High-temperature phase transitions in SrBi₂Ta₂O₉film: a study by THz spectroscopy

Abstract

Time-domain THz transmission experiment was performed on SrBi 2 Ta 2 O 9 thick film (deposited on sapphire substrate) at temperatures between 30 and 923 K and complex dielectric spectra of the film were determined. The lowest frequency optic phonon near 28 cm -1 reveals slow monotonous decrease in frequency on heating with no significant anomaly near either the ferroelectric or ferroelastic phase transitions. An additional relaxation mode was resolved in the THz dielectric spectra at all temperatures and its slowing down to ferroelectric phase transition explains dielectric anomaly near T c1 . Second harmonic generation signal was observed in single crystal not only below T c1 but also below T c2 , which confirms the presence of intermediate ferroelastic phase.

Dynamics of the phase transitions in Bi-layered ferroelectrics with Aurivillius structure: Dielectric response in the terahertz spectral range

“…Let us stress another feature: The phonon contribution to the permittivity Ј in the ab plane does not exceed 200, while ab Ј ͑1 MHz͒ reaches ϳ2500 at T c . 29 Clearly, the dielectric strength of polar phonons cannot explain the dielectric anomaly near T c in SBN crystal: similarly as in SBT 23 an additional lower frequency excitation ͑probably with a relaxational character͒ is required.…”

“…An Optistat CF cryostat with thin mylar windows was used for measurements down to 10 K. For sample heating, we used an adapted commercial high-temperature cell ͑SPECAC P/N 5850͒ with 1-mm-thick sapphire windows. 23 The dielectric response of the BBN ceramics was investigated from 10 to 700 K using low-frequency impedance analyzer HP 4192A ͑100 Hz-1 MHz͒ and highfrequency impedance analyzer Agilent 4291B ͑1 MHz-1.8 GHz, 100-500 K͒ with a coaxial measuring cell. The composite dielectric resonator method and network analyzer Agilent E8364B was used at 8.8 GHz and temperatures 100-380 K. 32…”

“…10,[21][22][23] Surprisingly, the soft mode with frequency near 28 cm −1 has shown no anomaly around T c . Only a weak gradual softening of this mode was observed in the THz spectra during heating up to 950 K and, consequently, its contribution to the low frequency permittivity is much smaller than that needed to account for the reported dielectric anomaly near T c = 600 K. An additional relaxation was identified in the THz spectra and it was shown that its dielectric contribution is presumably responsible for the dielectric anomaly near T c .…”

“…Therefore it was proposed that the phase transition in SBT is prevailingly of order-disorder type. 23 Since no other Bi-layered Aurivillius compound was investigated in a similar way, we have performed such kind of studies on several materials, which are summarized, together with the list of their phase transition temperatures and space groups, in Table I. Our experimental results, along with recent first-principle studies, 24-28 enable us to discuss the microscopic origin of the ferroelectric transitions in these compounds.…”

Dielectric Relaxation Phenomenon in Ferroelectric Perovskite-related Structures

“…A relaxor behavior has been reported for these ceramics, suggesting a random electric field as the responsible of the observed behavior. The complex dielectric permittivity spectrum in the THz region was studied in SrBi 2 Ta 2 O 9 films (Kadlec et al, 2004). The lowest-frequency optical phonon revealed a slow monotonic decrease in frequency on heating with no significant anomaly near the phase transitions.…”