Dynamical mechanism of the ferroelectric phase transition ofSrTi18O3studied by light scattering

Abstract

A light scattering experiment has been performed on two SrTi( 18 O x 16 O 1Ϫx ) 3 single crystal plates with x ϭ0.86 ͑STO18-86͒ and xϭ0.53 ͑STO18-53͒. Each of the c 44 pure transverse acoustic mode and the antiferrodistortive E g mode splits into two modes below T c ; T c ϭ27Ϯ1 K for STO18-86 and T c ϭ20Ϯ2 K for STO18-53. These splits strongly imply that the crystal symmetry becomes lower than tetragonal one below T c , and that the phase transition really occurs at T c . Moreover, the ferroelectric E u soft mode is found above T c , but its softening is not complete. This fact indicates that the phase transition mechanism of SrTi( 18 O x 16 O 1Ϫx ) 3 is of displacive type ͑the instability of the E u mode causes the ferroelectric phase transition͒, but is not the ideal one. The displacive character is found to become stronger with decreasing exchange rate x in the range xϾ0.33.

Perfect Softening of the Ferroelectric Mode in the Isotope-Exchanged Strontium Titanate ofSrTiO318Studied by Light Scattering

“…2 show the soft mode behavior expected in a typical displacive-type mechanism of a ferroelectric phase transition. The present results are completely different from the previous ones observed above 5 cm ÿ1 in STO18 [7][8][9][10][11]. The damping constant E u keeps a small value even just above T c .…”

“…The observation of Raman scattering by inactive A 2u and E u modes above T c was reported in SrTi 18 O x 16 O 1ÿx 3 [15]. An anomalous behavior of the acoustic c 44 mode has been reported around T c by Brillouin scattering [8,16]; the drastic decrease of the value of frequency shift and the anomalous increase of intensity were presented. However, we have not observed such anomalous behavior in the c 44 mode in the high quality STO18 samples of 95% and 96% concentrations in the present study.…”

“…Since then, the phase transition mechanism of the isotope-exchanged strontium titanate SrTi 18 O 3 (hereafter abbreviated as STO18 and the ordinary one as STO16) has attracted the intense interest of many researchers in order to elucidate its phase transition mechanism related to the quantum fluctuations. The lowest energy phonon mode in STO18 has been searched for and analyzed to elucidate the soft mode dynamics near T c by many authors [7][8][9][10][11]. However, any clear softening behavior has not been reported: By Raman scattering studies, the observation of the Raman inactive soft E u mode has been tried, but the assignment of certain spectral feature to the E u mode has not been convincing and unambiguous.…”

Broad Doublet and Partially Softened Acoustic Mode in SrTiO3

“…[10] The c 55 mode should not be observed in the present scattering geometry. Since a ferroelectric E u (x, y) soft mode of Slater type is thought to cause the ferroelectric phase transition of STO18-99, spontaneous Downloaded by [George Mason University] at 08:18 20 December 2014 polarization directs randomly along two independent axes in xy plane to make a complicated ferroelectric domain state (CDS) below T c [13]. This CDS breaks a mode selection rule and the c 55 mode can be observed as a result.…”

Broad Doublet and Plasmon Peak in Perovskite-Type Oxides

“…Very recently, Tsujimi and Itoh proposed their new hypothesis for the origin [10,11]. It has been reported from the spectra for SrTi( 18 O 16 0.99 O 0.01 ) 3 (STO18-99) that the c 44 transverse acoustic mode splits into two modes (TA1 and TA2 modes; a pair of c 44 and c 55 ) at T c = 28 ± 2 K, and only the TA2 mode softens partially below T c due to the coupling with a soft ferroelectric E u (x, y) phonon of Slater type [12,13]. Since the frequency shift of the TA2 mode becomes almost constant below T ∼ 22 K, the TA2 mode observed below this temperature has been named a "STA" (partially softened transverse acoustic) mode.…”