Crystal Structure and the Spectral Response of the Ba‐Doped SrTiO₃ Incipient Ferroelectrics

Abstract

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.

Raman Response of Quantum Critical Ferroelectric Pb-Doped SrTiO3

“…As can be seen in Figure 5, the bending points at the x = 0.005, 0.02, and 0.04 are observed at temperatures of ~90 K, ~75 K, and ~100 K, respectively. As was shown for the Sr 1−x Ba x TiO 3 solid solutions with small Ba content [3], the presence of prohibited lines in the paraelectric phase points out the existence of the polar nanoregions in the system. The abrupt growth in their intensity corresponds to the expansion of their dimensions.…”

“…The appearance of these modes in the Raman spectra of par perovskite materials is usually attributed to the pre-transitional polarization fluc For instance, in the Sr1-xCaxTiO3 solid solutions [11] the appearance of these mod sociated with the ferroelectric phase transition [33]. Additionally, it was shown th pure SrTiO3 ceramics or the BSTx solid solutions, the TO2 and TO4 lines indicate ence of the polar nanoregions originated from the microdeformations between chastically oriented microcrystallites, impurities, or oxygen vacancies [3].…”

“…Critical lattice dynamics reveals similar behavior. It has been observed that, unlike in conventional ferroelectrics, the frequency of the soft mode does not become unstable but also saturates at low temperatures [2,3]. It is widely accepted [1,4,5] that ferroelectric ordering in SrTiO 3 at low temperatures is suppressed by the quantum fluctuations of the polar Ti ions, preventing their stabilization in the off-center potential energy minima positions, hence the formation of the ferroelectric state.…”

Functional properties of Sr1-xGdxTiO3 ceramics synthesized by solid state reaction method

“…[1], showing a cuboid form include submicronsized voids inside the grains in the composition of x = 0.03 and x = 0.11 [1]. The microstructure of prepared compositions showed stable structural composition at very high sintering temperatures [10,24].…”

“…The electrical properties of Perovskite structured ceramic oxide STO can be enhanced by the induction of heavy metal ions on A or B site substitution [16][17][18][19][20]. For example, Sr 2+ is substituted by Ca 2+ , Ba 2+ , Bi 3+ , and Pb 2+ on A-sites, similarly, Al 3+ and Zr 4+ are substituted on B-sites by modifying the dielectric and energy storage properties of STO compound [21][22][23][24][25][26][27][28][29][30][31][32][33].…”