The crystal structure and lattice dynamics of quantum paraelectric BaxSr1−xTiO3 (x = 0, 0.01, 0.02) solid solutions are studied using X‐ray diffraction (XRD), Raman and terahertz‐infrared (THz‐IR) spectroscopies in a temperature range of 4–300 K. XRD and Raman spectroscopy reveal the cubic‐to‐tetragonal nonpolar structural phase transition at about 100 K. At the same time, Raman spectra manifest the presence of polar modes, TO2 and TO4, normally prohibited in paraelectric phase. Emergence of these modes indicates the appearance of the polar nanoregions in a broad temperature range. The modes become more intensive at low temperatures, and temperature dependence of their intensities on cooling reveals the kink‐like change of the slope from flat to steep, indicating on activation of polar nanoregions. The transmission THz‐IR spectra show that squared frequency of the polar TO1 soft mode, responsible for ferroelectric transition, follows Cochran's behavior at high temperatures. However, at low temperatures, it doesn't vanish at extrapolated Curie temperature but saturates, demonstrating the plateau feature below 20 K. This behavior, coherent with known saturation of the dielectric constant, indicates that transition to ferroelectric phase in BaxSr1−xTiO3 is suppressed by quantum fluctuations and the system stays in quantum paraelectric state at very low temperatures.
A quantum paraelectric SrTiO3 is a material situated in close proximity to a quantum critical point (QCP) of ferroelectric transition in which the critical temperature to the ferroelectric state is suppressed down to 0 K. However, the understanding of the behavior of the phase transition in the vicinity of this point remains challenging. Using the concentration x of Pb in solid solution Sr1−xPbxTiO3 (PSTx) as a tuning parameter and applying the combination of Raman and dielectric spectroscopy methods, we approach the QCP in PSTx and study the interplay of classical and quantum phenomena in the region of criticality. We obtain the critical temperature of PSTx and the evolution of the temperature-dependent dynamical properties of the system as a function of x to reveal the mechanism of the transition. We show that the ferroelectric transition occurs gradually through the emergence of the polar nanoregions inside the non-polar tetragonal phase with their further expansion on cooling. We also study the ferroelastic cubic-to-tetragonal structural transition, occurring at higher temperatures, and show that its properties are almost concentration-independent and not affected by the quantum criticality.
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