H. K. Poswal

Protein crystallography beamline (PX‐BL21) at Indus‐2 synchrotron

The protein crystallography beamline (PX-BL21), installed at the 1.5 T bending-magnet port at the Indian synchrotron (Indus-2), is now available to users. The beamline can be used for X-ray diffraction measurements on a single crystal of macromolecules such as proteins, nucleic acids and their complexes. PX-BL21 has a working energy range of 5-20 keV for accessing the absorption edges of heavy elements commonly used for phasing. A double-crystal monochromator [Si(111) and Si(220)] and a pair of rhodium-coated X-ray mirrors are used for beam monochromatization and manipulation, respectively. This beamline is equipped with a single-axis goniometer, Rayonix MX225 CCD detector, fluorescence detector, cryogenic sample cooler and automated sample changer. Additional user facilities include a workstation for on-site data processing and a biochemistry laboratory for sample preparation. In this article the beamline, other facilities and some recent scientific results are briefly described.

Investigation of structure and hydrogen bonding of superhydrous phase B (HT) under pressure using first-principles density functional calculations

High-pressure behaviour of superhydrous phase B (high temperature; HT) of Mg10Si3O14(OH)4 (Shy B) is investigated with the help of density functional theory-based first-principles calculations. In addition to the lattice parameters and equation of state, we use these calculations to determine the positional parameters of atoms as a function of pressure. Our results show that the compression induced structural changes involve cooperative distortions in the full geometry of the hydrogen bonds. The bond-bending mechanism proposed by Hofmeister et al. (Vibrational spectra of dense hydrous magnesium silicates at high pressure: Importance of the hydrogen bond angle, Am. Miner. 84 (1999), pp. 454–464) for hydrogen bonds to relieve the heightened repulsion due to short H- - -H contacts is not found to be effective in Shy B. The calculated O–H bond contraction is consistent with the observed blue shift in the stretching frequency of the hydrogen bond. These results establish that one can use first-principles calculations to obtain reliable insights into the pressure-induced bonding changes of complex minerals.

Low-temperature and high-pressure Raman and X-ray studies of pyrochlore Tb 2 Ti 2 O 7 : phonon anomalies and possible phase transition

We have carried out temperature- and pressure-dependent Raman and x-ray measurements on single crystals of Tb2Ti2O7. We attribute the observed anomalous temperature dependence of phonons to phonon-phonon anharmonic interactions. The quasiharmonic and anharmonic contributions to the temperature-dependent changes in phonon frequencies are estimated quantitatively using mode Gruneisen parameters derived from pressure-dependent Raman experiments and bulk modulus from high-pressure x-ray measurements. Further, our Raman and x-ray data suggest a subtle structural deformation of the pyrochlore lattice at ~9 GPa. We discuss possible implications of our results on the spin-liquid behavior of Tb2Ti2O7.

High pressure behavior of nano-crystalline CeO2 up to 35 GPa: a Raman investigation

The present paper reports the results of in situ Raman studies carried out on nano-crystalline CeO2 up to a pressure of 35 GPa at room temperature. The material was characterized at ambient conditions using X-ray diffraction and Raman spectroscopy and was found to have a cubic structure. We observed the Raman peak at ambient at 465 cm−1, which is characteristic of the cubic structure of the material. The sample was pressurized using a diamond anvil cell using ruby fluorescence as the pressure monitor, and the phase evolution was tracked by Raman spectroscopy. With an increase in the applied pressure, the cubic band was seen to steadily shift to higher wavenumbers. However, we observed the appearance of a number of new peaks around a pressure of about 34.7 GPa. CeO2 was found to undergo a phase transition to an orthorhombic α -PbCl2-type structure at this pressure. With the release of the applied pressure, the observed peaks steadily shift to lower wavenumbers. On decompression, the high pressure phase existed down to a total release of pressure.

The study of pressure induced structural phase transition in spin-frustrated Yb2Ti2O7 pyrochlore

Our in situ x-ray diffraction and Raman measurements of Yb2Ti2O7 pyrochlore show that it undergoes a reversible structural phase transition from cubic pyrochlore to a monoclinic phase at similar to 28.6 GPa. Analysis of the x-ray data shows the transition to be thermodynamically first order and the high pressure phase to be substitutionally disordered. These experimental results are supported by our first principles calculations

Pressure induced structural phase transition in triglycine sulfate and triglycine selenate

To elucidate the cause of destruction of ferroelectricity with pressure in triglycine sulfate and triglycine selenate, we have investigated these compounds with the help of Raman measurements as well as first principles total energy and structural optimization calculations. Our results show that, beyond the critical pressures, the loss of ferroelectricity in these compounds is due to the conformational change in one of the three glycine ions of these crystals. Our studies suggest that pressure induced phase transition might be of displacive nature unlike the temperature induced ferroelectric phase transition in these crystals which is known to be of order-disorder type.

In-situ energy dispersive x-ray diffraction study of the growth of CuO nanowires by annealing method

The in-situ growth of CuO nanowires was studied by Energy Dispersive X-ray Diffraction (EDXRD) to observe the mechanism of growth. The study was carried out for comparison at two temperatures—at 500 °C, the optimum temperature of the nanowires growth, and at 300 °C just below the temperature range of the growth. The in situ observation revealed the successive oxidation of Cu foil to Cu2O layer and finally to CuO layer. Further analysis showed the presence of a compressive stress in CuO layer due to interface at CuO and Cu2O layers. The compressive stress was found to increase with the growth of the nanowires at 500 °C while it relaxed with the growth of CuO layer at 300 °C. The present results do not support the existing model of stress relaxation induced growth of nanowires. Based on the detailed Transmission Electron Microscope, Scanning Electron Microscope, and EDXRD results, a microstructure based growth model has been suggested.

High-pressure studies on the properties of FeGa3 : Role of on-site Coulomb correlation

High pressure X-ray diffraction measurements have been carried out on the intermetallic semiconductor FeGa

Pressure-induced structural transformations in the low-cristobalite form of AlPO4

_3

Behaviour of NTE Material Ag3[Co(CN)6] under Pressure

and the equation of state for FeGa