Comprehensive characterization of the structure of Zr-based metallic glasses.

Structure of metallic glasses fascinates as the generic amorphous structural template for ubiquitous systems. Its specification necessitates determination of the complete hierarchical structure, starting from short-range-order (SRO) → medium-range-order (MRO) → bulk structure and free volume (FV) distribution. This link has largely remained elusive since previous investigations adopted one-technique-at-a-time approach, focusing on limited aspects of any one domain. Reconstruction of structure from experimental data inversion is non-unique for many of these techniques. As a result, complete and precise structural understanding of glass has not emerged yet. In this work, we demonstrate the first experimental pathway for reconstruction of the integrated structure, for Zr 67 Ni 33 and Zr 52 Ti 6 Al 10 Cu 18 Ni 14 glasses. Our strategy engages diverse (× 7) multi-scale techniques [XAFS, 3D-APT, ABED/NBED, FEM, XRD, PAS, FHREM] on the same glass. This strategy complemented mutual limitations of techniques and corroborated common parameters to generate complete, self-consistent and precise parameters. Further, MRO domain size and inter-void separation were correlated to identify the presence of FV at MRO boundaries. This enabled the first experimental reconstruction of hierarchical subset: SRO → MRO → FV → bulk structure. The first ever image of intermediate region between MRO domains emerged from this link. We clarify that determination of all subsets is not our objective; the essence and novelty of this work lies in directing the pathway towards finite solution, in the most logical and unambiguous way.

Band-Gap Energy and Electronic d-d Transitions of NiWO4 Studied under High-Pressure Conditions.

We report an extensive study of the optical and structural properties of NiWO4 combining experiments and density functional theory calculations. We have obtained accurate information on the pressure effect on the crystal structure determining the equation of state and compressibility tensor. We have also determined the pressure dependence of the band gap finding that it decreases under compression because of the contribution of Ni 3d states to the top of the valence band. We report on the sub-band-gap optical spectrum of NiWO4 showing that the five bands observed at 0.95, 1.48, 1.70, 2.40, and 2.70 eV correspond to crystal-field transitions within the 3d8 (t2g6eg2) configuration of Ni2+. Their assignment, which remained controversial until now, has been resolved mainly by their pressure shifts. In addition to the transition energies, their pressure derivatives are different in each band, allowing a clear band assignment. To conclude, we report resistivity and Hall-effect measurements showing that NiWO4 is a p-type semiconductor with a resistivity that decreases as pressure increases.

Assessing the prospect of XAFS experiments of metalloproteins under in vivo conditions at Indus-2 synchrotron facility, India.

The feasibility of X-ray absorption fine-structure (XAFS) experiments of ultra-dilute metalloproteins under in vivo conditions (T = 300 K, pH = 7) at the BL-9 bending-magnet beamline (Indus-2) is reported, using as an example analogous synthetic Zn (0.1 mM) M1dr solution. The (Zn K-edge) XAFS of M1dr solution was measured with a four-element silicon drift detector. The first-shell fit was tested and found to be robust against statistical noise, generating reliable nearest-neighbor bond results. The results are found to be invariant between physiological and non-physiological conditions, which confirms the robust coordination chemistry of Zn with important biological implications. The scope of improving spectral quality for accommodation of higher-shell analysis is addressed.

Adapting a continuous flow cryostat and a plate DAC to do high pressure Raman experiments at low temperatures.

We present a method for modifying a continuous flow cryostat and a steel plate DAC (Diamond Anvil Cell) to perform high pressure micro-Raman experiments at low temperatures. Despite using a steel DAC with a lower specific heat capacity (∼335 J/kg K), this setup can routinely perform high pressure (∼10 GPa) measurements at temperatures as low as 26 K. This adaptation is appropriate for varying the temperature of the sample while keeping it at a constant pressure. We determined that the temperature variation across the sample chamber is about 1 K using both direct temperature measurements and finite element analysis of the heat transport across the DAC. We present Raman spectroscopy results on elemental selenium at high pressures and low temperatures using our modified setup.

First high-pressure XAFS results at the bending-magnet-based energy-dispersive XAFS beamline BL-8 at the Indus-2 synchrotron facility.

The static focusing optics of the existing energy-dispersive XAFS beamline BL-8 have been advantageously exploited to initiate diamond anvil cell based high-pressure XANES experiments at the Indus-2 synchrotron facility, India. In the framework of the limited photon statistics with the 2.5 GeV bending-magnet source, limited focusing optics and 4 mm-thick diamond windows of the sample cell, a (non-trivial) beamline alignment method for maximizing photon statistics at the sample position has been designed. Key strategies include the selection of a high X-ray energy edge, the truncation of the smallest achievable focal spot size to target size with a slit and optimization of the horizontal slit position for transmission of the desired energy band. A motor-scanning program for precise sample centering has been developed. These details are presented with rationalization for every step. With these strategies, Nb K-edge XANES spectra for Nb2O5 under high pressure (0-16.9 GPa) have been generated, reproducing the reported spectra for Nb2O5 under ambient conditions and high pressure. These first HPXANES results are reported in this paper. The scope of extending good data quality to the EXAFS range in the future is addressed. This work should inspire and guide future high-pressure XAFS experiments with comparable infrastructure.

Remarkable resilience of the formate cage in a multiferroic metal organic framework material: dimethyl ammonium manganese formate (DMAMnF).

Dimethyl ammonium (DMA) metal formate, an important member of the dense metal organic framework (MOF) family, is known to exhibit a low temperature ferroelectric transition, caused by the ordering of the hydrogen bonds. In this study, we probed the effect of pressure on the disordered hydrogen bond and the HCOO- linkers of DMA manganese formate, with the help of XRD, IR and Raman spectroscopic studies up to ∼20 GPa. We observed that though a phase transition was initiated at ∼3.4 GPa, it was complete only by 6 GPa, indicating its first order nature. Beyond 7 GPa, this compound becomes highly disordered and shows an almost amorphous character, indicating a total collapse of the formate network. The reversibility of the initial structure of DMAMnF on the release of pressure from 20 GPa (i.e. from a highly disordered phase) shows the remarkable resilience of the formate cage. At the first crystal to crystal transition at 3.4 GPa, the distortion of the formate cage causes the ordering of the dynamically disordered hydrogen bond, resulting in a rearrangement of the DMA+ cation. Lifting of the mutual exclusivity of the Raman and IR modes (C-H out of plane and O-C-O bending modes) of HCOO- linkers, at this transition, indicates that the high pressure phase may be non-centro-symmetric.

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 ßa = ßb ≥ ßc in the tetragonal phase and becomes more anisotropic with ßa ≪ ßb < ßc 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.

Proton transfer aiding phase transitions in oxalic acid dihydrate under pressure.

Oxalic acid dihydrate, an important molecular solid in crystal chemistry, ecology and physiology, has been studied for nearly 100 years now. The most debated issues regarding its proton dynamics have arisen due to an unusually short hydrogen bond between the acid and water molecules. Using combined in situ spectroscopic studies and first-principles simulations at high pressures, we show that the structural modification associated with this hydrogen bond is much more significant than ever assumed. Initially, under pressure, proton migration takes place along this strong hydrogen bond at a very low pressure of 2 GPa. This results in the protonation of water with systematic formation of dianionic oxalate and hydronium ion motifs, thus reversing the hydrogen bond hierarchy in the high pressure phase II. The resulting hydrogen bond between a hydronium ion and a carboxylic group shows remarkable strengthening under pressure, even in the pure ionic phase III. The loss of cooperativity of hydrogen bonds leads to another phase transition at ∼ 9 GPa through reorientation of other hydrogen bonds. The high pressure phase IV is stabilized by a strong hydrogen bond between the dominant CO2 and H2O groups of oxalate and hydronium ions, respectively. These findings suggest that oxalate systems may provide useful insights into proton transfer reactions and assembly of simple molecules under extreme conditions.

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.

The role of Jahn-Teller distortion in insulator to semiconductor phase transition in organic-inorganic hybrid compound (p-chloroanilinium)2CuCl4 at high pressure.

(p-Chloroanilinium)2CuCl4(C2H14Cl6CuN2) is from an important family of organic-inorganic layered hybrid compounds which can be a possible candidate for multiferroicity. In situ high pressure FTIR, Raman and resistivity measurements on this compound indicate the weakening of Jahn-Teller distortion and the consequent removal of puckering of the CuCl6(4-) octahedra within the layer. These effects trigger insulator to semiconductor phase transition along with a change in the sample colour from yellow to dark red. This article explains the crucial role of the anisotropic volume reduction of the CuCl6(4-) octahedron (caused due to the quenching of Jahn-Teller distortion) in the observed insulator to semiconductor phase transition.

Synthesis, antifungal activity and structure-activity relationships of vanillin oxime-N-O-alkanoates.

Vanillin oxime-N-O-alkanoates were synthesized following reaction of vanillin with hydroxylamine hydrochloride, followed by reaction of the resultant oxime with acyl chlorides. The structures of the compounds were confirmed by IR, 1H, 13C NMR and mass spectral data. The test compounds were evaluated for their in vitro antifungal activity against three phytopathogenic fungi Macrophomina phaseolina, Rhizoctonia solani and Sclerotium rolfsii by the poisoned food technique. The moderate antifungal activity of vanillin was slightly increased following its conversion to vanillin oxime, but significantly increased after conversion of the oxime to oxime-N-O-alkanoates. While vanillin oxime-N-O-dodecanoate with an EC50 value 73.1 microg/mL was most active against M. phaseolina, vanillin oxime-N-O-nonanoate with EC50 of value 66.7 microg/mL was most active against R. solani. The activity increased with increases in the acyl chain length and was maximal with an acyl chain length of nine carbons.

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.

Raman and x-ray diffraction investigations on BaMoO(4) under high pressures.

X-ray diffraction and Raman scattering studies on the scheelite structured barium molybdate show that, at ∼5.8 GPa, it undergoes a first order phase transition to the fergusonite structure (I 2/a,Z = 4)-as also observed in iso-structural barium tungstate. At still higher pressures, barium molybdate transforms to another phase between ∼7.2 and 9.5 GPa. On release of pressure from 15.8 GPa, the initial phase is recovered, implying that the observed structural modifications are reversible.

Pressure-induced structural phase transformations in silicon nanowires.

High-pressure structural behavior of silicon nanowires is investigated up to approximately 22 GPa using angle dispersive X-ray diffraction measurements. Silicon nanowires transform from the cubic to the beta-tin phase at 7.5-10.5 GPa, to the Imma phase at approximately 14 GPa, and to the primitive hexagonal structure at approximately 16.2 GPa. On complete release of pressure, it transforms to the metastable R8 phase. The observed sequence of phase transitions is the same as that of bulk silicon. Though the X-ray diffraction experiments do not reveal any size effect, the pressure dependence of Raman modes shows that the behavior of nanowires is in between that of the bulk crystal and porous Si.