Surinder M. Sharma

Magnetic behavior of iron-filled multiwalled carbon nanotubes

Using vibrating-sample magnetometry, magnetic properties of iron-filled multiwalled carbon nanotubes have been investigated. The field dependence of dc magnetization at high magnetic fields suggests that these tubes behave as a one-dimensional exchange-coupled ferromagnetic system. At 5K, the saturation magnetization (MS) of the nanowires is found to be 85emu∕g, which is much less than the expected bulk value ∼210emu∕g. The observed exchange bias, in spite of the small fraction of γ‐Fe in our samples, implies that γ‐Fe may not be the only antiferromagnetic component responsible for the exchange bias in these Fe-filled carbon nanotubes. Quantitative study on the temperature dependence of saturation magnetization, remanent magnetization and coercivity has been carried out.

The hydrogen bond under pressure

The hydrogen bonds are quite pervasive in several classes of materials. Its parameters are known to show systematic variations with hydrogen bond length, and pressure variable is thus a natural way for studying hydrogen bonded substances. In this article, we review the unifying features as obtained through several experimental and theoretical investigations. Amongst other things, it is examined whether the observed pressure-induced variations in parameters of hydrogen bonds are consistent with the co-relations known on different chemical substances at normal pressure. In particular, the controversies on variations of O–H and H- - -O pairs with pressure and symmetrization of hydrogen bond have been resolved. The effects of close packing promoted by pressure such as formation of muli-centered hydrogen bonds and steric repulsions and the way the hydrogen bonds counter these in different ways are also examined.

Pressure-Induced Structural Transformations in Bis(glycinium)oxalate

We report in situ high-pressure Raman spectroscopic as well as X-ray diffraction measurements on bis(glycinium)oxalate, an organic complex of glycine, up to 35 GPa. Several spectral features indicate that at ∼1.7 GPa it transforms to a new structure (phase II) which is characterized by the loss of the center of symmetry and the existence of two nonidentical glycine molecules. Across the transition, all the N-H···O bonds are broken and new weaker N-H···O bonds are formed. Our high-pressure X-ray diffraction studies support the possibility of a non-centrosymmetric space group P2(1) for phase II. Across 5 GPa, another reorganization of N-H···O hydrogen bonds takes place along with a structural transformation to phase III. The C-C stretching mode of oxalate shows pressure-induced softening with large reduction from the initial value of 856 to 820 cm(-1) up to 18 GPa, and further softening is hindered at higher pressures.

High Pressure Raman Spectroscopic Study of Deuterated γ-Glycine

Raman spectroscopic investigations of deuterated gamma-glycine, carried out up to 21 GPa, indicate emergence of a new phase, which is similar to the delta-phase, reported to be formed from the undeuterated gamma-glycine at 3 GPa and the transformation to this phase is complete by 6 GPa. Observed pressure -induced variations in CD2 and N-D stretching modes indicate significant changes in the hydrogen-bonding interactions. Around approximately 15 GPa, splitting of CD2 and C-C stretching modes and discontinuous changes in the slope of CO2 and N-D stretching modes indicate another structural rearrangement across this pressure. The Raman spectra of retrieved phase at ambient conditions suggest that it may be a layered structure similar to the zeta-phase reported to be formed on decompression of the nondeuterated delta-glycine.

Pressure Effects on Single Wall Carbon Nanotube Bundles

We report high pressure Raman studies on single wall carbon nanotube bundles under hydrostatic conditions using two different pressure transmitting media, alcohol mixture and pure water. The radial and tangential modes show a blue shift when SWNT bundle is immersed in the liquids at ambient pressures. The pressure dependence of the radial modes is the same in both liquids. However, the pressure derivatives dw/dP of the tangential modes are slightly higher for the water medium. Raman results are compared with studies under non-hydrostatic conditions and with recent high-pressure X-ray studies. It is seen that the mode frequencies of the recovered sample after pressure cycling from 26 GPa are downshifted by

Pressure induced amorphization of AlPO4

~7-10 cm^{-1}

Structural changes in single-walled carbon nanotubes under non-hydrostatic pressures: x-ray and Raman studies

as compared to the starting sample.

The behaviour of alpha -quartz and pressure-induced SiO2 glass under pressure: a molecular dynamical study

AlPO4 has been compressed to pressures of 16 GPa in a diamond anvil cell and its X-ray diffraction pattern studied by the energy-dispersive technique. The compound is observed to become amorphous at ∼ 12 GPa. This explains the loss of Raman spectrum of AlPO4 reported by Jayaraman and coworkers (1987).

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

Using in situ x-ray diffraction and Raman scattering techniques, we have investigated the behaviour of single-walled carbon nanotubes bundles under non-hydrostatic pressures. It is seen that the diffraction line corresponding to the two-dimensional triangular lattice in the bundles is not reversible for pressures beyond 5 GPa, in sharp contrast to earlier results under hydrostatic pressure conditions. Most interestingly, radial breathing and tangential Raman modes of the pressure-cycled samples from 21 and 30 GPa match very well with those of the starting sample. Raman and x-ray results put together clearly suggest that the ordering of tubes in the bundles is only marginally regained with a very short coherence length on decompression.

Hydrogen Bond Symmetrization in Glycinium Oxalate under Pressure.

The authors have carried out extensive molecular dynamical calculations on alpha -quartz and pressure-induced glass and have related these to the experimental observations under static and shock pressure loading. In the crystalline quartz, densification and amorphization take place sharply around 20 GPa and are related to the changes in the Si coordination. The pressure-induced glass is considerably less compressible than the fused silica, showing a gradual change in the Si coordination, and is unlike the glass studied earlier by Tse, Klug and Le Page (1992). However even this glass shows a densification, similar to that of quartz as well as fused silica. Retrieval of the four-coordinated state, in both cases, requires annealing at high temperatures. Just before amorphization of alpha -quartz, O atoms are still far from the recently proposed BCC packing.