Temperature Dependence of Raman and Rayleigh Scattering in LiNbO3and LiTaO3

Abstract

The Raman spectra of Liwb03 and LiTa03 have been investigated between 90 and 1100'K. The dependence of optic-mode frequencies and Raman-scattering efBciencies on temperature indicates that the ferroeiectric transition (T, =900'K for LiTaO&, =1480'K for LiNb03) is second order and is associated with an optic phonon mode whose frequency becomes small as the Curie temperature is approached from below. Critical Rayleigh scattering is observed for LiTa03 and the shape of the anomaly is attributed to the combined eRects of lattice polarization Quctuation and the discontinuity in electro-optic eRect at the transition. The absolute scattering efBciency and linewidth data indicate that the stimulated Raman gain for several lattice modes is comparable with or greater than that for the 655-cm ' CS2 line, and calculations show that cw Raman oscillation in these modes is feasible.tional modes for the lowand high-temperature phases are listed in Table I. The nonzero components of infrared absorption and Raman scattering tensors are also noted. In the ferroelectric phase, 22 optic modes are both Ramanand infrared-active (4Ai+9E), and in the paraelectric phase nine of these become Ramanand 13 infrared-active. The macroscopic Geld produced by the infrared-active phonons raises the degeneracy between longitudinal (L) and transverse (T) modes, thereby doubling the number of observed frequencies for these modes. Above T, the Raman-active phonons are nonpolar, so that the TL degeneracy is restored and there are only five (1Ato+4Eo) characteristic Raman shifts expected.The above discussion is based on the point-group symmetry (3ttt below T, and 3m above T,) and is only approximately applicable for the Gnite-wavelength phonons actually observed. In particular, attention must be restricted to phonons propagating along or perpendicular to the unique crystal (c) axis, since for other directions phonons to not in general possess a well-deGned L or T polarization. Even with this restriction, there are additional considerations. For example, a transverse E phonon of 6nite wavelength can propagate either parallel or perpendicular to the optic axis and may have a different frequency in each case. Similarly, transverse A & modes may have different frequencies for propagation along (010) and (110).These latter effects, however, were not observed in the present experiment.The strength of a given Raman-active vibration is given by the scattering efliciency 5/ldQ, where 8 is the fraction of incident power that is scattered into a solid angle dQ near a normal to the optical path length E. A semiclassical expression for (Stokes-shifted) scattering eKciency is4

Comparison of the electronic structures and energetics of ferroelectricLiNbO3andLiTaO3Inbar1, Cohen2 1996Phys. Rev. B15711910View full textAdd to dashboardCite

“…Also, a comparison of the Ta-O distances in KTaO 3 and LiTaO 3 shows exactly this same effect. In KTaO 3 , the oxygen octahedra is larger than the oxygen octahedra in LiTaO 3 , and yet, in KTaO 3 the B atom never displaces to the off center position, while in LiTaO 3 , the B cation exhibit a ferroelectric distortion.…”

“…They have many applications in optical, electro-optical and piezoelectric devices, but the fundamental physics that leads to their ferroelectric behavior have not been studied. Their transition temperatures, which are among the highest known ferroelectric transition temperatures, are quite different, 1480 K for LiNbO 3 and 950 K for LiTaO 3 . The electronic origin of their different T C is a mystery since Nb 5+ and Ta 5+ behave very similarly, and structurely these materials are almost identical.…”

“…1b). Glass in 1968 and later Johnston et al have determined, using dielectric and thermal measurements, that the phase transformation in these systems is continuous 2,3 .…”

Ultrafast zone-center coherent lattice dynamics in ferroelectric lithium tantalate

“…The excitation of coherent phonons basically obeys the Raman selection rule, that is, all excited coherent phonons are Raman active. There is a disagreement in the literature on the assignment of the long-wavelength optical phonons in LiTaO 3 [5][6][7][8][9][10], [15][16][17][18][19]. Our assignment is mainly based on the Raman results by Kaminow and Johnston [15], which are further supported by the first-principle calculations [20].…”

“…In contrast, the importance of studying the transient phonon characteristics at the center of the Brillouin zone (BZ) has been underappreciated. On the one hand, it is now well accepted that the relaxation of transverse optical modes at the BZ center is directly responsible for the ferroelectric-paraelectric phase transition, although the exact transformation mechanism is still under debate [5][6][7][8][9][10]. As far as other modes are concerned, one can expect them to contribute significantly at temperatures much lower or higher than the Curie point T C .…”