Search

Hydrogen bonds are one of the most enigmatic intermolecular interactions where the strength of interaction depends very intimately on the local environment of the bonded atoms. One can change the e...

Studies on α-nickel sulfate hexahydrate (NSH) crystals grown under different conditions are undertaken to investigate how changes in growth conditions affect crystal properties and whether or not there is any modification of the average crystal structure due to changes in crystallization conditions. Thermogravimetric and microhardness studies were carried out on the crystals grown from two different aqueous solutions, one of them containing an excess of sulfuric acid. Raman spectra were recorded and a single-crystal neutron diffraction investigation was conducted on both crystals. A detailed comparison between the two crystal structures and their Raman spectra showed that, although the two crystal structures are very similar, there are slight differences, such as the change in unit-cell volume, differences in the ionic structure, particularly of the sulfate ions, and changes in the hydrogen-bonding network. During solution crystal growth of a salt like NSH, varying the ionic environment around the solute ions influences the interionic interactions between them. Hence it is suggested that the above-mentioned structural differences result from a fine-tuning of the interionic interaction between the cations and anions of NSH in the solution phase. This difference is finally carried over to the crystalline phase. The resulting small crystal structure differences are enough to produce measurable changes in the thermal stability and fragility of the crystals. These differences in crystal properties can be explained on the basis of the observed structural differences between the two crystals grown under different conditions.

Single-crystal neutron diffraction investigation on Cs4(HSO4)3(H2PO4) and Cs6H(HSO4)3(H2PO4)4 superprotonic crystals, belonging to CsHSO4–CsH2PO4–H2O salt system, is under taken to elucidate the precise hydrogen atom positions in these crystals. The investigation revealed that these crystals are very sensitive to the ambient conditions and can undergo deterioration due to fluctuation in air moisture content. Cs6H(HSO4)3(H2PO4)4 crystals are more stable as compared to Cs4(HSO4)3(H2PO4). Crystal structure of Cs6H(HSO4)3(H2PO4)4 is obtained both before and after deterioration, it is found that the asymmetric O–HO hydrogen bond between the PO4 and SO4 ions of this crystal becomes stronger after deterioration. This led to the shrinkage of the unit cell, and most likely prevented further deterioration. Diabatic state model for hydrogen bonds is used to obtain the energy contour for the O–H···O hydrogen bond of Cs6H(HSO4)3(H2PO4)4 crystal. The influence of this change in the hydrogen bonding on the proton conduction ability of the crystal is discussed.

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 SO4 2− 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.

A new salt, 4-aminopyridinium 3-(4-aminopyridinium) succinate tetra hydrate was synthesized from 4-aminopyridine and maleic acid in 2:1 stoichiometric ratio. The salt was characterised using single crystal X-ray diffraction, Raman scattering and DSC/DTA techniques. Intermolecular interactions in the salt investigated through Hirshfeld surfaces and 2D finger plots, showed that the C..H, N..H, O..H and H…H interactions were 15.4%, 2.2%,37.6% and 41.3% respectively. The water molecules occupy around 19.2% of the total volume of the unit cell and also contribute significantly to the stability of the crystal. As the salt crystallizes in a non-centerosymmetric space group, it can exhibit second order non-linear optical properties. MOPAC was used to calculate the hyperpolarizability, it was found to be 3.29378 10−30 esu. A database analysis to compare the bond parameters and the physical properties like hyperpolarizability etc. in 4-aminopyridine and its complexes was undertaken. It was found that, in general, complexation or salt formation leads to higher average polarizability, hyperpolarizability and dipole moment, and our salt also has higher hyperpolarizablilty than that of parent compound 4-aminopyridine.

l -histidine tetra fluroborate (L-HTFB) is a semi-organic NLO material. Ab-initio computations were performed at CAM-B3LYP/ 6–311++g(d,p) level of theory to arrive at the structures, energies, and vibrational wave numbers. The experimental FT-IR and FT-Raman spectra of L-HTFB have been recorded and analyzed. It is compared with the simulated spectra. The scaled wave numbers obtained are in good agreement with the experimental values. Hirshfeld surface analysis represented in the 2D fingerprint plot reveals the interaction within the compound. Optimized geometry reveals that the complexes l -histidine (L-H) and tetrafluoroborate (TFB) are linked by H-F bond which provides inter and intra molecular hydrogen bonded interactions such as N10-H16⋯F25, N11-H17⋯F22, N11-H14⋯F24, N11-H14⋯N10 and N11-H15⋯O12. The inter-molecular distances H17⋯F22, H14⋯F24 and H16⋯F25 are in between 1.82 A and 2.53 A. The intra molecular distances H14⋯N10 and H15⋯O12 are found to be at 2.17 A and 1.71 A which stabilizes lone pair electron of N10 and O12 with energies 11.89 and 59.91 KJ/mol respectively. The intra and inter molecular hyperconjugations responsible for the stability of the molecule are well identified theoretically using the NBO analysis. Third order optical nonlinearity is measured by means of the open aperture Z-scan technique which reveals that the crystal has optical limiting property. Photoluminescence results clearly indicate the use of L-HTFB as a new violet-light emitting material.

Potassium dihydrogen phosphate (KDP) family of hydrogen-bonded crystal constitutes an important family of crystals not only because of their immense importance in the field of nonlinear optics (NLOs) but also due to the fact that hydrogen bonds of these crystals provide us with a rare opportunity to understand the fundamental nature of hydrogen bonds, such as the effect of local chemical environment on the strength of hydrogen bonds and nuclear quantum effect on strong, moderate and weak hydrogen bonds. Keeping this aim in mind, we have undertaken detailed single-crystal neutron diffraction (SCND) investigations on ammonium dihydrogen phosphate (ADP) and KDP along with their deuterated analogue crystals under ambient conditions. Fine differences in the hydrogen bonds of the above-mentioned crystals are analysed in the light of a simple diabatic two-state theoretical model for hydrogen bonds. It is proposed that the presence of a partially covalent N–H $$_\mathrm{N}$$ –O bond in ADP has a very significant effect on its O–H bond making it highly anharmonic. It is this higher bond anharmonicity in ADP that is most likely responsible for its larger NLO coefficient compared to KDP.

One of the most intriguing facts about hydrogen bonds is that bonds formed between the same couple of donor and acceptor atoms can have large variation in the geometry as well as energies; this is mainly because hydrogen bonds are highly sensitive to their environment. We report here a comparative study of neutron structures of mixed crystals of potassium–ammonium dihydrogen phosphate. Basic structural framework of crystals belonging to potassium dihydrogen phosphate family is constructed by a network of zigzag O–H–O hydrogen bonded chains of $$\hbox {PO}_{2}\hbox {(OH)}_{2}$$ anions, with cations occupying the interspaces of the zigzagged anionic chains. These crystals having simple crystal structure primarily determined by the O–H–O hydrogen bonds connecting $$\hbox {PO}_{2}\hbox {(OH)}_{2}$$ anions provide us with a unique opportunity to tune the hydrogen bond geometry by varying the cationic occupancy and study the effect of this change on the average structure of the crystal. The cation–anion interaction is found to have a direct influence on the O–H–O hydrogen bonds of the mixed 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.

A new third order nonlinear optical single crystal of Morpholin-4-ium hydrogen tartrate has been grown by the slow evaporation method. The lattice parameters and crystalline purity of this orthorhombic crystal were measured by single crystal X-ray diffraction and powder X-diffraction technique respectively. The optimized geometry shows that the morpholinium ring exist in a chair form where tartrate anions are linked via N H⋯O hydrogen bonds, forming chains. These chains are linked via N H⋯O and O H⋯O hydrogen bonds, involving the morpholinium cation. The vibrational spectral investigation predicts that the red shifting of O H and N H stretching wavenumbers are due to O H⋯O and N H⋯O intermolecular interactions, respectively which are also evident form NBO analysis. The photoluminescence spectrum shows luminescence at the blue region with an appreciable lifetime leading to applications in blue OLEDs. Third order nonlinear optical properties like self-defocusing, saturable absorption and optical limiting at 532 nm are identified by Z-scan analysis. The laser damage threshold (LDT) energy have been measured by using Nd:YAG laser of wavelength 532 nm. The field dependent nature of first and second order hyperpolarizabilities and luminescence properties suggest that MHT crystal is a potential candidate for OLED and nonlinear optical applications.

Protonated crystals of glycine silver nitrate (C4H10Ag2N4O10) undergo a displacive kind of structural phase transition to a ferroelectric phase at 218 K. Glycine silver nitrate (GSN) is a light-sensitive crystal. Single-crystal X-ray diffraction investigations are difficult to perform on these crystals due to the problem of crystal deterioration on prolonged exposure to X-rays. To circumvent this problem, single-crystal neutron diffraction investigations were performed. We report here the crystal structure of GSN in a ferroelectric phase. The final R value for the refined structure at 150 K is 0.059. A comparison of the low-temperature structure with the room-temperature structure throws some light on the mechanism of the structural phase change in this crystal. We have attempted to explain the structural transition in GSN within the framework of the vibronic theory of ferroelectricity, suggesting that the second-order Jahn-Teller (pseudo-Jahn-Teller) behavior of the Ag(+) ion in GSN leads to structural distortion at low temperature (218 K).

Hydrogen bonding interactions are one of the most important chemical interactions among materials, especially biological materials, which help confer specificity, which is crucial for their efficient functioning. Recently, low-barrier hydrogen bonds (LBHBs) have been proposed to play a critical role in enzyme catalysis. In this review, tools to identify LBHBs are described, along with analyses of neutron crystal structures of small molecules to identify geometric parameters characteristic of LBHBs, which are assumed to be characterized by dynamic disorder along the hydrogen bond (H-bond) of the bonding hydrogen atom. The analysis of protein structures determined by neutron diffraction indicates that LBHBs are found to occur in both active site and non-active site regions of a protein. Moreover, very short H-bonds are observed in the vicinity of folding cores identified through nuclear magnetic resonance studies on two proteins, SUMO-1 and HIV-1 protease. This observation suggests that LBHBs may also be im...

Crystals of a new salt in 2:1 ratio of 2-aminopyridine and malonic acid are grown by slow evaporation. These crystals of bis(2-aminopyridinium) malonate are orthorhombic and belong to the non-centrosymmetric space group, Fdd2 with parameters a = 22.0786(6), b = 23.0218(6), c = 5.5595(1)angstrom and Z=8 at 300 K. The crystals are isostructural to those of bis(2-aminopyridinium) maleate, which is a NLO material. The isostructurality index between bis(2-aminopyridinium) maleate and bis(2-aminopyridinium) malonate was also calculated. The hyperpolarizability calculated using semi empirical method MOPAC2009 showed that bis(2-aminopyridinium) malonate has slightly higher beta value compared to that of bis(2-aminopyridinium) maleate. (C) 2011 Elsevier B.V. All rights reserved.

Four new Cu(II) complexes of 4,4′-diaminodiphenyl sulfone (DDS) with different anions (chloride, sulfate, nitrate or acetate) were synthesized and characterized by elemental analysis, electronic absorption, IR, magnetic moment, thermal analysis and powder X-ray diffraction studies. It was found that the pure DDS and complex [Cu2(DDS)2(NO3)2].(NO3)2 (3) crystallize in orthorhombic system while the complexes [Cu2(DDS)2].Cl4 (1), [Cu2(DDS)2].(SO4)2 (2), and [Cu2(DDS)2].(CH3COO)4 (4) crystallize in monoclinic system. The crystallite sizes of complexes have smaller values as compared to pure DDS. Infrared studies suggest that the coordination of NH2 of DDS with Cu(II) ion. The binding of the complexes with calf thymus (CT) DNA was investigated by electronic absorption titration, fluorescence measurements and DNA thermal denaturation. The spectroscopic studies together with the DNA melting studies indicated that the complexes may bind to CT-DNA in a non-intercalative mode.

Recent spectroscopic investigations of various amino acids report intriguing high-pressure and low-temperature behavior of NH3+ groups and their influence on various hydrogen bonds in the system. In particular, the variation of the intensity of NH3+ torsional mode at different temperatures and pressures has received much attention. We report here the first in situ Raman investigations of fully deuterated α-glycine up to ∼20 GPa. The discontinuous changes in COO− and ND3+ modes across ∼3 GPa indicate subtle structural rearrangements in fully deuterated α-glycine. The decrease in the intensity of ND3+ torsional mode is found to be similar to that of undeuterated α-glycine. The pressure-induced stiffening of ND and CD2 stretching modes are discussed in the context of changes in the hydrogen-bonding interactions. Copyright © 2011 John Wiley & Sons, Ltd.

2-Aminopyridinium maleate (2APM) is a recently discovered organic acid-base complex that crystallizes in a non-centrosymmetric space group Pc and hence has a nonzero second-order optical susceptibility. In 2APM the cation (aminopyridinium ion) is connected to the anion (maleate ion) through hydrogen bonds (N1-H10–O2 and N2-H12–O1), we have calculated the molecular properties of 2APM for varying strength of these hydrogen bonds. It is observed that the molecular hyperpolarizabilty varies for varying strength of these hydrogen bonds. These observations are interpreted in terms of the change in nature of the hydrogen bond interaction as its strength is varied. Macropscopic second-order optical susceptibility (d 0) of 2APM is obtained using the calculated value of molecular hyperpolarizability and the reported crystal structure of 2APM. In order to study the influence of molecular packing on the macroscopic non-linear optical property of the crystal, susceptibility calculations are repeated for 2-methyl-4nitr...

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.

An accurate X-ray diffraction study of a La3Ta0.25Zr0.50Ga5.25O14 single crystal (a = 8.2574(4) A, c = 5.1465(4) A, sp. gr. P321, Z = 1, R/Rw = 0.62/0.57% for 4144 unique reflections and 91 parameters) has been performed with a simultaneous neutron diffraction analysis. Tantalum, zirconium, and gallium atoms are found to occupy the mixed octahedral position (symmetry 32). Gallium atoms and a few zirconium atoms are in the position on axis 2 in the tetrahedron. The tetrahedral position on axis 3 is completely occupied by gallium atoms, while the large polyhedron on axis 2 is occupied by lanthanum atoms. The high resolution and averaging of the results obtained in two independent X-ray experiments with the same sample provided accu- rate structural data, in particular, on the anharmonicity of thermal atomic vibrations (atomic displacements). The X-ray and neutron diffraction data on the atomic displacements are compared.

The structural investigation of the ferroelectric phase transition in glycine silver nitrate has revealed that the transition at T c = 218 K is due to the displacement of the Ag+ ions from the plane made by the carboxyl oxygens of glycine zwitterions coordinated to it. Since the transition takes place between two ordered structures the thermal anomaly at T c is very weak, the transition enthalpy and transition entropy were found to be Δ H = 6.6 J / mol and the transition entropy Δ S = 0.03 J K − 1 mol − 1 respectively. These crystals are held together by a network of hydrogen bonds. In order to study these interactions the Raman spectrum of GSN was recorded and discussed in the light of ferroelectricity in glycine complexes in general.

Single-crystal neutron diffraction investigation of bis(glycinium) oxalate was undertaken in order to study its hydrogen-bonding network, particularly the very short hydrogen bond between the glycinum and oxalate ions, indicated by the X-ray diffraction study. The non-existence of any phase transition in these crystals was attributed to the fact that the short hydrogen bond in bis(glycinium) oxalate is asymmetric in nature, with no hydrogen disorder. The potential energy landscape for the above-mentioned H atom was found to have a single minimum closer to the glycinium ion. IR and Raman investigations of the title complex supported the above result.

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 Raman spectrum of the crystalline complex thiourea:diethyloxalate (2:1) is reported. The Raman wavenumbers in the complex are compared with those of crystalline thiourea (TU) and diethyloxalate (DEO), and the observed differences are understood from the point of view of the hydrogen bonding in the system. From the wavenumbers of the NH stretching vibrations, it is inferred that the strengths of hydrogen bonds in the complex are smaller compared to those in TU. The evolution of the Raman spectrum at high temperature reveals that the NH··· O hydrogen bonds, which are a key to the formation of the complex, are broken above 397 K. The spectrum corresponding to the crystalline complex reappears below 375 K when cooled from temperatures below melting. Copyright © 2006 John Wiley & Sons, Ltd.

The crystal structure of thiourea:diethyl oxalate in 2:1 ratio is the first of its kind where the diethyl oxalate exists as a solid which otherwise is a liquid at room temperature. It crystallizes in triclinic centrosymmetric space group P-1 with the following unit cell dimensions a = 7.1870(7) A, b = 7.4890(8) A, c = 8.3637(7) A, α = 63.783(7)∘, β = 67.41(1)∘, γ = 64.933(7)∘. The R-factor = 0.0386 for 1850 Fo > 4σ (Fo) and 0.0417 for all 2020 data. There is a center of inversion at the center of the C–C bond of diethyl oxalate. This system is stabilized by N–H ⋅s S and N–H ⋅s O hydrogen bonds.

Metal-semiconductor multilayers are interesting, artificial structures as prospective candidates for spin injection devices. A Fe–Ge multilayer sample with very thin individual layers (few crystallographic planes) has been deposited by sputtering on Si[1 0 0] substrate. We have characterized the structure of this multilayer sample using X-ray diffraction, X-ray reflectometry and neutron reflectometry. The magnetic moment density in the ferromagnetic Fe layer has been obtained by polarized neutron reflectometry and the bulk magnetic behavior of the thin film by SQUID magnetometer measurements. We found that the film is a soft ferromagnet at room temperature with a substantially reduced magnetic moment of the Fe atoms.

The basic result of carboxylic group that the oxygen atom of the -OH never seems to be a hydrogen bond acceptor is violated in the cases, namely urea oxalic acid and bis urea oxalic acid complexes, where the hydroxyl oxygen atom is an acceptor of a weak N—H… O hydrogen bond. The parameters of this hydrogen bond, respectively in these structures are: hydrogen acceptor distance 2.110 A and 2.127 A and the bending angle at hydrogen, 165.6° and 165.8°. The bond strength around the hydroxyl oxygen is close to 1.91 valence units, indicating that it has hardly any strength left to form hydrogen bonds. These two structures being highly planar, force the formation of this hydrogen bond. As oxalic acid is the common moiety, the structures of the two polymorphs, α-oxalic acid and β-oxalic acid, also were looked into in terms of hydrogen bonding and packing.

The crystal structure of ZTS has been determined by neutron diffraction with a finalR-value of 0.026. Using the structural parameters, the contributions from the structural groups to the linear optical susceptibility and linear electro-optic coefficients have been evaluated. Results showed a significant contribution from the hydrogen bonds in the structure.

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.

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.

X-ray powder diffraction and Raman measurements were performed on the mixed crystals of deuterated potassium dihydrogen phosphate (DKDP) and deuterated ammonium dihydrogen phosphate (DADP) grown at our lab. These crystals are known to behave like deuteron glasses due to frustration between ferroelectric and antiferroelectric ordering. Both spectral as well as structural studies indicate that crystals belonging to the glassy regions of the crystal composition have stronger O-D-O hydrogen bonds as compared to those belong to the ferroelectric or antiferroelectric regions of the crystal composition.

Zinc sulphide quantum dots chemically capped with thioglycerol having two different sizes have been synthesized. The particles have a disordered sphalerite structure and are slightly contracted by 1% against the bulk. Small angle X-ray scattering investigations reveal that powders of these nanocrystallites are mass fractals that aggregate via a reaction-limited process to form irregular but rather dense networks with a fractal dimensionality of Df = 2.7 and 2.1, respectively.

The experimental electron density of ammonium dihydrogen phosphate (ADP) crystal in the paraelectric phase (155 K) as well as antiferroelectric phase (100 K) is obtained from its high resolution X-ray diffraction data using the maximum entropy method. Marked redistribution of electron density has been observed in ADP crystals as the crystal temperature is lowered below the phase transition temperature Tc = 148 K. The nature of very strong O–H–O hydrogen bonds between phosphate anions changes from an ideal covalent interaction to a polar covalent interaction as the temperature is altered from 155 to 100 K. The influence of intermolecular interaction like the dipolar interaction on the electron density particularly in the intermolecular region is clearly visible in the electron density maps. One of the most striking features of the electron density of ADP is the presence of non nuclear maxima (NNM) within the “ab” planes. It is argued that the appearance of these NNMs is a normal consequence of the chemical bonding between homonuclear groups in ADP. The manuscript describes the experimental electron density of ammonium dihydrogen phosphate (ADP) obtained from its high resolution X-ray diffraction data recorded at two crystal temperatures namely 100 and 155 K.

A series of lanthanide oxides was incorporated in a vitreous phosphate host network. Molar constituents of the glasses were typically (La2O3)10(RxOy)10(Al2O3)5(P2O5)75. Each glass had a different lanthanide (R atom) from the series; La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er and the values of x and y depended on the valency of the rare-earth atom. Both X-ray and neutron diffraction were employed in examining their structures. The results indicate that the basic PO4 tetrahedral unit remains unaltered with an average P–O distance of 1.54±0.02 A and predominant Q2 linkages to its neighbouring units so as to form a continuous network while accommodating the included lanthanides. In accordance with this model, the average distance of rare-earth (comprising La and a second type of R atom) to oxygen decreased from 2.44 to 2.26 A, a trend to be expected from the lanthanide contraction. The average oxygen coordination around the rare-earth was found to vary in the range of 6–8. With these average parameters, a small (74 atom) hand-built model was made to check the feasibility of constructing a continuous random network. Optical transmission measurements show all these glasses to absorb strongly in the UV region and to have marked absorption resonances in the visible region of 400–1000 nm except for the La, Ce, Eu, Tb containing glasses which have low or negligible absorption in the latter range.

Crystallization of the completely deuterated glycine silver nitrate (DGSN) from D2O gave a new polymorph with the crystal structure significantly differing from that of the nondeuterated form (GSN): space group P21/c with polymeric two-dimensional layers parallel to the ab plane. In DGSN, the silver ions are mono-nuclear, whereas in GSN Ag–Ag dimers are present. In contrast to GSN, DGSN does not undergo any phase transitions on cooling at least till 100 K.

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.

Crystals of arachidic acid (CH3(CH2)18COOH) were obtained by slow evaporation of as-obtained arachidic acid (AA) dissolved in each of the solvents: chloroform, benzene, cyclohexane and ethanol respectively. They were characterised and compared with as-obtained AA using differential scanning calorimetry (DSC), micro-Raman spectroscopy and X-ray diffraction. A pre-transition observed in DSC suggested the presence of a polymorphic phase probably due to conformational changes commonly observed in such long chain fatty acids. This was supported by reduced d-spacing values acquired from diffraction data of crystallised AA indicating tilted molecular packing.

Quenching of dynamic disorder in glassy systems is termed as the glass transition. Ferroic glasses belong to the class of paracrystalline materials having crystallographic order in-between that of a perfect crystal and amorphous material, a classic example of ferroic glass is the solid solution of ferroelectric deuterated potassium dihydrogen phosphate and antiferroelectric deuterated ammonium dihydrogen phosphate. Lowering temperature of this ferroic glass can lead to a glass transition to a quenched disordered state. The subtle atomic rearrangement that takes place at such a glass transition can be revealed by careful examination of the temperature induced changes occurring in the x-ray powder diffraction (XRD) patterns of these materials. Hence we report here results of a complete diffraction line shape analysis of the XRD patterns recorded at different temperatures from deuterated mixed crystals DK x A1−x DP with mixing concentration x ranging as 0 < x < 1. Changes observed in diffraction peak shapes have been explained on the basis of structural rearrangements induced by changing O–D–O hydrogen bond dynamics in these paracrystals.

A new complex of 2-aminopyridine and malonic acid in a 1:1 stoichiometric ratio is obtained. The complex crystallizes in a centrosymmetric P21/c space group with a = 3.8522(2)A, b = 17.5226(8)A, c = 13.7317(7)A and β = 94.418(3)°. This complex is held by network of hydrogen bonds between the protonated 2-aminopyridinium cation and the malonate anion. There is no phase transition observed in this complex, which is verified using DSC and single crystal X-ray measurement studies. Unlike 2-aminopyridinium maleate, this complex crystallizes in the centrosymmetric space group and hence, this complex will not exhibit any non-linear optical property.

Single-crystal neutron-diffraction studies were conducted on alanine-doped triglycine sulphate (TGS) crystals to understand how alanine substitution results in prevention of depoling in TGS. The results of refinement indicated that the most likely site of L-alanine substitution in TGS is the zwitterionic site G2.

Disodium uridine 5′-monophosphate heptahydrate (5′-UMPNa2), Na2[C9H11N2O9P].7H2O, crystallises in space group C2221 with a=22.985, b=8.911 and c=19.494 A. A neutron beam of λ=1.216 A was used; Z=8 and V=3992.75 A3. Data consisted of 1785 unique reflections. Na ions were connected to the main molecule through water molecules and sugar oxygens. One of the Na ions occupied a special position, and the other at a general position was partially disordered. The uracil base was planar, and had anti conformation about the glycosidic bond. The sugar had C(2′) endo conformation and was gauche–gauche.

Four new Cu(II) complexes of 4,4□‐diaminodiphenyl sulfone(DDS) with different anions(chloride, sulphate or acetate) were synthesized and characterized by X‐ray diffraction studies. It is observed that all the complexes crystallize in monoclinic system unlike the pure DDS which crystallizes in orthorhombic system. Complexes of DDS with CuSO4 in 1:1 and 2:1 stoichiometric ratio crystallize in monoclinic system with much larger volume compared to other DDS complexes.

Arachidic acid (AA), commonly used in building multi dimensional lamellar structures through Langmuir‐Blodgett technique, has been recrystallized from different solvents and studied for structural variations. To the best of our knowledge there is no structural characterization of bulk AA documented in the literature. Hence the characterization study undertaken in the present work, is the first of its kind and will provide useful insight into the crystal structure of AA. Future studies can then relate this information to the analysis of structural formation of Langmuir‐Blodgett films. On comparison of powder x‐ray diffraction patterns with the as‐obtained powder, it appeared that the synthesized crystals form weakly bound inclusion complexes with the respective solvents.

Crystals of the langasite family are of interest as they are piezoelectric in different devices. The properties of these classes of crystals can be modified within certain limits by isomorphous substitution. Single-crystal neutron diffraction studies were carried out for LGT (La3Ga5.5Ta0.5O14), LGST (La3Ga5.25Ta0.25Si0.5O14) and LGZrT (La3Ga5.25Ta0.25Zr0.5O14) as the neutron study gives a better average picture of the crystal properties over a macroscopic region of the grown crystal. The effect of small substitutions at various sites on the piezoelectric properties of the crystal was studied.

Charge-density analysis of hydrogen-bonded complexes of glycine is undertaken in order to elucidate the chemical nature of very strong hydrogen bonds present in these complexes. We have chosen two glycine complexes triglycine sulphate (TGS) and Bis (glycinium) oxalate (DGO) which are known to exhibit very strong hydrogen bonding. Electron density maps demonstrate the covalent nature of very strong O–H...O bond found in DGO and TGS complexes. Strong hydrogen bonds in TGS have significant covalent character. This charge-density analysis confirmed the existence of the bifurcated acceptor hydrogen bond which is responsible for the coupling between the two components of structural disorder found at high temperatures (T > 49 °C) in TGS crystals, namely disorder of amino group of one of the glycine ions (GI) and the hydrogen atom disorder between the carboxylic group of other two glycines GII and GIII. Recent charge density studies have demonstrated that hydrogen bonds are more convincingly reproduced by the model independent maximum entropy method (MEM) than by multipole models, hence we have used MEM procedure in our charge-density analysis.

A new complex of 2-aminopyridine and maleate in 1:1 stoichiometric ratio was obtained. The complex crystallizes in non-centrosymmetric space group Pc. This complex is held by network of hydrogen bonding between the protonated 2-aminopyridinium cation and maleate anion and also by pi–pi stacking. There is no phase transition observed in this complex, which has been verified using DSC and single crystal X-ray measurement studies. A comparison between 2-aminopyridinium maleate and bis(2-aminopyridinium) maleate structure shows that the positive charge resides on the ring nitrogen in the present complex unlike the case in bis(2-aminopyridinium) maleate where the charge is on the amino nitrogen. This complex has higher meting point compared to that of bis(2-aminopyridinium) maleate which is due to the difference in hydrogen bonding pattern between the two complexes.

The title complex, 2CH4N2S.C4H6O4, is a host-guest system. The asymmetric unit consists of one complete thiourea molecule and one-half of a dimethyl oxalate molecule lying on an inversion centre. The host thiourea molecules are connected to form zigzag chains by N-H...S hydrogen bonds. The guest dimethyl oxalate molecules provide O-atom acceptors for N-H...O hydrogen bonds, thus interconnecting the chains of thiourea molecules to form completely connected sheets. The reduction in temperature from 300 to 100 K leaves the structure unchanged and still isostructural with that previously determined for the analogous thiourea-diethyl oxalate (2/1) complex. It does, however, induce closer packing of the molecules, general shrinkage of the unit cell and shortening of the hydrogen bonds, these last two to the extent of 1-2%.

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.

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.

Crystal structures of triglycine selenate (TGSe) and triglycine sulfate (TGS) obtained from single crystal neutron diffraction are compared. The double well single cell local potential experienced by the non-planar amino group of one of the three glycine ions (GI) of these two isostructural crystals is obtained using their crystal structure. It is suggested that the change in the nature of the ferroelectric phase transition as one goes from TGS to TGSe is due to the increase in the zero point energy resulting due to the change in the shape and height of the double well local potential of these crystals. Substitution of a selenate ion (SeO(4)(2-)) in TGSe by a sulfate ion (SO(4)(2-)) is considered as a source of an effective chemical pressure that can be utilized to tune the ferroelectric phase boundary in these crystals. The influence of alanine substitution on the ferroelectric phase transition in these crystals is investigated using differential scanning calorimetry.

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.