charcoal, shale and calcareous schist we found significant deviations of the exponents from mean-field values in agreement with recent acoustic emission experiments. (paper)

[en] Although filled skutterudites have a high figure of merit, ZT, it is still an aim to further improve their thermoelectric behavior. One successful route is to homogeneously disperse nanoparticles into the skutterudite matrix and thereby to reduce the lattice thermal conductivity without disturbing the electrical conductivity and the Seebeck coefficient. In this work Ta_0.8Zr_0.2B was dispersed into n-type (Mm,Sm)_yCo₄Sb₁₂ powder by ball-milling or high energy ball-milling prior to hot-pressing. Structural and physical properties, covering also the low temperature range, were studied and compared with the recently investigated impact of nano Ta_0.8Zr_0.2B on p-type skutterudites. ZT was enhanced by 16%. As for thermoelectric applications good mechanical properties are important, we investigated the influence of Ta_0.8Zr_0.2B nanoparticles, added to (Mm,Sm)_yCo₄Sb₁₂, on the hardness, elastic moduli and fracture toughness and found an enhancement of all three mechanical properties. It is well known that p-type skutterudites generally have a higher thermal expansion coefficient than their n-type counterpart. We show that dispersed borides in both n-type (Mm,Sm)_yCo₄Sb₁₂ and p-type DD_0.7Fe₃CoSb₁₂ can level out this discrepancy.