Hyper-Raman scattering experiments suggest that a splitting of the lowest F1u-symmetry mode of PbMg1/3Nb2/3O3 crystal occurs in a wide temperature range around its Burns temperature Td≈630  K. The upper-frequency component, earlier investigated by inelastic neutron scattering experiments above Td, appears to be underdamped even hundred of degrees below Td. The lower-frequency component, known below Td from far-IR spectroscopy, actually becomes underdamped above Td. This suggests that the lower-frequency mode is the “primary” polar soft mode of PbMg1/3Nb2/3O3, responsible for the Curie-Weiss behavior of its dielectric permittivity above Td.

Hyper-Raman scattering is a non-linear-optic spectroscopy where two incident photons scatter one photon after interaction with an excitation in the media [1]. One major interest of this technique is its high sensitivity to all polar vibrations, including soft polar modes [2], and to excitations that are inactive in both infrared absorption and Raman scattering. These specificities provide hyper-Raman with a very powerful tool for the investigation of local and average structure of ferroelectric-type materials such as relaxors or multiferroic systems. We will focus on hyper-Raman results obtained on three single crystals of the relaxor PbMg1/3Nb2/3O3 (PMN)[3,4]. The relative scattering intensities of the band near 250 cm−1 in various polarization geometries are fully compatible with the hyper-Raman tensor of the F2u ‘silent’ mode of the parent Oh cubic structure. The temperature dependence of the three F1u-symmetry polar modes was investigated between 20 K and 800 K. Some of the transverse (TO) and longitudinal (LO) components are splitted up to the highest temperatures, confirming the existence of local lattice distortions from the cubic symmetry well above the Burns temperature Td ≈ 620 K. The splitting of the LO2-mode strongly increases below Td, a behaviour which likely relates to the growth of the nano-domains. The soft TO-mode is also clearly observed, with a frequency decreasing to zero near Td. Same behaviours have been observed in another relaxor-type compound, PbMg1/3Ta2/3O3. Finally, a weak but clear transition-like anomaly in the temperature dependence of the lowest frequency LO-mode (LO1) is observed near the Curie temperature Tc = 210 K. These experimental observations will be compared to Raman and neutron scattering literature data, and confronted to the proposed scenarios for the evolution of the local structure of PMN with temperature. [1] V. N. Denisov, B. N. Marvin, and V. B. Podobedov, Phys. Rep. 151, 1 (1987). [2] H. Vogt, Coherent and Hyper-Raman Techniques, in Topics in Applied Physics Light Scattering in Solids II, Vol. 50, edited by M. Cardona and G. Güntherdot (Springer, Berlin, 1982). [3] B. Hehlen, G. Simon, and J. Hlinka, Phys. Rev. B 75, 052104 (2007) [4] A. Al-Zein, B. Hehlen, J. Rouquette, and J. Hlinka, Phys. Rev. B 78, 134113 (2008).

Résumé (à fournir)

The boson peak (BP) of silica at several densities and of lithium borate glasses has been measured using hyper-Raman scattering (HRS). In the former case, the HRS spectral responses greatly differ from Raman ones but match very well the neutron-scattering measurements. In borate glasses, the Raman and hyper-Raman BPs are rather similar. These results are consistent with the picture that librational motions of rigid structural units (i.e. tetrahedra in silica and triangles and rings in borates) contribute to the excess of low frequency modes. In disordered networks, rigid librations are expected to couple with rigid translations (the second family of “external” modes), leading to extended hybrid excitations. The temperature dependence of the BP of vitreous silica was also measured from 20 to 800 K. It can be scaled with the very high frequency limit of the sound velocity. This indicates that this limit velocity provides a measure of temperature-dependent structural changes in silica.

Hyper-Raman scattering is a non-linear-optic spectroscopy where two incident photons scatter one photon after interaction with an excitation in the media. One major interest of this technique is its high sensitivity to all polar vibrations including soft polar modes, and also to excitations that are inactive in both infrared absorption and Raman scattering. These specificities provide hyper-Raman with a very powerful tool for the investigation of the local and average structure of ferroelectric relaxors. The hyper-Raman spectroscopy of three single crystals of the relaxor PbMg1/3Nb2/3O3 has been performed [1,2]. The relative scattering intensities obtained for the band near 250 cm−1 in various polarization geometries are fully compatible with the hyper-Raman tensor of the F2u ‘silent' mode of the parent Oh cubic structure. The temperature dependence of the three F1u-symmetry polar modes was investigated between 20 K and 800 K. Some of the transverse (TO) and longitudinal (LO) components are splitted up to the highest temperatures, confirming the existence of local lattice distortions well above the Burns temperature Td ≈ 620 K. The splitting of the LO2-mode near 400 cm-1 strongly increases below Td, a behaviour which likely relates to the growth of the nanodomains. The soft TO-mode is also clearly observed, with a frequency decreasing to zero near Tb. Same behaviours have been observed in another relaxor-type compound, PbMg1/3Ta2/3O3. Finally, a weak but clear transition-like anomaly in the temperature dependence of the lowest frequency LO-mode (LO1) is observed near the Curie temperature Tc=210 K. These observations will be compared to Raman and neutron scattering literature data, and confronted to the proposed scenarios for the evolution of the local structure of PMN with temperature. [1] B. Hehlen, G. Simon, and J. Hlinka, Phys. Rev. B 75, 052104 (2007) [2] A. Al-Zein, B. Hehlen, J. Rouquette, and J. Hlinka, Phys. Rev. B 78, 134113 (2008).

The D1 and D2 vibrational ring modes of normal and densified silica are investigated using a time resolved pump-probe technique and spontaneous Raman spectroscopy. The vibrational response function of both techniques is derived and compared to the experimental results obtained on three samples of silica of increasing density. Comparison of the amplitudes of the time- and frequency-domain responses reveals a much larger increase in concentration of three-membered rings as compared to four-membered ones upon densification. In addition, measurements of the Raman-scattering intensity in SiO2 also provide a mean to identify bending motions and to estimate the variation of the average Si-O-Si bond angle in densified samples.