Accurate determination of the far-infrared dispersion in SrTiO3by hyper-Raman spectroscopy

Abstract

We use the fluctuation-dissipation theorem to relate the hyper-Raman scattering efficiency of transverse and longitudinal-optical phonons to the infrared dielectric function e(Q) = e'(D) +i@"(0).The relation is applied to SrTiO, at room temperature in order to clarify the discrepancies between various far-infrared reflectivity studies. We take advantage of the hyper-Raman selection rules allowing the observation of Raman-forbidden modes. e "(0) is derived from-accurately measured hyper-Raman lines and combined with compatible infrared reflectivity data to give e'(Q). Below 100 cm ' the dielectric dispersion can be adequately described by a classical dispersion formula with a quasiharmonic-oscillator frequency of 87.9 cm ' and a single damping constant of 23.7 cm '. This approximation fails above 100 cm '.

Disorder driven structural and dielectric properties of silicon substituted strontium titanate

“…3), there are two more relatively sharp modes at 174.9 and 788.1 cm À1 that are not assignable to second-order scattering. We ascribe the former to TO 2 and the later to LO 4 firstorder Raman scattering modes 13,14 originating from lowering of the crystal symmetry with the silicon impurity doped STO ceramics as a result of breaking of inversion symmetries and it is consistent with the findings from the XRD analysis. The "silent" TO 2 and LO 2 modes are both at 173 cm À1 in undoped STO, as first determined by the early electric-field induced Raman scattering studies 15,16 by Worlock and Fleury.…”

High-temperature relaxor ferroelectric behavior in Pr-dopedSrTiO3

“…Their occurrence is made possible by relaxation of these rules by unintentional and intentional structural defects. By comparison with previously published Raman and infrared scattering results on bulk STO [16][17][18][19][20][21] and on thin STO films 22 we identified the four Raman modes near 175, 475, 540, and 795 cm −1 as TO 2 , LO 3 , TO 4 , and LO 4 , modes, respectively. The appearance of TO 2 and TO 4 modes, has been considered as evidence of polar nanoregions ͑PNRs͒ which characterize a relaxor ferroelectric system.…”

“…27 A mode near 80 cm −1 is most pronounced for the compositions x = 0.00 and 0.025. This mode corresponds to an IRactive soft TO 1 mode 3,17,19 which is known to exhibit a softening tendency.…”

Hyper‐Raman spectra and frequency dependence of soft mode damping in SrTiO3

“…Therefore, with decreasing temperature, as the soft mode shifts to smaller frequencies, its linewidth must narrow more rapidly than that predicted by Eqn. (11) with ro(0)=constant. This is in complete accord with the behavior shown in Fig.…”