Hyper-Raman Scattering: A Powerful Tool To Probe The Structure In Glasses

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 (HRS) 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. The anomalous behaviours observed can be explained in term of local structural changes. At low frequency, HRS reveals a clear splitting of the soft TO-mode. The high frequency component corresponds to previous neutron scattering expectations while the low frequency one was not observed so far [3]. Its frequency decreases to zero at T0 @ 400 K. The results strongly suggest that this vibration is the primary soft mode of PMN. Similar observations have been made in another relaxor-type compound, PbMg1/3Ta2/3O3, thus suggesting a universal behaviour of the low-frequency dynamics of cubic relaxors. [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) [3] A. Al-Zein, J. Hlinka, J. Rouquette, and B. Hehlen, Phys. Rev. Lett. 105, 017601 (2010)

Terahertz time-domain spectroscopy has been used to extract the light-to-vibration coupling coefficient for sodosilicate glasses, from which it was possible to calculate the variance in the distribution of uncorrelated charges. It was found that increasing the sodium content of the glasses increased the standard deviation of the uncorrelated charge distribution in a linear fashion, and was almost an order of magnitude higher when compared to the charge distribution for pure silica reported previously, in agreement with previously published simulation data.

Perovskite (ABO3) ferroelectric materials exhibit a Ferroelectric-Paraelectric (Ferro-Para) phase transition at a critical temperature TC (and a critical pressure PC), which can be defined as a phase transition from a non-centrosymmetric to a centrosymmetric structure due to the disappearance of the spontaneous polarization (PS) characterizing these materials. Neutron diffraction is particularly useful to investigate this Ferro-Para phase transition in pressure-temperature space due to the relatively strong scattering of oxygen compared to XRD. It enables 1) to calculate PS with the dipole equation moment and also 2) to detect phase transitions involving octahedral tilting. In this study, high-temperature and high-pressure neutron powder diffraction patterns of typical perovskites PbTiO3 and PbZr0.52Ti0.48O3 contain specific reflections, (h, k, 2l or h, 2k, 2l), strongly linked to polar shifts. The progressive disappearance of these reflections is due to the anti-phase scattering of A (Pb), B (Ti, Zr) and O atoms. In other words, this disappearance is related to the decrease in the imaginary part of the form factor corresponding to these reflections which becomes zero at the Ferro-Para phase transition. In addition, the temperature- and pressure- dependence of the calculated PS is different from the behavior of the c/a ratio which should be proportion to √(P_C-P) in the case of purely displacive transition.

Ferroelectric materials with the perovskite structure have drawn considerable attention for many years. Among them, PbZr1−xTixO3 (PZT) materials are particularly interesting due to their high piezoelectric response in the so-called “morphotropic phase boundary” (MPB) region. Recently, synchrotron X-ray powder diffraction experiments done by Noheda et al. showed the existence of a monoclinic (M) phase (near x ~ 0.5) between the tetragonal and rhombohedral phases1,2. The effect of high pressure on the physical properties of PZT has been studied recently3. Experiments based on electron diffraction4 and neutron diffraction studies5, Raman scattering results6, as well as theoretical calculations7 have indicated the existence of an intrinsic short range dynamical disorder over nearly the entire PZT solid solution. Rouquette et al. observed the presence of diffuse X-ray/neutron scattering in PbZr0.52Ti0.48O3 diffraction patterns which is an evidence of local lattice deformations with respect to the average structure. Additional high-pressure high-temperature investigations showed the disappearance of the polar phonons A1(TO) (which is the origin of the ferroelectric instability) upon compression around 5 GPa in agreement with neutron diffraction data. High-pressure high-temperature EXAFS experiments were carried out on BM29 at the ESRF to follow the corresponding changes in short-range order around Zr at the ferro-to-paraelectric phase transition. The EXAFS results at 300 K isotherm show a change in the fine structure that starts at 5.3 GPa and this is in consistent with the value of the curie pressure Pc obtained by neutron diffraction3 and Resonance Raman Spectroscopy3 experiments. By fitting the first coordination shell (Zr,Ti)O6 octahedra using a cubic model, the values of the Debye-Waller (DW) factor at different pressure points were obtained. A noticeable drop in the DW factor above 6 GPa was observed, and these values agree with the value of the DW for BaZrO38 that has a centro-symmetric structure with no shift in the position of the Zr atom. This drop at an isotherm can be considered as a representative of the polar displacement and therefore of the ferro-to-paraelectric phase transition. Below 6 GPa, the obtained values of DW are in good agreement with what was obtained for PZT9. 1B. Noheda, D. E. Cox, G. Shirane, J. A. Gonzalo, L. E. Cross, and S.-E. Park, Appl. Phys. Lett. 74, 2059 (1999). 2B. Noheda, J. A. Gonzalo, L. E. Cross, R. Guo, S.-E. Park, D. E. Cox, and G. Shirane, Phys. Rev. B 61, 8687 (2000). 3J. Rouquette, J. Haines, V. Bornand, M. Pintard, Ph. Papet, R. Astier, J. M. Léger, and F. Gorelli, Phys. Rev. B 65, 2141021 (2002); J. Rouquette, J. Haines, V. Bornand, M. Pintard, Ph. Papet, W. G. Marshall, and S. Hull, Phys. Rev. B 71, 024112 (2005). 4 A.M. Glazer, P.A. Thomas, K. Z. Baba-Kishi, G. K. H. Pang, and C. W. Tai, Phys. Rev. B 70, 184123 (2004). 5 D. L. Corker, A. M. Glazer, R. W. Whatmore, A. Stallard, and F. J. Fauth, J. Phys.: Condens. Matter 10, 6251 (1998). 6 J. Frantti, S. Ivanov, J. Lappalainen, S. Eriksson, V. Lantto, S. Nishio, K. Kakihana, and H. Rudlöf, Ferroelectrics 266, 73 (2002). 7 I. Grinberg, V. R. Cooper, and A. M. Rappe, Nature 419, 909 (2002). 8 C.Laulhe, F.Hippert, J.Kreisel, M.Maglione, A.Simon, J.L.Hazemann, and V.Nassif, Phys. Rev. B 74, 014106, (2006). 9 D.Cao, I.-K.Jeong, R.H.heffner, T.Darling, J.-K.Lee, F.Bridges, J.-S.Park, and K.-S.Hong , Phys. Rev. B 70, 224102, (2004).

Lattice vibrations of PbMg1/3Nb2/3O3 relaxor and re- lated materials have been already subject of numerous theoretical and experimental studies, but despite of these efforts, so far none of the traditional techniques was capa- ble to disclose a comprehensive and broadly acceptable picture of the dynamical processes associated with the order parameter dynamics in such systems. For com- putational methods starting from first-principles, this is a difficult task since the characteristic time and length- scales of the structural fluctuations, which are believed to be responsible for the relaxor behavior, are beyond the capabilities of the standard brut-force methods. Perhaps the most definite observation is the very existence of a soft dynamics in the system. The "royal" experi- mental lattice dynamics technique, the inelastic neutron scattering, has brought the first demonstration that the A-site cation vibration modes are showing at least partially the classical softening and hardening behavior at high and low temperatures, and these observations were later fully supported also by the "most proper" technique for studies of ferroelectric soft modes, the far-infrared spectroscopy. Nevertheless, there is a more than a hundred K broad range of the intermediate temperatures, where the situation is far from being clear. Among others, it was obscured by the continuing discussions about the so called waterfall effect and various spurious phenomena. But the analysis is mainly difficult because the soft mode response cannot be well modeled by a single damped harmonic oscillator response: the mode seems to be split and/or coupled to other excitations of vibrational and/or relaxational nature (central peaks). Moreover, for both far-infrared and inelastic neutron scattering techniques it is technically very di±culties to disclose neatly the be- havior at very low frequencies in these materials. At the same time, several rather fine spectral features could be identified in Brillouin and Raman spectra, and very detailed temperature-dependent experiments could reveal anomalies linked to the characteristic "critical" temperatures established independently from other techniques. Unfortunately, there is no consensus about the assignment of these features and precise meaning of these temperatures. In fact, even the mechanism responsible for activation of the Raman scattering is a matter of controversy. In order to broaden the perspective for the considerations about this subject, we shall offer an overview of our recent studies of PMN lattice dynamics by the hyper-Raman scattering technique, which, according to our opinion, provides rather interesting new insights in the relaxor soft mode problem. Hyper-Raman scattering (HRS) spectroscopy is based on a non-linear optical effect where two incident pho- tons produce one scattered photon after interaction with a phonon or other excitation. One of the interest of this technique is that its selection rules are different from RS and IR ones. For example, in the Pm3m simple cubic perovskite, the polar F1u modes are active both in IR and HRS, while the "silent" F2u mode is active only in HRS. Here we mostly exploit the fact that hyper-Raman technique combines advantage of infrared spectroscopy to probe the polar modes irrespectively on the structural °uctuation in the structure (in contrast with the the Raman spectroscopy), that the same modes can be inves- tigated in several nonequivalent geometries (similarly to neutron scattering), and, at the same time, this technique allows to probe the spectral profiles with a sufficient de- tail down to frequencies of a few cm-1. The key results to be discussed are the following: 1. there is a remarkable correspondence between the LO band intensities (!) in the HRS and Im[1/epsilon] spectra 2. experiments reveal enhancement of the intensity of the LO modes in the forbidden backscattering geometry, 3. the pronounced mode observed in cross-polarized spectra near 250 cm-1 is assigned as F2u mode, 4. soft mode is splitting is clearly observed and the soft mode spectra could be followed well below the Burns temperature, 5. temperature dependence of soft mode intensity re- veals freezing of the relaxor order parameter in a very similar way as the refractive index data does it in the original works of Burns and coworkers. Although all the observations are not fully understood, we are convinced that the HRS scattering has a potential to become extremely useful tool for studying polar modes in lead-based relaxor materials.

PZT is the most commonly used piezoelectric material. In spite of intensive experimental and theoretical studies, no model at the atomic scale has been proposed to explain the large piezoelectric effect in PZT, (d33= 520 pC/N), compared to the low piezoelectric response in the simple end-member lead titanate PbTiO3 with no-Zr (d33= 35 pC/N). X-ray absorption spectroscopy (XAS) appears as the technique of choice not only to clarify the role of Zr, but also to quantify the Zr displacement through the Ferroelectric-Paraelectric (F-P) transition. We clearly show evidence of the polar character of the Zr-atoms in PZT with a Zr-shift which will produce a small polarization. Such an atomic configuration for one type of atoms leads to relatively easy switching, i.e. relatively low electric field to align the Zr-polar atoms, which will create a favorable energetic situation for the cooperative switching of the strongly polar Ti-O dipole and would therefore account for the large piezoelectric effect in PZT1. In mixed B-cation systems of the perovskite ABO3 systems, such local configurations for one type of B cations leads to relatively easy switching giving rise to exceptional ferro/piezoelectric properties. Our recent “in-situ” XAS measurements of the giant piezoelectric effect in PbZn1/3Nb2/3O3 single crystal as a function of electric field confirms that large Zn atoms (small polarization) are easily poled and cooperatively align the strong Nb-O dipoles. 1 PRB 81, 174110 (2010)