Nandini Garg

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.

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

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.

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 phase transformations in α-AlPO4: an x-ray diffraction investigation

We have re-investigated the high pressure behaviour of berlinite AlPO4 (α-AlPO4) with x-ray diffraction using a powerful synchrotron x-ray source SPring-8. Our results show that it transforms to a crystalline phase beyond ~13 GPa. Our data seem to be consistent with a CrVO4 type of structure in the Cmcm space group, similar to the high pressure phase observed in some isostructural phosphate compounds. The persistence of the diffraction pattern up to 40 GPa establishes that the previously accepted amorphization of AlPO4 around 12-18 GPa is incorrect. Experimental results suggest that the so-called memory glass effect observed earlier may in fact be the reversibility of the α-phasecrystalline phase transformation. Comparisons of our experimental and theoretical results raise serious doubts about the theoretical understanding of the high pressure behaviour of α-AlPO4.

Pressure induced crystallization in amorphous silicon

We have investigated the high pressure behavior of amorphous silicon (a-Si) using x-ray diffraction and Raman scattering techniques. Our experiments show that a-Si undergoes a polyamorphous transition from the low density amorphous to the high density amorphous phase, followed by pressure induced crystallization to the primitive hexagonal (ph) phase. On the release path, the sequence of observed phase transitions depends on whether the pressure is reduced slowly or rapidly. Using the results of our first principles calculations, pressure induced preferential crystallization to the ph phase is explained in terms of a thermodynamic model based on phenomenological random nucleation and the growth process.

A molecular dynamical investigation of high pressure phase transformations in berlinite (alpha-AlPO4)

Since 1990, berlinite ( -AlPO4 ) has been believed to have a memory glass property under high pressures. Recent high pressure Raman scattering experiments have raised serious doubts in our understanding of the high pressure behaviour of -AlPO4 . We have now carried out extensive molecular dynamical calculations to understand the nature of structural changes in -AlPO4 under high pressures. Our simulations show that around 15 GPa the oxygen sublattice becomes disordered and the intensities of the Bragg diffraction peaks are reduced. High pressure causes a monotonic increase in the distortion of the AlO4 and PO4 tetrahedra; and, as observed in earlier MD calculations, -AlPO4 undergoes a first order phase transformation at ~30 GPa to a disordered structure. However, even beyond 30 GPa, the calculated diffraction pattern of this phase continues to show sharp diffraction peaks. At higher compression, this diffraction pattern shows a systematic reduction in the intensity and beyond 45 GPa, most of the peaks vanish except (10 = 12) and (10 = 14). These calculations show the persistence of translational order well beyond the generally accepted pressure of amorphization and support the recent Raman scattering results. Further, this disordered phase does not transform to any new crystalline phase on annealing at high pressures. Our simulations employing instantaneous compression confirm the earlier result that, beyond 12 GPa, the Cmcm phase is more stable than the -phase. However, this phase transforms to a four coordinated disordered phase at ambient conditions and can only be stabilized on compression beyond 20 GPa. Our results, presented here, strongly suggest the need for a re-investigation of -AlPO4 by x-ray diffraction under high pressures.

Equation of state of scheelite-structured ZrGeO4 and HfGeO4

The high-pressure behaviour of scheelite-structured ZrGeO4 and HfGeO4 has been investigated with the help of angle-dispersive powder x-ray diffraction measurements. Our results show that these compounds do not undergo any phase transition up to the pressures of 20.7 and 19.0 GPa, respectively. The isothermal bulk modulus and its pressure derivatives are found to be 238 GPa and 4.5 for ZrGeO4, and 242 GPa and 4.8 for HfGeO4, implying that these germanates are highly incompressible.

High pressure structural stability of BaLiF3

High pressure x-ray diffraction studies on inverse-perovskite BaLiF3 show that this compound is structurally stable up to ∼50 GPa. The bulk modulus of BaLiF3 is determined to be 75.9 GPa which is in close agreement with that determined from a semi-empirical formulation. Our ab initio calculations show that among the three alkaline earth fluoro perovskites (ALiF3, A = Ba, Ca, and Sr) which crystallize in the inverse-perovskite structure, BaLiF3 is the least brittle at ambient conditions and also that the degree of brittleness decreases at high pressures. The behavior of the elastic constants at high pressure accompanied by a reduction in the bandgap indicates a decrease in the directional nature of the bonding.

Search for a precursor crystal-to-crystal phase transition to amorphization in α-GeO2 and α-AlPO4 under pressure

Recent X-ray diffraction studies on α-quartz (SiO2) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced amorphization, α-GeO2 and α-AlPO4 (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO2.

Classical molecular dynamical simulations of high pressure behavior of alpha cristobalite (SiO2)

Static and dynamic high pressure experiments reveal that the behavior of α-cristobalite (SiO 2 ) depends on the rate and the nature of stress loading. To understand this behavior we have carried out extensive molecular dynamics simulations. The response to rapid and simultaneous increase of the pressures and temperatures to the expected Hugoniot values was calculated. These simulations along the simulated Hugoniot path revealed that alpha cristobalite transforms to a new six coordinated phase beyond 16 GPa, 1410.3 K. We find that between 16 and 18 GPa, even after equilibrating for a very long time, the disorder in the stishovite-like phase persists. On release of pressure, this disordered state is retained, in agreement with the experimental observations. Simulations under slow pressure increase show that the observation of Cmcm is sensitive to pressure steps and that stishovite phase arises through the Cmcm phase.