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Polarized Raman spectra of piezoelectric bismuth germanate single crystal were recorded using vortex light in order to verify the predicted symmetry selection rules for Raman scattering of photons with nonzero orbital angular momentum. At first, the phonon spectrum is fully characterized by the infrared reflectivity and Raman scattering with ordinary light. Then, vortex light Raman spectra were compared with the ordinary ones, but no influence of the orbital momentum on the Raman spectra of Bi$_4$Ge$_3$O$_{12}$ was observed. In particular, we experimentally negate the theoretical prediction that Raman scattering using vortex light can yield frequencies of the silent optic modes of Bi$_4$Ge$_3$O$_{12}$. We also have not seen evidence for the predicted symmetry-lowering of the Raman scattering tensors of $E$ modes. We propose that the orbital momentum effects are inaccessible to the ordinary Raman spectroscopy experiments because of the tiny magnitudes of typical phonon coherence lengths., Comment: 6 pages, 6 figures

Using a broad band dielectric spectroscopy approach (1 to 10^14 Hz) we prove that the tungsten bronze Ca0.3Ba0.7Nb2O6 (CBN-30) displays a ferroelectric phase transition of mixed displacive and order-disorder character, and its paraelectric phase does not show traces of relaxor behaviour but precursor effects as polar fluctuations below about 550 K. The analysis of the sub-MHz dielectric response together with infrared and Raman spectroscopy reveals that simultaneous polarization mechanisms are responsible for the phase transition. The comparison of the excitations found in CBN-30 with those of (Sr,Ba)Nb2O6 reveals that these mechanisms are congruous, although in CBN-30 the main relaxation process behaves differently due to the different domain structure. The excitations are phenomenologically assigned to phonons, to an anharmonic vibration of cationic origin which plays the role of a soft central mode, and to a relaxation in the GHz range probably due to polarization fluctuations of nanometric size which carries the main part of the permittivity and splits below TC into several weaker excitations with different polarization correlation lengths. The overall dielectric response is therefore explained by the coexistence of several excitations with different thermal behaviors, corroborating the complexity of the tetragonal tungsten bronze structures., Comment: 21 pages, 7 figures, 1 table

Neutron pair distribution function analysis and first principles calculations have been employed to study short-range correlations in heavily disordered dielectric material Sr$_x$Ba$_{1-x}$Nb$_2$O$_6$ ($x=0.35, 0.5$ and 0.61). The combination of methods has been fruitful in pinpointing main local-structure features, their temperature behaviour and interrelation. A rather complex system of tilts is found to be both temperature and Sr-content sensitive with the biggest tilt magnitudes reached at low temperatures and high $x$. Relative Nb-O$_6$ displacements, directly responsible for material's ferroelectric properties, are shown to be distinct in two octahedra sub-systems with different freezing temperatures and disparate levels of deviation from macroscopic polarization direction. Intrinsic disorder caused by Sr, Ba and vacancy distribution is found to introduce local strain to the structure and directly influence octahedra tilting. These findings establish a new atomistic picture of the local structure -- property relationship in Sr$_x$Ba$_{1-x}$Nb$_2$O$_6$.

Polarized Raman, IR and time-domain THz spectroscopy of orthorhombic lead zirconate single crystals yielded a comprehensive picture of temperature-dependent quasiharmonic frequencies of its low-frequency phonon modes. It is argued that these modes primarily involve vibration of Pb and/or oxygen octahedra librations and their relation to particular phonon modes of the parent cubic phase is proposed. Counts of the observed IR and Raman active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. The most remarkable finding is the considerably enhanced frequency renormalization of the y-polarized polar modes, resulting in a pronounced low temperature dielectric anisotropy. Results are discussed in terms of contemporary phenomenological theory of antiferroelectricity.