Vanadium pentoxide polymorphs (α-, β-, γ'-, and ε'-VO) have been studied using the Raman spectroscopy and quantum-chemical calculations based on density functional theory. All crystal structures have been optimized by minimizing the total energy with respect to the lattice parameters and the positions of atoms in the unit cell. The structural optimization has been followed by the analysis of the phonon states in the Γ-point of the Brillouin zone, and the analysis has been completed by the computation of the Raman scattering intensities of the vibrational modes of the structures. The optimized structural characteristics compare well with the experimental data, and the calculated Raman spectra match the experimental ones remarkably well. With the good agreement between the spectra, a reliable assignment of the observed Raman peaks to the vibrations of specific structurals units in the VO lattices is proposed. The obtained results support the viewpoint on the layered structure of vanadium pentoxide polymorphs as an ensemble of VO chains held together by weaker interchain and interlayer interactions. Similarities and distinctions in the Raman spectra of the polymorphs have been highlighted, and the analysis of the experimental and computational data allows us, for the first time, to put forward spectrum-structure correlations for the four VO structures. These findings are of the utmost importance for an efficient use of Raman spectroscopy to probe the changes at the atomic scale in the VO-based materials under electrochemical operation.

Structure, electronic states, and vibrational dynamics of γ-LiV 2 O 5 were studied by combined use of the quantum-chemical calculations and Raman spectroscopy. The spin-polarized DFT+U calculations correctly mimic the structural changes induced by the Li intercalation into the V 2 O 5 framework. The analysis of the density of electronic states shows that the electrons of Li atoms are transferred to the Vb atoms and are aligned in ferromagnetic order. The charge distribution in the system reflects the change of valence state of the Vb atoms from 5+ to 4+ and it is in line with changes of Vb-O bond lengths. The calculated Raman spectrum of the γ-LiV 2 O 5 structure is in line with the experimental Raman spectra that allows a reliable assignment of all prominent Raman peaks. The comparison of the spectra of γ-LiV 2 O 5 and γ ′ -V 2 O 5 indicates spectral signatures of structural changes induced by the Li insertion into the γ ′ -V 2 O 5 lattice. Results of the study opens the opportunity of using the Raman spectroscopy for characterization of structural modifications of the vanadate framework upon intercalation of guest species.

We report the results of experimental and theoretical studies of phonon modes in GaN/AlN superlattices (SLs) with a period of several atomic layers, grown by submonolayer digital plasma-assisted molecular-beam epitaxy, which have a great potential for use in quantum and stress engineering. Using detailed group-theoretical analysis, the genesis of the SL vibrational modes from the modes of bulk AlN and GaN crystals is established. Ab initio calculations in the framework of the density functional theory, aimed at studying the phonon states, are performed for SLs with both equal and unequal layer thicknesses. The frequencies of the vibrational modes are calculated, and atomic displacement patterns are obtained. Raman spectra are calculated and compared with the experimental ones. The results of the ab initio calculations are in good agreement with the experimental Raman spectra and the results of the group-theoretical analysis. As a result of comprehensive studies, the correlations between the parameters of acoustic and optical phonons and the structure of SLs are obtained. This opens up new possibilities for the analysis of the structural characteristics of short-period GaN/AlN SLs using Raman spectroscopy. The results obtained can be used to optimize the growth technologies aimed to form structurally perfect short-period GaN/AlN SLs.

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The (a,b) plane angular dependence of the third-order nonlinear optical susceptibility, χ(3), of a c-cut paratellurite (α-TeO2) single crystal was quantitatively evaluated here by the Z-scan technique, using a Ti:sapphire femtosecond laser operated at 800 nm. In particular, the mean value Re(⟨χ(3)⟩a,b)(α-TeO2) of the optical tensor has been extracted from such experiments via a direct comparison with the data collected for a fused silica reference glass plate. A Re(⟨χ(3)⟩(a,b)(α−TeO2)):Re(χ(3))(SiO2 glass) ratio roughly equal to 49.1 is found, and our result compares thus very favourably with the unique experimental value (a ratio of ∼50) reported by Kim et al. [J. Am. Ceram. Soc. 76, 2486 (1993)] for a pure TeO2 glass. In addition, it is shown that the angular dependence of the phase modulation within the (a,b) plane can be fully understood in the light of the strong dextro-rotatory power known for TeO2 materials. Taking into account the optical activity, some analytical model serving to estimate the diagonal and non-diagonal components of the third order nonlinear susceptibility tensor has been thus developed. Finally, Re(χxxxx(3)) and Re(χxxyy(3)) values of 95.1×10−22  m2/V2 and 42.0×10−22 m2/V2, respectively, are then deduced for a paratellurite single crystal, considering fused silica as a reference.