C. M. R. Remédios

Effect of Ni(II) doping on the structure of L-histidine hydrochloride monohydrate crystals

In this paper, we study the effect of Ni(II) doping on the structure of L-histidine hydrochloride monohydrate crystals using x-ray diffraction and Raman spectroscopy. X-ray powder diffraction shows no significant change in the unit cell parameters of the doped single crystal, whereas x-ray multiple diffraction using synchrotron radiation indicates that the Ni ions are located in interstitial positions in the crystal lattice. The temperature-dependent Raman spectra reveal a structural phase transition in the 10–300 K temperature range. The proposed mechanism of this phase transition supports the suggestion that the Ni ions occupy interstitial positions.

X-ray phase measurements as a probe of small structural changes in doped nonlinear optical crystals

X-ray multiple diffraction experiments with synchrotron radiation were carried out on pure and doped nonlinear optical crystals: NH4H2PO4 and KH2PO4 doped with Ni and Mn, respectively. Variations in the intensity profiles were observed from pure to doped samples, and these variations correlated with shifts in the structure factor phases, also known as triplet phases. This result demonstrates the potential of X-ray phase measurements to study doping in this type of single crystal. Different methodologies for probing structural changes were developed. Dynamical diffraction simulations and curve fitting procedures were also necessary for accurate phase determination. Structural changes causing the observed phase shifts are discussed.

Absolute refinement of crystal structures by X-ray phase measurements

A pair of enantiomer crystals is used to demonstrate how X-ray phase measurements provide reliable information for absolute identification and improvement of atomic model structures. Reliable phase measurements are possible thanks to the existence of intervals of phase values that are clearly distinguishable beyond instrumental effects. Because of the high susceptibility of phase values to structural details, accurate model structures were necessary for succeeding with this demonstration. It shows a route for exploiting physical phase measurements in the crystallography of more complex crystals.

Analyzing structure factor phases in pure and doped single crystals by synchrotron X-ray Renninger scanning

X-ray multiple diffraction has been applied to study the substitutional incorporation of Mg2+ ions into NSH crystals (nickel sulfate hexahydrate, NiSO4·6H2O). Intensity profiles provide information on invariant phases, while angular positions of the multiple diffractions allow accurate determination of lattice parameters. By increasing the atomic disordering only of O2− sites in model structures of doped NSH, the sense and magnitude of induced phase shifts match those necessary to justify the observed changes in the intensity profiles. Causes of disordering and lattice parameter variation are discussed. Although the amount of extra oxygen disordering is relatively large with respect to the small difference in the ionic radii of the metallic ions, this disordering is beyond the resolution power achievable by analyzing diffracted intensities of isolated reflections, such as in standard crystallographic techniques.

X-ray dynamical diffraction in amino acid crystals: a step towards improving structural resolution of biological molecules via physical phase measurements

X-ray phase measurements have been applied to study hydrogen bonds and radiation damage in amino acid crystals.

New approach for piezoelectric resonances and phase transitions in KDP and ADP crystals

In this work we introduce a new experimental setup, called here the three-electrode system (TES), which is very useful for analyzing and comparing piezoelectric crystals, as well as for studying their phase transitions and measuring their piezoelectric coefficients. ADP and KDP, which are very well-known crystals, were used in this work as good examples of piezo-crystals. The principal characteristic of TES is the application of an electric field to a small region of the crystal and the observation of the electric field at another region, usually located at the extremes of the crystal. In this way we can see several interesting effects, like piezoelectric resonances, phase transitions and harmonic generation, which are dependent on voltage and frequency conditions. We consider the piezoelectric concept in developing a mathematical model for the generated elastic wave, starting at the excitation electrode and traveling to the second electrode, where the signal depends on the piezoelectric coefficient. Our results have shown that this technique is very efficient for the determination of phase transitions.

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.

Low-temperature Raman spectra of the 2-(α-methylbenzylamino)-5-nitropyridine crystal

The polar organic 2-(α-methylbenzylamino)-5-nitropyridine crystal (MBANP) has been studied by Raman spectroscopy at low temperatures (from 300 to 10K). The effect of temperature change on the vibrational spectrum is discussed with the aid of DFT calculations. The behavior of the Raman spectra indicates that MBANP molecules present a different conformation at low temperatures associated with the rotation of the phenyl and pyridine rings. Temperature-dependent X-ray measurements and differential scanning calorimetry (DSC) analysis were utilized as complementary techniques to investigate the structural stability of MBANP crystal.

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.