P. U. Sastry

Zinc (tris) thiourea sulphate (ZTS): A single crystal neutron diffraction study

The crystal structure of ZTS has been determined by neutron diffraction with a finalR-value of 0.026. Using the structural parameters, the contributions from the structural groups to the linear optical susceptibility and linear electro-optic coefficients have been evaluated. Results showed a significant contribution from the hydrogen bonds in the structure.

Phase transition in triglycine family of hydrogen bonded ferroelectrics: An interpretation based on structural studies

Using the crystal structure, a comprehensive interpretation of the origin of ferroelectricity in the hydrogen bonded triglycine family of crystals is given. Our detailed analysis showed that the instability of nitrogen double well potential plays a driving role in the mechanism of the ferroelectric transitions in these crystals.

A facile fabrication of a uniform and homogeneous CNT–TiO2 composite: a microscopic and scattering investigation

We demonstrated a facile and robust two-step method to fabricate a carbon nanotube–titania nano composite using spray hydrolysis and a chemical vapour deposition (CVD) technique with control over the morphology and deposition chemistry of reaction. Hierarchically structured NiO doped TiO2 submicrospheres have been realized by hydrolysis of titanium alkoxide at the water–oil interface of micrometer sized droplets. Utilizing these TiO2 microspheres, a composite of TiO2–CNT has been synthesized through the CVD technique. Mesoscopic structural investigations have been carried out on composites using ultra small-angle neutron scattering, and small-angle X-ray scattering. There exist three distinct length scales in the composite microspheres. Thermal evolution of microstructure was investigated by in situ X-ray diffraction which clearly indicates an amorphous to crystalline transition of TiO2. It is established that the polymorphic phase transition of TiO2 nanoparticles is associated with the increase in its size. Microscopic studies showed that the TiO2 substrate has given a uniform growth surface for CNTs. Furthermore, their growth mechanism has been discussed.

Electrosynthesis and characterization of nanocrystalline UO2 coating from aqueous alkaline electrolyte

In this study, UO2 coating was deposited by direct current (DC) electrolysis from an aqueous electrolyte containing UO2(NO3)2 as source of uranium. Deposition parameters such as electrolyte pH, temperature and current density were optimized in order to obtain a smooth adherent UO2 coating on metallic substrates. The effect of pH, temperature and current density on the deposition rate and on faradic current efficiency (CE) was studied in detail. The study revealed that the control over pH of the electrolyte was more important in comparison to temperature and current density to obtain a smooth deposition of UO2 from this uranyl nitrate–oxalate complex electrolyte. Grazing incidence X-ray diffraction (GI-XRD) of the as-deposited coating with a broad peak confirmed amorphous to ultra-nanocrystalline deposition. Upon vacuum annealing at higher temperature, the as-deposited coating transformed into nanocrystalline fcc UO2 with the appearance of sharper (111), (220), (220), (222) XRD reflections. The average grain size obtained was 13.2 nm for the sample annealed at 700 °C. In the presence of air, the fcc UO2 coating was transformed largely into the hcp U3O8 phase with a small percentage of hcp UO3 at temperatures higher than 400 °C. The surface morphology of the as-deposited coating showed a granular morphology with the appearance of few cracks. Upon annealing at 700 °C, the film showed a network of cracks throughout the surface with numerous pores inside the UO2 matrix. Composition analysis by EDS of the film confirmed the presence of uranium and oxygen. Cross-sectional FESEM analysis of the focused ion beam (FIB) cut samples confirmed deposition of a 6 μm thick UO2 coating. Upon annealing at 700 °C, it converted into a highly porous UO2 coating with improved adhesion with the substrate.

Pressure and Temperature Dependent Structural Studies on Hollandite Type Ferrotitanate and Crystal Structure of a High Pressure Phase

The structural stability and phase transition behavior of tetragonal (I4/m) hollandite type K2Fe2Ti6O16 have been investigated by in situ high pressure X-ray diffraction using synchrotron radiation and a diamond anvil cell as well as by variable temperature powder neutron and X-ray diffraction. The tetragonal phase is found to be stable in a wider range of temperatures, while it reversibly transforms to a monoclinic (I2/m) structure at a moderate pressure, viz. 3.6 GPa. The pressure induced phase transition occurs with only a marginal change in structural arrangements. The unit cell parameters of ambient (t) and high pressure (m) phases can be related as am ∼ at, bm ∼ ct, and cm ∼ bt. The pressure evolution of the unit cell parameters indicates anisotropic compression with βa = βb ≥ βc in the tetragonal phase and becomes more anisotropic with βa ≪ βb < βc in the monoclinic phase. The pressure-volume equations of state of both phases have been obtained by second order Birch-Murnaghan equations of state, and the bulk moduli are 122 and 127 GPa for tetragonal and monoclinic phases, respectively. The temperature dependent unit cell parameters show nearly isotropic expansion, with marginally higher expansion along the c-axis compared to the a- and b-axes. The tetragonal to monoclinic phase transition occurs with a reduction of unit cell volume of about 1.1% while the reduction of unit cell volume up to 6 K is only about 0.6%. The fitting of temperature dependent unit cell volume by using the Einstein model of phonons indicates the Einstein temperature is about 266(18) K.

Mechanical and tribological properties of rhombohedral diamond-rich coatings grown on stainless steel by hot filament CVD

In this study, rhombohedral diamond-rich coatings were grown directly on stainless steel (SS) substrate and also on sputter deposited titanium nitride interlayer, by hot filament chemical vapour deposition technique. Grazing incidence X-ray diffraction measurement of the coatings confirmed the formation of polycrystalline rhombohedral phases of diamond. Field emission scanning electron microscopy revealed a jelly type morphology of the coatings. The coating deposited with titanium nitride interlayer showed improved resistance against scratch indentation, lower wear rate and had lower value of coefficient of friction in comparison to the coating deposited directly on SS.

Effect of Chemical Pressure at the Boundary of Mott Insulator to Itinerant Electron Limit Transition in Spinel Vanadates

The effect of chemical pressure on the structural, transport, magnetic and electronic properties of ZnV2O4 has been investigated by doping Mn and Co onto the Zn sites of ZnV2O4. With Mn doping the V-V distance increases and with Co doping it decreases. The resistivity and thermoelectric power data indicate that, as the V-V distance decreases, the system moves towards quantum phase transition. The transport data also indicate that the conduction is due to small polaron hopping. The chemical pressure shows a non-monotonous behaviour of charge gap and activation energy. On the other hand, when Ti is doped on the V-site of ZnV2O4, the metal metal distance decreases and, at the same time, T-N also increases.