R. Chitra

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.

Stacking interaction between homostacks of simple aromatics and the factors influencing these interactions

The intermolecular π–π stacking interactions within the homostacks (X–X type stacking) of simple aromatic molecules in crystal structures extracted from the Cambridge crystallographic database are reported here. It is found that the stacking interactions in crystals of simple aromatic hydrocarbons become important only for molecules with more than three rings. Whereas for crystals of nitrogen substituted heterocyclic aromatic molecule, the stacking interactions become important for doubly substituted single ringed molecules itself. We investigated some of the factors that affect the stacking interactions between these molecules viz the substitution of a hydrogen atom of a simple aromatic hydrocarbons and the formation of a hydrogen bonds were the nitrogen substituted heterocyclic aromatic molecule acts as a hydrogen bond acceptor. Molecular electron density parameters obtained from the theoretical quantum calculations using the package GAMESS UK are used to explain the results obtained from data mining.

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.

Investigation of the structure and properties of (Kx(NH4)1 − x)3H(SO4)2 single crystals

The influence of isomorphous replacement in the cation sublattice on the kinetics of the phase transition in single crystals of the solid solutions (Kx(NH4)1 − x)mHn(SO4)(m + n)/2 · yH2O belonging to the K3H(SO4)2-(NH4)3H(SO4)2-H2O salt system was studied. Superproton phase transitions for the end compositions of this system have been found earlier. The optical and thermal properties of crystals with the composition (K,NH4)3H(SO4)2 in the temperature range from 295 to 500 K were investigated, and the crystal structure was determined at 295 K. The results of the study and the comparison with the literature data show that the replacement of potassium atoms with ammonia leads to a fundamental change in the kinetics of the phase transition, the phase-transition temperature remaining virtually unchanged.

The role of the double-well potential seen by the amino group in the ferroelectric phase transition in triglycine sulfate

The two most important molecular movements which bring about the order–disorder ferroelectric phase transition in the hydrogen-bonded ferroelectric triglycine sulfate (TGS) are the swinging of the amino group (−NH3+) of one of its three glycine ions, namely GI, and the tunnelling of hydrogen in the hydrogen bond between its other two glycine ions, GII and GIII (GII–H–GIII). The potential function for bent hydrogen bonds is used along with the structural parameters of the TGS crystal to model the double-well potential (U) seen by the amino group (−NH3+) of GI in TGS. The ferroelectric phase transition in TGS is investigated from the point of view of the double-well instability. Results obtained are in good agreement with those obtained earlier using the Ising-type theoretical model. Correlation between the two crucial molecular movements in TGS, namely swinging of the −NH3+ group of GI and tunnelling of hydrogen in the hydrogen bond GII–H–GIII of TGS, is established.

Influence of N?H?O hydrogen bonds on the structure and properties of (K1?x(NH4)xH2PO4) proton glasses: a?single crystal neutron diffraction study

It has been known for quite some time now that proton dynamics plays a key role in the structural ferroelectric (FE)/antiferroelectric (AFE) phase transition in the crystals belonging to the potassium dihydrogen phosphate crystal family. Mixed crystals belonging to this family having the composition M(1-x)(NW(4))(x)W(2)AO(4), where M = K, Rb, Cs, W = H, D, and A = P, As, exhibit proton glass behavior due to frustration between FE and AFE ordering; these proton glasses do not undergo any structural phase change but retain their room temperature structure down to very low temperatures. Single crystal neutron diffraction investigations of four mixed crystals with composition (K(1-x)(NH(4))(x)H(2)PO(4)), where x = 0.0, 0.29, 0.67 1.0, were undertaken with the intention to investigate the effect of the local structural deviations on the overall average structure of the crystals and correlate these structural changes to the presence or absence of a structural phase transition in these crystals. Hydrogen bonding is shown to play a key role in the changing nature of the mixed crystals as the composition varies from the potassium rich ferroelectric region to the proton glass region to the ammonium rich antiferroelectric region.

Pressure induced structural phase transition in triglycine sulfate and triglycine selenate

To elucidate the cause of destruction of ferroelectricity with pressure in triglycine sulfate and triglycine selenate, we have investigated these compounds with the help of Raman measurements as well as first principles total energy and structural optimization calculations. Our results show that, beyond the critical pressures, the loss of ferroelectricity in these compounds is due to the conformational change in one of the three glycine ions of these crystals. Our studies suggest that pressure induced phase transition might be of displacive nature unlike the temperature induced ferroelectric phase transition in these crystals which is known to be of order-disorder type.

Ferroelectric phase transition in triglycine selenate: an interpretation based on its structure and its comparison with triglycine sulphate

Triglycine selenate (TGSe) is a hydrogen-bonded ferroelectric, which undergoes a structural phase transition at T c  = 295 K. It is isomorphous to triglycine sulphate (TGS) which is a very well studied order–disorder ferroelectric extensively used in infrared detection. The crucial molecular unit from the point of view of ferroelectric phase transition in these crystals is the group of one of the three glycine ions GI, which has two equivalent positions in an asymmetric unit. This group gets disordered between its two equivalent positions for temperatures above T c . The potential energy of this group as a function of the distance between its equivalent positions was modelled, and the phase transition in TGSe was interpreted using “the coupled anharmonic oscillator model” of ferroelectrics proposed by Y. Onodera. Similarities as well as differences between the ferroelectric phase transition in TGS and TGSe are discussed.

Investigation of diffraction line broadening due to compositional fluctuations in L-alanine-doped triglycine sulfate

Broadening of X-ray powder diffraction peaks as a result of compositional disorder in L-alanine-doped triglycine sulfate crystals is investigated using the Williamson-Hall method. The analysis indicates that L-alanine substitution in triglycine sulfate crystals leads to anisotropic strain in the crystal.

Effect of cationic substitution on the double-well hydrogen-bond potential in [K1−x(NH4)x]3H(SO4)2 proton conductors: a single-crystal neutron diffraction study

The structure of the mixed crystal [K1-x(NH4)x]3H(SO4)2 as obtained from single-crystal neutron diffraction is compared with the previously reported room-temperature neutron structure of crystalline K3H(SO4)2. The two structures are very similar, as indicated by the high value of their isostructurality index (94.8%). It was found that the replacement of even a small amount (3%) of K+ with NH4+ has a significant influence on the short strong hydrogen bond connecting the two SO42- ions. Earlier optical measurements had revealed that the kinetics of the superionic transition in the solid solution [K1-x(NH4)x]3H(SO4)2 are much faster than in K3H(SO4)2; this reported difference in the kinetics of the superionic phase transition in this class of crystal is explained on the basis of the difference in strength of the hydrogen-bond interactions in the two structures.