Raman Spectra Of Crystals Research Articles

Comparative investigation of LiNbO3 crystals Raman spectra in the temperature range 100–400 K

We have investigated Raman spectra of congruent and stoichiometric LiNbO3 crystals in the temperature range 100–450 K. Slope gradient is greater for the temperature dependence of band width associated with Nb5+ ions vibrations than that associated with Li+ ions vibrations in a lithium niobate crystal structure. This fact indicates that the anharmonicity of Nb5+ ions vibrations along the polar axis is greater compared to Li+ ions vibrations. It is likely that O2– ions contribute to this anharmonicity. The O2– ions vibrations are characterized by an anharmonic potential in the LiNbO3 crystal structure. The O2– ions vibrations according to ab initio calculations strongly interact with vibrations of Nb5+ ions. We have found that the temperature dependence of the fundamental bands intensity is nonmonotonic and the “extra bands” intensity is strictly linear.

Crystal structure and Raman spectra of molecular complex [{Cd(Et2dtc)}2• DABCO]• C60• (DABCO)2 at high pressure

Crystal structure and Raman spectra of molecular complex [{Cd(Et2dtc)}2• DABCO]• C60• (DABCO)2 at high pressure

Фотополимеризация в слоях фуллерена молекулярного донорно-акцепторного комплекса \Pt(nPr-=SUB=-2-=/SUB=-dtc)-=SUB=-2-=/SUB=-\·(C-=SUB=-60-=/SUB=-)-=SUB=-2-=/SUB=-

AbstractWe measured Raman spectra in crystals of molecular donor–acceptor fullerene complexes { Me ( n Pr_2 dtc )_2} · (C_60)_2 ( Me = Ni, Cu, Pt). In the spectra of the {Pt( n Pr_2 dtc )_2} · (C_60)_2 complex under prolonged irradiation with a laser with λ = 532 nm, characteristic changes in the photopolymerization of fullerene are observed, associated with the splitting of degenerate phonon Hg modes and softening of Ag modes of the C_60 molecule. The kinetics of photopolymerization under conditions of weak irradiation at room temperature is studied. It was found that thermal destruction of the photopolymer with increasing temperature leads to a decrease in its concentration in the final photopolymerization product. The kinetics of thermal destruction is described by the Arrhenius equation, with the activation energy E _A of (0.68 ± 0.03) eV; the dimers are destructed to a concentration of 1% within 15 min at ~114°C.

Light scattering from polar and pseudoscalar modes in doped LiNbO3 and LiTaO3 monocrystals

The paper deals with spontaneous scattering in LiNbO3 and LiTaO3 crystals we synthesized to contain different dopants (Mg2+, Zn2+, Gd3+, B3+, Cu+ and others). We investigated photoinduced light scattering patterns in doped LiNbO3 crystals. We recorded Raman spectra at different light scattering geometries and temperatures. We detected pseudoscalar modes in the Raman spectra of doped LiNbO3 and LiTaO3 crystals.

On the dimorphism of Pr6Mo10O39

Abstract Attempts to synthesize Pr4Mo7O27 using Pr, Pr6O11 and MoO3 in a molar ratio of 8:6:77 led to a main product of scheelite-type Pr0.667[MoO4] and few single crystals of the triclinic A-type Pr6Mo10O39. The latter crystallizes in space group P1̅ (a=945.25(1), b=1058.49(2), c=1815.16(3) pm; α=104.149(1), β=95.220(1), γ=102.617(1)°, Z=2). Its crystal structure comprises six crystallographically independent Pr3+ cations, eight tetrahedral [MoO4]2− units, and one [Mo2O7]2− entity. The cations display coordination numbers of seven (1×) and eight (5×), while the [MoO4]2− tetrahedra are surrounded by five Pr3+ cations each. The [Mo2O7]2− anions exhibit a coordination environment of seven Pr3+ cations. The attempt to synthesize PrF[MoO4] using PrOF (from in situ thermal decomposition of PrF[CO3]) as reagent did not lead to the desired product but to monoclinic B-type Pr6Mo10O39. This slightly less dense modification compared to its triclinic analogue crystallizes in space group C2/c (a=1247.93(3), b=1989.68(6), c=1392.52 (4) pm, β=100.505(2)°, Z=4) with three crystallographically independent Pr3+ cations, four [MoO4]2− tetrahedra, and again one [Mo2O7]2− unit in the crystal structure. Thus, both Pr6Mo10O39 modifications are better described with the structured formula Pr6[MoO4]8[Mo2O7]. The coordination numbers around the Pr3+ cations are seven (1×) and eight (2×) while all four [MoO4]2− anions are again surrounded by five Pr3+ cations each. Six of the latter represent the coordination environment around the [Mo2O7]2− entities. Besides the thorough comparison of the crystal structures single crystal Raman spectra were recorded for both Pr6Mo10O39 phases.

Modern level for properties prediction of iodine-containing organic compounds: the halogen bonds formed by iodine

The modern concepts on specific features of anisotropy of the electron density of valence shells of bound atoms and chemical bonds providing the successful description of the properties of halogen bonds formed by the iodine molecule in compounds with useful physicochemical properties are generalized in the review. The prognoses of basicity by the diiodine basicity scale for the nitrogen-containing compounds are illustrated. The regularities and peculiarities of shifting bands in Raman spectra of iodine-containing organic crystals are considered. The methodical and specific problems of modeling appeared when the electronic characteristics of the iodine compounds are estimated and briefly discussed.

Evolution of crystal structures in GeTe during phase transition

We investigated changes in the crystal structure of GeTe during its phase transition. Using density functional theory (DFT) calculations, four possible crystal structures were identified: R3m, P1, Cm, and Fm3m. Among these, P1 and Cm were examined here for the first time. By calculating the internal energy of the crystal volume change, we verified that P1, R3m, and Cm can coexist in crystalline GeTe. The X-ray diffraction spectra of annealed and laser-irradiated GeTe films revealed coexisting P1 or R3m and Cm. In addition, we confirmed that Cm transforms into P1 or R3m after laser irradiation. The presence of these new structures was revealed in the crystal Raman spectra. Many of the Raman peaks in the crystalized GeTe could be explained by the coexistence of various structures. By calculating the band gaps of these structures, we also found that a structural transformation induces a change in the crystal resistance, owing to differences in the band gaps of individual structures. The generation of new crystal structures suggests a facile phase change and instability during the structural transformation.

Clustering/anticlustering effects on the GeSi Raman spectra at moderate (Ge,Si) contents: Percolation scheme vs. ab initio calculations

We test a presumed ability behind the phenomenological percolation scheme used for the basic description of the multi-mode Raman spectra of mixed crystals at one dimension along the linear chain approximation, to determine, via the Raman intensities, the nature of the atom substitution, as to whether this is random or due to local clustering/anticlustering. For doing so, we focus on the model percolation-type GeySi1−y system characterized by six oscillators {1×(Ge−Ge),3×(Ge−Si),2×(Si−Si)} and place the study around the critical compositions y ∼ (0.16, 0.71, and 0.84) corresponding to nearly matching of intensities between the like Raman modes from a given multiplet (Ge−Si triplet or Si−Si doublet). The interplay between the GeySi1−y Raman intensities predicted by the percolation scheme depending on a suitable order parameter κ of local clustering/anticlustering is found to be consistent with ab initio calculations of the GeySi1−y Raman spectra done with the Ab Initio Modeling PROgram code using large (64-, 216-, and 512-atoms) disordered cubic supercells matching the required (y,κ) values. The actual “percolation vs. ab initio” comparative insight at moderate/dilute-(Ge,Si) limits, with an emphasis on the κ-induced intra-bond transfer of oscillator strength, extends a pioneering one earlier achieved at an intermediate composition (y ∼ 0.50) by using small (32-atom) supercells [O. Pagès et al., J. Appl. Phys. 114, 033513 (2013)], mainly concerned with the inter-bond transfer of oscillator strength, providing altogether a complete picture.

Pentazole and Ammonium Pentazolate: Crystalline Hydro-Nitrogens at High Pressure.

Two new crystalline compounds, pentazole (N5H) and ammonium pentazolate (NH4)(N5), both featuring cyclo-N5- are discovered using a first-principles evolutionary search of the nitrogen-rich portion of the hydro-nitrogen binary phase diagram (NxHy, x ≥ y) at high pressures. Both crystals consist of the pentazolate N5- anion and ammonium NH4+ or hydrogen H+ cations. These two crystals are predicted to be thermodynamically stable at pressures above 30 GPa for (NH4)(N5) and 50 GPa for pentazole N5H. The chemical transformation of ammonium azide (NH4)(N3) mixed with dinitrogen (N2) to ammonium pentazolate (NH4)(N5) is predicted to become energetically favorable above 12.5 GPa. To assist in identification of newly synthesized compounds in future experiments, the Raman spectra of both crystals are calculated and mode assignments are made as a function of pressure up to 75 GPa.

Tunable UV-visible absorption of SnS2 layered quantum dots produced by liquid phase exfoliation.

4H-SnS2 layered crystals synthesized by a hydrothermal method were used to obtain via liquid phase exfoliation quantum dots (QDs), consisting of a single layer (SLQDs) or multiple layers (MLQDs). Systematic downshift of the peaks in the Raman spectra of crystals with a decrease in size was observed. The bandgap of layered QDs, estimated by UV-visible absorption spectroscopy and the tunneling current measurements using graphene probes, increases from 2.25 eV to 3.50 eV with decreasing size. 2-4 nm SLQDs, which are transparent in the visible region, show selective absorption and photosensitivity at wavelengths in the ultraviolet region of the spectrum while larger MLQDs (5-90 nm) exhibit a broad band absorption in the visible spectral region and the photoresponse under white light. The results show that the layered quantum dots obtained by liquid phase exfoliation exhibit well-controlled and regulated bandgap absorption in a wide tunable wavelength range. These novel layered quantum dots prepared using an inexpensive method of exfoliation and deposition from solution onto various substrates at room temperature can be used to create highly efficient visible-blind ultraviolet photodetectors and multiple bandgap solar cells.

Recovery and analysis of Raman spectra obtained using a static Fourier transform spectrometer

The recovery of Raman spectra in the 800-1050-nm spectral range from a prototype static Fourier-transform spectrometer is described. Distortion effects introduced by the optical system of the static Fourier-transform spectrometer are studied. A method of correcting optical distortion is suggested. The proposed algorithm is tested using a monochromatic radiation source. A two-dimensional map of period distribution in the distorted interference pattern is plotted. A mapping that enables optical distortions of the two-dimensional interferogram to be corrected is proposed. The correction results are demonstrated by plotting interference fringe profiles. A comparison of various well-known phase correction methods in order to identify an optimal algorithm is presented. A matching criterion for the obtained spectra is proposed. Raman spectrum of a stilbene crystal obtained by a diffraction spectrometer with a resolution of δν = 14 cm-1 is used as a reference.

Structure and dynamics of the Lu2Si2O7 lattice: Ab initio calculation

The ab initio calculations have been carried out for the crystal structure and Raman spectrum of a single crystal of lutetium pyrosilicate Lu2Si2O7. The types of fundamental vibrations and their frequencies and intensities in the Raman spectrum for two polarizations of the crystal have been determined. The calculations have been performed within the framework of the density functional theory (DFT) using the hybrid functionals. The ions involved in the vibrations have been identified using the method of isotopic substitution. The results of the calculations are in good agreement with the experiment.

First principle calculation of structure and lattice dynamics of Lu2Si2O7

Ab initio calculations of crystal structure and Raman spectra has been performed for single crystal of lutetium pyrosilicate Lu 2 Si 2 O 7 . The types of fundamental vibrations, their frequencies and intensities in the Raman spectrum has been obtained for two polarizations. Calculations were made in the framework of density functional theory (DFT) with hybrid functionals. The isotopic substitution was calculated for all inequivalent ions in cell. The results in a good agreement with experimental data.

Raman spectra of lithium niobate crystals heavily doped with zinc and magnesium

We have examined the Raman spectra of heavily doped lithium niobate single crystals (at close-to-threshold concentrations of doping cations): LiNbO3:Zn (4.5 mol % ZnO), LiNbO3:Mg (5.01 mol %):Fe (0.005 mol %), LiNbO3:Mg (5.1 mol %), and LiNbO3:Mg (5.3 mol % MgO). Low-intensity lines with frequencies at 209, 230, 298, 694, and 880 cm–1 have been revealed for the first time. Analysis of the data from the literature on lattice dynamics calculations from first principles (ab initio) does not make it possible to unambiguously state that these lines correspond to fundamental vibrations of the А2 symmetry species, which are forbidden for the С3V6 (R3c) space group. At the same time, ab initio calculations unambiguously indicate that the experimentally observed low-intensity “superfluous” lines with the frequencies at 104 and 119 cm–1 cannot correspond to vibrations of the А2 symmetry species. It is most likely that they correspond to two-particle states of acoustic phonons with a total wave vector equal to zero.

Boson peak in low-frequency Raman spectra of polymethylmethacrylate

The role of the local field, anharmonicity, and degree of disorder in the formation of the boson peak has been studied. The low-frequency Raman spectrum of a model polymethylmethacrylate crystal with disorder elements was calculated using the method of atom-atom potentials taking into account the effect of local fields on the spectrum. The calculations were carried out for a crystal containing rigid molecules of an isotactic polymethylmethacrylate oligomer and for a structure with flexible molecules.

Growth of LiNbO3:Er Crystals and concentration dependences of their properties

A series of lithium niobate (LiNbO3) crystals of congruent and stoichiometric compositions, doped with erbium, have been grown under non-steady-state thermal conditions. A series of LiNbO3:Zn crystals, nominally pure LiNbO3 crystals of congruent and stoichiometric compositions, and a LiNbO3:B crystal have also been grown. Both growth conditions and concentration dependences of physicochemical, ferroelectric, and structural characteristics of LiNbO3:Er crystals are investigated. The growth regular domain microstructures and periodic nanostructures in LiNbO3:Er crystals are analyzed by optical microscopy and atomic force microscopy (AFM). A comparative study of the optical homogeneity and photorefractive properties of LiNbO3:Er crystals of congruent and stoichiometric compositions and the Raman spectra of LiNbO3 crystals of different compositions is performed.

Phonon spectra of eulytite crystals Bi4M3O12 (M = Ge,Si): ab initio study

In this paper we present the results of ab initio DFT calculation of phonon spectra for bismuth ortho-germanate Bi4Ge3O12 and bismuth ortho-silicate Bi4Si3O12 crystals, in the center of the first Brillouin zone. First, the geometry optimization was performed using the analytical energy gradients, with respect to atomic coordinates and unit cell parameters. Vibrational frequencies and normal modes were calculated within the harmonic approximation by diagonalizing the mass-weighted Hessian matrix. The IR and Raman spectra of both crystals were simulated with the periodic ab initio Crystal 09 code and B3LYP hybrid functional and the two sets of Transverse-Optical and Longitudinal-Optical frequencies are generated, together with their intensities. Also, the influence of isotopic substitution for Bi, Ge and O in phonon modes and the picture with values of frequencies shift in each mode by isotopic substitution were calculated. The obtained results are discussed and the comparision between the computed spectra and experimental data is quite satisfactory, which justifies the model and simulation scheme used for the title systems.

Raman spectroscopy of SrB4O7 single crystals in the temperature range 300–1273 K

The polarized Raman spectra of SrB4O7 (SBO) single crystals are studied in detail in the temperature range of 300–1273 K. The TO, LO, and IO phonon lines of A 1, A 2, B 1, and B 2 symmetries of rhombic SBO at 300 K are identified. The behavior of the Raman spectra of SBO crystals is studied upon heating up to their melting. The relation of Raman spectra with the structure of boron–oxygen fragments, as well as the transformation of spectra in the process of melting of SBO crystals, is discussed.

Study of Raman spectra in the processes of glass formation, crystallization, and melting of strontium diborate

The behavior of the Raman spectra of SrB4O7 (SBO) and Li2B4O7 crystals upon melting and glass formation is studied by high-temperature Raman spectroscopy (HTRS). The character of formation of boron–oxygen fragments in different aggregate states of SBO is discussed based on an analysis of the Raman spectra in the crystalline, glassy, and molten states.

Phase transitions in (NH4)2MoO2F4 crystal

The mechanisms of temperature and high pressure phase transitions have been studied by Raman spectroscopy. Room temperature (295 K) experiments under high hydrostatic pressure up to 3.6 GPa for (NH4)2 MoO2 F4 have been carried out. Experimental data indicates a phase transition into a new high-pressure phase for (NH4)2 MoO2 F4 at 1.2 GPa. This phase transition is related to the ordering anion octahedron groups [MoO2 F4]2− and is not associated with ammonium group. Raman spectra of small non-oriented crystals ranging from 10 to 350 K have been observed. The experiment shows anion groups [MoO2 F4]2− and ammonium in high temperature phase are disordered. The phase transition at T1 = 269.8 K is of the first-order, close to the tricritical point. The first temperature phase transition is related to the ordering anion octahedron groups [MoO2 F4]2−. Second phase transitions T2 = 180 K are associated with the ordering of ammonium. The data presented within this study demonstrate that 2D correlation analysis combined with traditional Raman spectroscopy are powerful tool to study phase transitions in the crystals.