• V SRIHARI

Articles written in Bulletin of Materials Science

• High pressure studies on core/shell amorphous carbon nanostructures

High pressure behaviour of core/shell amorphous carbon (a-C) nanostructures was studied using synchrotron X-ray diffraction (XRD) and Raman spectroscopy up to ${\sim}$25 GPa. These nanostructures having spherical a-C shells with ${\gamma}$-Fe$_2$O$_3$ nanoparticles at the core were synthesized by catalyst-assisted lamp black method. The typical size of core (${\gamma}$-Fe$_2$O$_3$)/shell (a-C) is ${\sim}$5 nm/20 nm. Comparative XRD of the core (${\gamma}$-Fe$_2$O$_3$) in opened- and closed-shell (a-C) shows that the carbon shells do not fully transmit the applied high pressure inwards, under hydrostatic conditions. Raman spectroscopy shows that the graphitic G-mode blue shifts reversibly with applied pressure with a coefficient ${\sim}$3.5 cm$^{-1}$ GPa$^{-1}$, which is ${\sim}$25 to 40% less than that for graphite and carbon nanotubes. It is suggested that a rigid and crosslinked structure of the carbon shell might be lossy for the observed pressure drop.

• Probing bipyramid microstructure of pure and Dy-doped HoMnO$_3$ using high pressure and low temperature studies

Bipyramidal distortions play the governing role in physical properties of HoMnO$_3$ multiferroics. We have carried out angle-dispersive high pressure synchrotron X-ray diffraction experiments on hexagonal HoMnO$_3$ to find the evolution of bipyramidal parameters upon external compression. It is found that the average bond-lengths and bond-angles between Mn and O atoms depict significant reduction, which results in the distortion in MnO$_5$ bipyramidal structure athigh pressures. Further, since subtle variations in these distortions are used to compare the properties of hexagonal HoMnO$_3$ manganite series, in-situ low temperature infrared spectroscopic studies of 10% Dy-doped HoMnO$_3$ have beencarried out to study the effect of further increase in R-site cationic radius on MnO$_5$ deformations. It is found that the phonon modes specifically related to the motion of Mn and O atoms in ab-plane display irregularities near magnetic transition temperatures ($T_N$ and T$_{SR}$) via spin phonon coupling. Even a small increase in average ionic radius of rare-earth atom suppresses this anomalous behaviour and follows the trend of undoped hexagonal rare-earth manganites.

• # Bulletin of Materials Science

Volume 46, 2023
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Continuous Article Publishing mode

• # Dr Shanti Swarup Bhatnagar for Science and Technology

Posted on October 12, 2020

Prof. Subi Jacob George — Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru
Chemical Sciences 2020