Ag2Mo3O10·2H2O nanorods under high pressure: In situ Raman spectroscopy.

This work presents a pressure-dependent behavior of silver trimolybdate dihydrate (Ag2Mo3O10·2H2O) nanorods using in situ Raman scattering. The Ag2Mo3O10·2H2O nanorods were obtained by the hydrothermal method at 140 °C for 6 h. The structural and morphological characterization of the sample was performed by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Pressure-dependent Raman scattering studies were performed on Ag2Mo3O10·2H2O nanorods up to 5.0 GPa using a membrane diamond-anvil cell (MDAC). The vibrational spectra under high pressure showed splitting and emergence of new bands above 0.5 GPa and 2.9 GPa. Reversible phase transformations under pressure were observed in silver trimolybdate dihydrate nanorods: Phase I - ambient phase (1 atm - 0.5 GPa) â†’ Phase II (0.8 GPa - 2.9 GPa) â†’ Phase III (above 3.4 GPa).

High-pressures study by Raman spectroscopy and DFT calculations of L-tyrosine hydrobromide crystal.

The high-pressure Raman spectra of L-tyrosine hydrobromide crystal (LTHBr) were obtained from 1.0 atm to 8.1 GPa in the 100-3200 cm-1 spectral region. The structural conformation and dimensions of the monoclinic unit cell were estimated using the powder X-ray diffraction (PXRD) method and Rietveld refinement using the GSAS program. At atmospheric pressure, the Raman spectrum was obtained in the spectral range of 100-3200 cm-1 and the assignment of the normal modes based on density functional theory calculations was provided. Large wavenumber shifts of modes at 106, 123, and 157 were observed, which were interpreted as the large displacement of the atoms, making the molecule a flexible structure. The change in the slope (dÉ· / dP) of these bands between the pressures of 3.0 and 4.0 GPa and the appearance of a mode of low wavenumber indicate the occurrence of a structural phase transition. A band initially observed at 181 cm-1 in the spectrum recorded at 0.7 GPa change the relative intensity with a band at 280 cm-1 (recorded at 5.8 GPa), indicating a conformational transition. In the region of the internal modes, the spectra show changes that reinforce the conformational phase transition since the bands initially at 1247 and 1264 cm-1 observed at 1.0 GPa have their intensities reversed, and at 3.0 GPa it is observed the fusion of the bands at 1264 and 1290 cm-1 (values recorded at ambient pressure). Thus, we can assume that the LTHBr crystal has undergone a structural phase transition and a conformational phase transition in the pressure range investigated.

Pressure-induced phase transition in Glycinium maleate crystal.

The multicomponent glycinium maleate single crystal was grown by the slow evaporation method. The crystal was submitted to pressures ranging from 1 atm to 5.6 GPa and Raman spectroscopy was used as a spectroscopic probe. The modifications of relative intensity bands related to the lattice modes at 0.3 GPa were associated with rearrangements of hydrogen bonds. Moreover, between 1.7 and 4.8 GPa the Raman results indicate that the crystal experience a long structural phase transition, which was confirmed by PCA analysis. DFT calculations gave us more precision in the assignments of modes. The behavior of the internal modes under pressure showed that the maleic acid molecule undergoes greater modifications than glycine amino acid. All observed modifications were reversible when the pressure was released.

Raman spectroscopy of captopril crystals under low-temperature conditions.

The polymorphism is a characteristic of several active principles, and can affect the bioavailability of a drug. Among the drugs used in the treatment of heart diseases, captopril is one of the most widely used in the world. Despite the knowledge of vibrational properties of captopril under high temperature and under high pressure, a lack of information impedes the understanding of the substance in the crystal form at low temperatures. In this research, we investigated the vibrational properties of captopril crystals under cryogenic conditions in the 300-8 K interval using Raman spectroscopy. By observing the behavior of the inter- and intra-molecular vibrations it was possible to infer that the captopril molecules suffered a rearranging into the unit cell due slight orientational changes mainly involving CH⋯O hydrogen bonds. The phenomenon occurs in a large temperature range. However, the observed changes do not suggest the occurrence of a structural phase transition and the Raman spectra indicate that the trans conformation is recorded down to the lowest temperature available in the experiments.

Lattice dynamics calculations and high-pressure Raman spectra of the ZnMoO4.

We report here the analysis of vibrational properties of the ZnMoO4 by using theoretical and experimental approaches, well as results of high pressure experiments in this system. The analysis of the lattice dynamics calculations through the classical rigid ion model, was applied to determine the mode assignment in the triclinic phase of the ZnMoO4. Additionally, the experimental high-pressure Raman spectra of the ZnMoO4 were carried out from 0 GPa up to 6.83 GPa to shed light on the structural stability of this system. The pressure-dependent studies showed that this crystal undergoes a first order phase transition at around 1.05 GPa. The Raman spectrum analysis of the new phase shows a significant change in the number of modes for the spectral range of 20-1000 cm-1. The instability of this phase occurs due to the decrease of the MoO bond lengths in the high-pressure phase, connected with tilting and/or rotations of the MoO4 tetrahedra leading to a disorder at the MoO4 sites. The second and third phase transformations were observed, respectively, at about 2.9 GPa and 4.77 GPa, with strong evidences, in the Raman spectra, of crystal symmetry change. The principal component analysis (PCA) and the hierarchical cluster analysis (HCA) were used in order to infer the intervals of pressure where the different phases do exist. Discussion about the number of non equivalent sites for Mo ions and the kind of coordination for molybdenum atoms is also furnished.

Temperature dependence Raman spectroscopy and DFT calculations of Bi2(MoO4)3.

This work reports a theoretical and experimental study on the electronic and vibrational properties of Bi2(MoO4)3. First-principle calculations were applied to increase the understanding on the properties of the chemical composition through the energy bands. The conduction band minimum (CBM) is found at the high symmetric Γ-point, while the valence-band maximum (VBM) is located between the Z and the Γ-points. Therefore, these facts confirm that the Bi2(MoO4)3 crystal is a semiconductor compound with an indirect band-gap of about 2.1 eV. Moreover, lattice dynamic properties were calculated using density functional perturbation theory (DFPT) in order to assign the experimental Raman bands. In addition, we performed temperature-dependent Raman spectroscopic studies in the Bi2(MoO4)3 crystals to obtain information on structural changes induced by effects of the temperature change. From the changes observed in the Raman spectra phase transitions at ∼ 668 and 833 K were inferred, with the last one possibly related to the disorder due to the heating process.

High-pressure studies on l,l-dileucine crystals by Raman spectroscopy and synchrotron X-ray diffraction combined with DFT calculations.

The vibrational properties of the dipeptide l-leucyl-l-leucine hydrate were investigated through Raman and infrared spectroscopy. With the aid of first principle calculations using the density functional theory, the assignment of the vibrational modes from the material was furnished. In addition, the behavior of the crystal under high pressure was investigated using Raman spectroscopy (~8 GPa) and synchrotron X-ray diffraction (~26 GPa). The results show significant changes in both the X-ray diffractogram and the Raman spectra, suggesting that l-leucyl-l-leucine hydrate undergoes a phase transition between 2.3 and 2.9 GPa. Finally, for pressures above 16 GPa the broadening of X-ray peaks suggests a disorder in the crystal lattice induced by high-pressure effects.

Phase transformation in the C form of myristic-acid crystals and DFT calculations.

In this study, the vibrational frequencies of myristic acid (CH3(CH2)12COOH) were obtained using density functional theory calculations, and the results were compared with experimental Raman and infrared data. Additionally, Raman spectra of crystalline myristic acid were recorded in the 300-20 K range. Raman spectroscopy gives important insights into the effect of low temperatures on its monoclinic phase. X-ray diffraction was performed from 298 to 133 K to provide additional information about the cryogenic behavior of the crystals. These undergo a phase transformation, which was confirmed by differential scanning calorimetry through an enthalpy anomaly observed at low temperatures. Raman spectra and X-ray diffraction refinement of the cell parameters in combination with differential scanning calorimetry at low temperatures revealed slight modifications, confirming a conformational change in the myristic acid molecules involving rearrangement of dimers within the unit cell.

Polarized Raman and Infrared Spectroscopy and ab Initio Calculation of Palmitic and Stearic Acids in the Bm and C Forms.

A complete experimental study on the vibrational properties of palmitic and stearic acids crystallized in the Bm and C forms, both belonging to the monoclinic system with the P21/a (C2h5) space group, through polarized Raman and infrared spectroscopy, is reported in this paper. Density functional theory calculations were also performed to assign the normal modes and to help in the interpretation of the experimental data. The different polarizations were compared and their influence on the spectral profiles, in both the lattice and the internal mode regions, was discussed. In general, the Raman and infrared spectra exhibit accentuated differences among the polymorphic forms, which are associated with the different molecular modifications, defined as gauche and all-trans conformations. Insights about interaction among different groups are also furnished.