Himal Bhatt

Hydrogen Bond Symmetrization in Glycinium Oxalate under Pressure.

The study of hydrogen bonds near symmetrization limit at high pressures is of importance to understand proton dynamics in complex bio-geological processes. We report here the evidence of hydrogen bond symmetrization in the simplest amino acid-carboxylic acid complex, glycinium oxalate, at moderate pressures of 8 GPa using in-situ infrared and Raman spectroscopic investigations combined with first-principles simulations. The dynamic proton sharing between semioxalate units results in covalent-like infinite oxalate chains. At pressures above 12 GPa, the glycine units systematically reorient with pressure to form hydrogen-bonded supramolecular assemblies held together by these chains.

High pressure structural and vibrational properties of the spin-gap system Cu2PO4(OH)

The structural and vibrational properties of the spin-gapped system Cu(2)PO(4)(OH) have been investigated at room temperature under high pressure up to ~20 GPa by Raman scattering and synchrotron-based x-ray diffraction and infrared (IR) spectroscopic measurements. The orthorhombic phase (space group Pnnm, z = 4) remains stable up to at least 7 GPa where it undergoes a weakly first order structural transition (with negligible volume drop) to a monoclinic phase (space group P2(1)/n, z = 4) with an abrupt monoclinic distortion. Refinement of atomic positions has been performed for the low pressure phase. The conspicuous changes in the vibrational spectra (Raman as well as far-IR) confirm this phase transition. At further higher pressures the monoclinic angle increases rapidly and the system transforms irreversibly into a disordered phase. Detailed vibrational analyses have been performed in the orthorhombic phase and pressure-induced structural evolution has been correlated with the vibrational modes corresponding to the Cu-O bonds. A strong negative pressure dependence of hydroxyl mode frequencies (as observed from the mid-IR absorption spectra) supports the pressure-induced structural disordering at higher pressures.

Structural and optical investigations of Fe1.03Se0.5Te0.5 under high pressure

Optimally doped iron-chalcogenide superconductor Fe1.03Se0.5Te0.5 has been investigated under high pressures using synchrotron-based x-ray diffraction and mid-infrared reflectance measurements at room temperature. The superconducting transition temperature (Tc) of the same sample has been determined by temperature-dependent resistance measurements up to 10 GPa. The tetragonal phase (P4/nmm) is found to exist in phase-separated states where both the phases have remarkably high compressibility. A first-order structural transition to the orthorhombic phase (Pbnm) is reported above 10 GPa. For the tetragonal phase, a strong correlation is observed between the Fe(Se,Te)4 tetrahedral deformation and the sharp rise of Tc up to ∼ 4 GPa, above which Tc shows marginal pressure dependence at least up to 10 GPa. The evolution with pressure of the optical conductivity shows that with increasing pressure the tetragonal phase approaches towards a conventional metallic state. Above ∼ 6 GPa, the Drude term reduces drastically, indicating poor metallic character of the high-pressure orthorhombic phase.

Spectroscopic studies of glycinium oxalate under pressure

We report the first high pressure study on Glycinium oxalate complex up to 6 GPa. High pressure infrared and Raman spectroscopic measurements have been carried out in the spectral range 600 – 3600 cm−1. Most of the skeletal and internal vibrational modes lying in the complex 600 – 1200 cm−1 region have been reassigned and their high pressure behavior has been discussed. The spectra indicate subtle pressure induced transformations at 0.7 GPa and 2.5 GPa, with significant changes in the hydrogen bonding network.

Hydrogen bonds in α-oxalic acid dihydrate-A Raman spectroscopic study

We have carried out Raman spectroscopic studies on α-oxalic acid dihydrate up to ∼15 GPa. Our data analysis indicates the occurrence of a phase transition across 2.2 GPa. The softening of O-H stretching mode, associated with water molecule, implies the strengthening of the O−H−−−O hydrogen bond and possible formation of hydronium ion.

A synchrotron infrared absorption study of pressure induced polymerization of acrylamide

The hydrogen bonded dimeric structure of the model amide based molecular crystal acrylamide has been investigated under pressure using micro-spectroscopy, employing synchrotron infrared radiation up to 24GPa at room temperature. The high pressure spectra indicate systematic evolution of new features above 4GPa, which have been identified to be due to the emergence of a polymeric phase. The polymerization gets completed up to 16.8GPa and the observed changes are found to be irreversible upon the release of pressure. The behavior of NH stretching modes indicate that the uniform inter- and intra-dimeric interactions, rather than depicting a drastic reconstruction across the phase transition, show subtle modifications and become diverse in the high pressure polymeric phase.

High pressure Raman spectroscopic studies of Pt(II) complex trans-PtCl2(PEt3)2

We report here the high pressure Raman spectroscopy of the Pt(II) complex trans-PtCl2(P(C2H5)3)2 up to 5 GPa. We have analyzed the metal-ligand stretching modes as well as ligand internal vibrational modes of the complex under pressure. Many characteristic Raman modes show pressure induced splitting at pressures as low as 1 GPa. On careful analysis of the skeletal region, the new modes appeared could be corroborated with the position of corresponding modes in the infrared spectrum, thus indicating a loss of inversion symmetry in the trans- isomer.

Phase transition in metal–organic complex trans-PtCl2(PEt3)2 under pressure: insights into the molecular and crystal structure

Structural studies on Pt(II) complexes which have direct correlation with their stereochemistry and microscopic interactions are of immense technological, catalytic and pharmacological importance. Here, we present high-pressure studies on trans-PtCl2(PEt3)2 (Et = C2H5) using infrared (IR) and Raman spectroscopy combined with powder X-ray diffraction (XRD) studies. The ambient structure was solved using single-crystal XRD studies and was further optimized using density functional theory (DFT) simulations. It has been shown that subtle molecular reorientations result in structural phase transition at pressures as low as ∼0.8 GPa. The emergence of Raman active modes in the IR spectra and vice versa indicates the loss of inversion symmetry across the phase transition. The crystal structure of the high-pressure phase has been found to be non-centrosymmetric using XRD studies, which suggest a change in space group from P21/n to P21 across 0.8 GPa. The spectroscopy results also indicate strengthening of inter- and intramolecular C–H⋯Cl hydrogen bonds resulting in a hydrogen bonded supramolecular network. On further compression up to 4.7 GPa, another phase transformation has been detected. The structure was completely retrieved on release of pressure. Thus, the present findings provide sound evidence of intramolecular rearrangements playing a decisive role in tuning bonding and structural characteristics and hence the physicochemical properties of Pt(II) complexes under varying environments.

High pressure infrared spectroscopy of Pt(II) complex cis-PtCl2(PEt3)2

We report here the high pressure infrared spectroscopy of the Pt(II) complex cis- PtCl2(P(C2H5)3)2 up to 12 GPa. We have analyzed the various ligand related vibrational modes of the complex under pressure. It has been observed that the cis- isomer, which preserved its structure on lowering the temperature, shows change in the rate of variation in vibrational frequencies at ~1 GPa. However, no new modes appeared upon compression, unlike in the trans isomer, up to the highest pressure. These observations indicate subtle pressure induced structural changes taking place in cis- PtCl2(P(C2H5)3)2. The highly complex C-H stretching spectral region has also been analyzed and the frequency variation of various modes has been described.

In situ high pressure study of an elastic crystal by FTIR spectroscopy

We performed an in situ high pressure FTIR spectroscopic study on a 2,3-dichlorobenzylidine-4-bromoaniline (DBA) crystal at pressures ranging from ambient pressure to 13.8 GPa at room temperature. The variations in the stretching frequency of the aromatic C–H, H–CN and C–Cl bands on compression showed significant molecular movement in the DBA crystal. Decompression was monitored on the aliphatic and aromatic C–H stretches which clearly show the reversibility of the molecular movements in the crystal lattice.