RESUMEN
Giant barocaloric effects were recently reported for spin-crossover materials. The volume change in these materials suggests that the transition can be influenced by uniaxial stress, and give rise to giant elastocaloric properties. However, no measurements of the elastocaloric properties in these compounds have been reported so far. Here, we demonstrated the existence of elastocaloric effects associated with the spin-crossover transition. We dissolved particles of ([Fe(L)2](BF4)2, [L=2,6di(pyrazol-1-yl)pyridine]) into a polymeric matrix. We showed that the application of tensile uniaxial stress to a composite film resulted in a significant elastocaloric effect. The elastocaloric effect in this compound required lower applied stress than for other prototype elastocaloric materials. Additionally, this phenomenon occurred for low values of strain, leading to coefficient of performance of the material being one order of magnitude larger than that of other elastocaloric materials. We believe that spin-crossover materials are a good alternative to be implemented in eco-friendly refrigerators based on elastocaloric effects.
RESUMEN
We study the relationship between avalanche criticality and the number of orientational domains in ferroelastic transitions. To this end, we use a general Ginzburg-Landau model appropriate for displacive transitions of the square lattice. The model includes disorder as a quenched distribution of local transition temperatures. We focus on the square-to-rectangle and the square-to-oblique ferroelastic transitions, which have two and four orientational domains, respectively, which in turn determine the corresponding degeneracy of the ground state of the system. The phase transitions are driven by temperature under the assumption of a strict athermal behavior. That is, we assume that thermal fluctuations do not play any role. Numerical results are obtained using a purely relaxational dynamics, and it is shown that both the square-to-rectangle and the square-to-oblique transitions occur intermittently in the form of avalanches. Avalanche sizes and avalanche energies are found to display power-law distributions, which corroborates avalanche criticality. We compare and contrast the dependence of avalanche criticality on the number of orientational domains of the low-symmetry phase. It is found that the critical exponents depend on that number, in agreement with recent experimental results.
RESUMEN
The piezoelectric response of BaTiO(3) is studied in the vicinity of the cubic to tetragonal phase transition, as a function of temperature and the applied electric field in the polar direction. We also investigate the influence of disorder. In the clean limit we obtain the divergence of the piezoelectric tensor at the critical point. The effect of a small amount of disorder is to translate the critical point in the temperature-electric field phase diagram. For large values of the disorder, the paraelectric to ferroelectric phase transition becomes diffuse but a maximum of the piezoelectric tensor is still obtained even though the divergence of the piezoelectric response is lost. These results are in agreement with experimental observations for the relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3). We use a Ginzburg-Landau model which explicitly includes the coupling of the polarization to the strain, the electrostatic interaction between polarizations, and a quenched random compressional stress field generated by point defects. The strain field and its associated elastic energy are written in terms of the stress field and the electric polarization by energy minimization subject to elastic compatibility.
RESUMEN
We study the influence of elastic anisotropy on nanoscale precursor textures that exist in some shape-memory alloys and show that tweed occurs in the limit of high elastic anisotropy while a nanocluster phase-separated state occurs for values of anisotropy inhibiting the formation of martensite. These results are consistent with specific heat data, elastic constant measurements, and zero-field cooling or field cooling experiments in nonstoichiometric NiTi alloys.