Phase changes of tris(glycinato)chromium(III) monohydrate crystal systematically studied by thermal analyses, XRPD, FTIR, and Raman combined with ab initio calculations.

Tris(glycinato)chromium(III) monohydrate [Cr(C2H4NO2)3·H2O] crystals were grown through the slow solvent evaporation method. The crystals were studied by Fourier transform infrared (FTIR) and Raman spectroscopy at room temperature. The assignments of vibration modes were performed using the Density Functional Theory (DFT). Thermal analyses (TGA, DTA, and DSC), X-ray diffraction (XRD), and Raman were used to study the phase changes on the crystals under high- and low-temperature conditions. Temperature-dependent XRPD measurements were carried out in the interval of 473-12 K. Several changes were observed in the patterns, like the appearance of new peaks and the disappearance of peaks occurring within 373-393 K due to water loss. In addition, the Raman measurements were performed in the 423-10 K interval. Several changes on the inter and intramolecular vibration bands during the cooling, such as decreasing bands' intensities, the appearance of vibration modes, and discontinuities on the modes' behavior, were observed. These spectral modifications occurred at about 370 K and within 120-220 K, thus, confirming that the crystals undergo two phase changes, one being structural and the other one conformational, respectively, at high- temperature and low-temperature conditions. Finally, thermal investigations corroborated the structural and vibrational results under high temperatures.

New structural phases of [bis(L-alaninato) diaqua] nickel(II) dihydrate crystal.

The study of [bis(L­alaninato) diaqua] nickel(II) dihydrate crystal using Raman scattering and X-ray diffraction as a function of temperature is reported in this paper. Thermal analysis (TGA and DSC) complementary measurements were also performed in order to obtain information on structural changes and mass loss occurred in this material. It was identified that the crystal undergoes loss of water at two different temperatures: ~340 and 393 K. X-ray diffraction measurements showed two phase transformations related to these two water loss events. After heating up to 423 K, the sample was cooled down to 298 K and its diffraction pattern presented the same pattern at 423 K, evidencing an irreversible phase transformation. The diffraction results also showed that crystal goes to monohydrate and anhydrous phases. Furthermore, cell lattice parameters and space groups of both phases were determined by applying Rietveld refinement through Le Bail method, demonstrating that their structures belong to the P21 and C2/c space groups, both with monoclinic symmetry. In addition, assignments of Raman spectra vibrational bands (at 300 K) are provided. The high-temperature Raman spectra were obtained in the 100-3500 cm-1 range, where it was observed several abrupt changes in the intensity of low-wavenumber bands and the appearance/disappearance of some vibrational modes that have coupling with OH⋯O hydrogen bonds. These spectral changes are in good agreement with X-ray diffraction and thermal analyses data. Finally, we obtained Raman measurements at low temperatures, from which we identified that the crystal structure is extremely stable throughout the temperature range of 293-10 K.

Structural, vibrational and thermal studies on bis(l-glutaminato)copper(II).

Copper(II) complexes of amino acids have been widely studied as potentials medicines and dietary supplementation, so the knowledge about the metal-ligand sites, thermal stability and behavior of these complexes is an important subject of study. Although the Raman spectroscopy could help to elucidate the nature of the interactions into crystal there are only few information about vibrational study of this compound in the literature and no data depending on the temperature. In addition, there is no temperature-dependent X-ray diffraction study of this material. We report here Raman Spectroscopy and Powder X-ray Diffraction measurements, both as a function of temperature and as a way of studying the thermal stability of the material. After the synthesis of the sample and confirmation of its crystal structure by Powder X-ray Diffraction, Raman measurements were performed in the 70-3600 cm-1 spectral region as a function of temperature from 10 up to 300 K. Some peaks become more evident during the cooling, due to a decrease in width and an increase in intensity. There is a discontinuity in the wavenumbers evolution around 110 K, that should be associated with a conformation of the structure. Optimized geometry and vibrational frequencies were obtained by means of Density Functional Theory and for the first time the analysis of the vibrational modes was done in terms of the Potential Energy Distribution. X-ray diffraction measurements as a function of temperature and Rietveld refinement showed discontinuities in the lattice parameters and degradation around 493 K (at air atmosphere) and 513 K (under vacuum). These results were corroborated by the thermal analysis which indicates that the compound is stable up to 493 K.

High pressure studies on bis(l-histidinate)nickel(II) monohydrate.

Raman spectra of bis(l-histidinate)nickel(II) monohydrate crystal were obtained for pressures up to 9.5GPa. Our results show the disappearance of some of the Raman modes and the appearance of other modes. These modifications evidence that the sample undergoes phase transitions at around 0.8 and 3.2GPa. The role played by the Ni ions and hydrogen bonds in the dynamics of the phase transitions is discussed. Under decompression, down to atmospheric pressure, the original Raman spectra are recovered, showing that both phase transitions are fully reversible.

Physicochemical analysis of Permian coprolites from Brazil.

In this paper we performed the study of two coprolites (fossilized feces) collected from the exposed levels of the Pedra de Fogo Formation, Parnaiba Sedimentary Basin, and Rio do Rasto Formation, Paraná Sedimentary Basin, both of the Palaeozoic era (Permian age). They were characterized using X-ray diffractometry, infrared, Raman and energy dispersive spectroscopy techniques in order to aid our understanding of the processes of fossilization and to discuss issues related to the feeding habits of the animals which generated those coprolites, probably cartilaginous fishes. The results obtained using a multitechnique approach showed that although these coprolites are from different geological formations, 3000km away from each other, they show the same major crystalline phases and elemental composition. The main phases found were hydroxyapatite, silica, calcite and hematite, which lead to infer that those coprolites were formed under similar conditions and produced by a similar group of carnivore or omnivore fishes.

Conformational change in the C form of palmitic acid investigated by Raman spectroscopy and X-ray diffraction.

Fatty acids are substances found in most living beings in nature. Here we report the effect of the low temperature in the vibrational and structural properties of the C form of palmitic acid, a fatty acid with 16 carbon atoms. The Raman spectra were obtained in the temperature interval from 300 to 18K in the spectral range between 30 and 3100 cm(-1). The assignment of the duly observed bands was done based on the density functional theory. On cooling, the main changes observed in the lattice mode region of the Raman spectra were interpreted as a conformational modification undergone by the palmitic acid molecules in the unit cell. The X-ray diffraction measurements were obtained from 290 to 80K showing a slight modification in the lattice parameters at about 210K. Differential scanning calorimetry (DSC) measurements were recorded between 150 and 300K and no enthalpic anomaly in the DSC thermogram was observed. These techniques provided strong evidence of the conformational change in the molecules of palmitic acid at low temperatures.

Low-temperature phase transformation studies in the stearic acid: C form.

This paper reports the temperature-dependent measurements in the C form of stearic acid. Raman scattering, X-ray diffraction, and differential scanning calorimetry measurements were performed at low temperatures. The polarized Raman spectra were measured for temperatures ranging from 8 to 300 K over the spectral range of 30-3000 cm(-1). The spectral changes observed in both the lattice vibrational modes and the internal vibrational modes regions of the Raman spectrum, allowed to identify a phase transition undergone by the stearic acid crystal occurring between 210 and 170 K and a change in the structure continues to be observed down to 8 K. The anharmonicity of some vibrational modes and the possible space groups presented by the crystal at low temperatures were also discussed. Low-temperature X-ray diffraction measurements were performed from 290 to 80 K and the results showed slight changes in the lattice parameters at ∼200 K. Furthermore, the evidence of the phase transformation was provided by the differential scanning calorimetry measurements, which identified an enthalpic anomaly at about 160 K.

KDP:Mn piezoelectric coefficients obtained by X-ray diffraction.

Crystals of pure potassium dihydrogen phosphate KH(2)PO(4) (KDP) and Mn-doped KDP (KDP:Mn) were grown from a water solution by the slow evaporation method and their piezoelectric properties were studied by X-ray diffraction methods. The results have shown an increase in the piezoelectric coefficients with the doping.