Abstract
This article summarizes the results of the investigations of the dynamics of ferroelectric (FE) phase transitions (PTs) obtained in Prague during the last 25 years. After a short introduction, explaining differences between displacive and order-disorder types of FE PTs, the results of the broadband dielectric, THz, and mainly IR spectroscopic investigations of hydrogen-bonded FEs, BaTiO3, relaxor FEs, strained incipient FEs, and various multiferroics are reviewed. The high sensitivity of the IR spectroscopy to polar phonons was demonstrated in ultrathin films, which allowed us to reveal strain-induced FE PTs. Electrically active magnons (i.e., electromagnons) can be observed in the IR and Raman spectra of multiferroics. Their frequencies soften on heating toward temperatures of magnetic PTs similarly as phonons in displacive FEs. As expected, the electromagnons can be dependent on the external magnetic field. This behavior has been demonstrated in BiFeO3, SrMn7O12, and multiferroics with Y- and Z-type hexaferrite crystal structures.
Highlights
Let us start first with a short historical remark
In Rochelle salt—the first known FE crystal—it has been shown that this material was for a long time considered a typical example of the system with an order–disorder phase transitions (PTs), two coupled polar phonons show anomalous softening with rising temperature and these phonons are coupled with the MW dielectric relaxation so that this system belongs to the crossover between order–disorder and displacive type of the PT.[28]
The relaxors are cubic in the whole temperature interval, the phonons are split at all temperatures due to the locally broken center of symmetry in polar nanodomains and the lowest frequency phonon undergoes a critical softening toward T∗ = 350 K–400 K, giving evidence about a local PT in polar nanodomains.[103,110]
Summary
The concept of FE soft mode, i.e., one or several polar phonons, whose frequency tends to zero or greatly decrease near the temperature of structural PT Tc, is very old, and it is frequently used for the explanation of the dynamics of PTs. Classical Cochran’s papers[1,2,3] from 1959 to 1961 are usually considered as the first papers about this topic, but Anderson[4] came with the idea of soft mode already in 1958. Most of the experimental works performed in the 1960s refer to soft mode studies of quantum paraelectrics SrTiO3 and KTaO3 probably due to the relatively high soft mode frequency and large temperature changes observed in these materials. We will review results mostly from the last 25 years
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