High-temperature superconductivity on the verge of a structural instability in lanthanum superhydride.

The possibility of high, room-temperature superconductivity was predicted for metallic hydrogen in the 1960s. However, metallization and superconductivity of hydrogen are yet to be unambiguously demonstrated and may require pressures as high as 5 million atmospheres. Rare earth based "superhydrides", such as LaH10, can be considered as a close approximation of metallic hydrogen even though they form at moderately lower pressures. In superhydrides the predominance of H-H metallic bonds and high superconducting transition temperatures bear the hallmarks of metallic hydrogen. Still, experimental studies revealing the key factors controlling their superconductivity are scarce. Here, we report the pressure and magnetic field dependence of the superconducting order observed in LaH10. We determine that the high-symmetry high-temperature superconducting Fm-3m phase of LaH10 can be stabilized at substantially lower pressures than previously thought. We find a remarkable correlation between superconductivity and a structural instability indicating that lattice vibrations, responsible for the monoclinic structural distortions in LaH10, strongly affect the superconducting coupling.

Superconductivity up to 243 K in the yttrium-hydrogen system under high pressure.

The discovery of superconducting H3S with a critical temperature Tc∼200 K opened a door to room temperature superconductivity and stimulated further extensive studies of hydrogen-rich compounds stabilized by high pressure. Here, we report a comprehensive study of the yttrium-hydrogen system with the highest predicted Tcs among binary compounds and discuss the contradictions between different theoretical calculations and experimental data. We synthesized yttrium hydrides with the compositions of YH3, YH4, YH6 and YH9 in a diamond anvil cell and studied their crystal structures, electrical and magnetic transport properties, and isotopic effects. We found superconductivity in the Im-3m YH6 and P63/mmc YH9 phases with maximal Tcs of ∼220 K at 183 GPa and ∼243 K at 201 GPa, respectively. Fm-3m YH10 with the highest predicted Tc > 300 K was not observed in our experiments, and instead, YH9 was found to be the hydrogen-richest yttrium hydride in the studied pressure and temperature range up to record 410 GPa and 2250 K.

A Boosted Critical Temperature of 166†K in Superconducting D3 S Synthesized from Elemental Sulfur and Hydrogen.

The discovery of superconductivity in H3 S at 203 K marked an advance towards room-temperature superconductivity and demonstrated the potential of H-dominated compounds to possess a high critical temperature (Tc ). There have been numerous reports of the H-S system over the last five years, but important questions remain unanswered. It is crucial to verify whether the Tc was determined correctly for samples prepared from compressed H2 S, since they are inevitably contaminated with H-depleted byproducts. Here, we prepare stoichiometric H3 S by direct in situ synthesis from elemental S and excess H2 . The Im 3 ‾ m phase of D3 S samples exhibits a Tc significantly higher than previously reported values (ca. 150 K), reaching a maximum Tc of 166 K at 157 GPa. Furthermore, we confirm that the sharp decrease in Tc below 150 GPa is accompanied by continuous rhombohedral structural distortions and demonstrate that the Cccm phase is non-metallic, with molecular H2 units in the crystal structure.

A novel crystal form of metacetamol: the first example of a hydrated form.

We report the crystal structure and crystallization conditions of a first hydrated form of metacetamol (a hemihydrate), C8H9NO2·0.5H2O. It crystallizes from metacetamol-saturated 1:1 (v/v) water-ethanol solutions in a monoclinic structure (space group P21/n) and contains eight metacetamol and four water molecules per unit cell. The conformations of the molecules are the same as in polymorph II of metacetamol, which ensures the formation of hydrogen-bonded dimers and R22(16) ring motifs in its crystal structure similar to those in polymorph II. Unlike in form II, however, these dimers in the hemihydrate are connected through water molecules into infinite hydrogen-bonded molecular chains. Different chains are linked to each other by metacetamol-water and metacetamol-metacetamol hydrogen bonds, the latter type being also present in polymorph I. The overall noncovalent network of the hemihydrate is well developed and several types of hydrogen bonds are responsible for its formation.

Superconducting phase diagram of H3S under high magnetic fields.

The discovery of superconductivity at 260 K in hydrogen-rich compounds like LaH10 re-invigorated the quest for room temperature superconductivity. Here, we report the temperature dependence of the upper critical fields µ0Hc2(T) of superconducting H3S under a record-high combination of applied pressures up to 160 GPa and fields up to 65 T. We find that Hc2(T) displays a linear dependence on temperature over an extended range as found in multigap or in strongly-coupled superconductors, thus deviating from conventional Werthamer, Helfand, and Hohenberg (WHH) formalism. The best fit of Hc2(T) to the WHH formalism yields negligible values for the Maki parameter α and the spin-orbit scattering constant λSO. However, Hc2(T) is well-described by a model based on strong coupling superconductivity with a coupling constant λ ~ 2. We conclude that H3S behaves as a strong-coupled orbital-limited superconductor over the entire range of temperatures and fields used for our measurements.

A new solvate of furosemide with dimethylacetamide.

The loop diuretic furosemide is used widely in the treatment of congestive heart failure and edema, and is practically insoluble in water. The physicochemical and pharmacokinetic properties of drugs can be modified by preparing the drug in an appropriate solid-state form. A new solvate of furosemide with dimethylacetamide (DMA) {systematic name: 4-chloro-2-[(furan-2-yl)methylamino]-5-sulfamoylbenzoic acid N,N-dimethylacetamide disolvate}, C12H11ClN2O5S·2C4H9NO, (I), is reported. The channeled structure formed on slow crystallization contains DMA solvent molecules in its channels. This structure adds to the evidence of varied conformations observed across all known structures, so supporting the idea that this flexible molecule has conformational lability. The current structure also differs from those of other previously known furosemide solvates in the number of solvent molecules per furosemide molecule, viz. 2:1 instead of 1:1. Desolvation of (I) gives the most stable form of furosemide, i.e. Form I.

Isoenergetic Polymorphism: The Puzzle of Tolazamide as a Case Study.

In the present case study of tolazamide we illustrate how many seemingly contradictory results that have been obtained from experimental observations and theoretical calculations can finally start forming a consistent picture: a "puzzle put together". For many years, tolazamide was considered to have no polymorphs. This made this drug substance unique among the large family of sulfonylureas, which was known to be significantly more prone to polymorphism than many other organic compounds. The present work employs a broad and in-depth analysis that includes the use of optical microscopy, single-crystal and powder X-ray diffraction, IR and Raman spectroscopies, DSC, semiempirical PIXEL calculations and DFT of three polymorphs of tolazamide. This case study shows how the polymorphs of a molecular crystal can be overlooked even if discovered serendipitously on one of numerous crystallizations, and how very different molecular packings can be practically isoenergetic but still crystallize quite selectively and transform one into another irreversibly upon heating.

High-pressure crystallography of periodic and aperiodic crystals.

More than five decades have passed since the first single-crystal X-ray diffraction experiments at high pressure were performed. These studies were applied historically to geochemical processes occurring in the Earth and other planets, but high-pressure crystallography has spread across different fields of science including chemistry, physics, biology, materials science and pharmacy. With each passing year, high-pressure studies have become more precise and comprehensive because of the development of instrumentation and software, and the systems investigated have also become more complicated. Starting with crystals of simple minerals and inorganic compounds, the interests of researchers have shifted to complicated metal-organic frameworks, aperiodic crystals and quasicrystals, molecular crystals, and even proteins and viruses. Inspired by contributions to the microsymposium 'High-Pressure Crystallography of Periodic and Aperiodic Crystals' presented at the 23rd IUCr Congress and General Assembly, the authors have tried to summarize certain recent results of single-crystal studies of molecular and aperiodic structures under high pressure. While the selected contributions do not cover the whole spectrum of high-pressure research, they demonstrate the broad diversity of novel and fascinating results and may awaken the reader's interest in this topic.

Contribution of weak S-H · · · O hydrogen bonds to the side chain motions in D,L-homocysteine on cooling.

Sulfhydryl groups play an important role in the formation of native structures of proteins and their biological functions. In the present work, we report for the first time the crystal structure of D,L-homocysteine and the results of a detailed study of the dynamics of its sulfhydryl group on cooling by precise single-crystal X-ray diffraction combined with polarized Raman spectroscopy of oriented single crystals. Although the crystal structures of both D,L-cysteine and D,L-homocysteine are layered, hydrogen bonds formed by -SH groups differ. In contrast with the crystal structure of D,L-cysteine with weak S-H · · · S hydrogen bonds between layers, D,L-homocysteine resembles the structures of amino acids with hydrophobic aliphatic side chains with no hydrogen bonds between the layers. The side chain of D,L-homocysteine forms a three-centered S-H · · · O hydrogen bond with carboxylate groups of two neighboring zwitterions. On cooling down, despite the shortening of the two S · · · O distances in the bifurcated S-H · · · O hydrogen bond, the wavenumber of the stretching vibrations of -SH groups increases. The same effect was also observed previously for other sulfhydryl containing amino acids, L-cysteine, and N-acetyl-L-cysteine on increasing pressure and is related to the strengthening of a three-centered bifurcated S-H · · · O hydrogen bond.

Effect of pressure on methylated glycine derivatives: relative roles of hydrogen bonds and steric repulsion of methyl groups.

Infinite head-to-tail chains of zwitterions present in the crystals of all amino acids are known to be preserved even after structural phase transitions. In order to understand the role of the N-H...O hydrogen bonds linking zwitterions in these chains in structural rearrangements, the crystal structures of the N-methyl derivatives of glycine (N-methylglycine, or sarcosine, with two donors for hydrogen bonding; two polymorphs of N,N-dimethylglycine, DMG-I and DMG-II, with one donor for hydrogen bond; and N,N,N-trimethylglycine, or betaine, with no hydrogen bonds) were studied at different pressures. Methylation has not only excluded the formation of selected hydrogen bonds, but also introduced bulky mobile fragments into the structure. The effects of pressure on the systems of the series were compared with respect to distorting and switching over hydrogen bonds and inducing reorientation of the methylated fragments. Phase transitions with fragmentation of the single crystals into fine powder were observed for partially methylated N-methyl- and N,N-dimethylglycine, whereas the structural changes in betaine were continuous with some peculiar features in the 1.4-2.9 GPa pressure range and accompanied by splitting of the crystals into several large fragments. Structural rearrangements in sarcosine and betaine were strongly dependent on the rate of pressure variation: the higher the rate of increasing pressure, the lower the pressure at which the phase transition occurred.

Weak hydrogen bonds formed by thiol groups in N-acetyl-(L)-cysteine and their response to the crystal structure distortion on increasing pressure.

The effect of hydrostatic pressure on single crystals of N-acetyl-l-cysteine was followed at multiple pressure points from 10(-4) to 6.2 GPa with a pressure step of 0.2-0.3 GPa by Raman spectroscopy and X-ray diffraction. Since in the crystals of N-acetyl-l-cysteine the thiol group is involved in intermolecular hydrogen bonds not as a donor only (bonds S-H···O) but also as an acceptor (bonds N-H···S), increasing the pressure does not result in phase transitions. This makes a contrast with the polymorphs of l- and dl-cysteine, in which multiple phase transitions are observed already at relatively low hydrostatic pressures and are related to the changes in the conformation of the thiol side chains only weakly bound to the neighboring molecules in the structure and thus easily switching over the weak S-H···O and S-H···S hydrogen bonds. No phase transitions occur in N-acetyl-l-cysteine with increasing pressure, and changes in cell parameters and volume vs pressure do not reveal any peculiar features. Nevertheless, a more detailed analysis of the changes in intermolecular distances, in particular, of the geometric parameters of the hydrogen bonds based on X-ray single crystal diffraction analysis, complemented by an equally detailed study of the positions of all the significant bands in Raman spectra, allowed us to study the fine details of subtle changes in the hydrogen bond network. Thus, as pressure increases, a continuous shift of the hydrogen atom of the thiol group from one acceptor (a carboxyl group) to another acceptor (a carbonyl group) is observed. Precise single-crystal X-ray diffraction and polarized Raman spectroscopy structural data reveal the formation of a bifurcated S-H···O hydrogen bond with increasing pressure starting with ∼1.5 GPa. The analysis of the vibrational bands in Raman spectra has shown that different donor and acceptor groups start "feeling" the formation of the bifurcated S-H···O hydrogen bond in different pressure ranges. The results are discussed in relation to some of the previously published data on the effect of high pressure on the polymorphs of l-cysteine, dl-cysteine, and glutathione, that show similarity with the effects reported here for N-acetyl-l-cysteine. The results obtained in this work allow one to suggest new models for the pressure-induced structural rearrangements in the whole family of cysteine-containing crystals.

Dynamics and thermodynamics of crystalline polymorphs. 2. ß-Glycine, analysis of variable-temperature atomic displacement parameters.

The molecular dynamics in the crystal and the thermodynamic functions of the ß-polymorph of glycine have been determined from a combination of molecular translation-libration frequencies reflecting the temperature dependence of atomic displacement parameters (ADPs), with frequencies derived from ONIOM(DFT:PM3) calculations on a 15-molecule ß-glycine cluster. ADPs have been obtained from variable-temperature diffraction data to 0.5 Å resolution collected with X-ray synchrotron (10-300 K) and sealed tube radiation (50-298 K). At the higher temperatures, the ADPs of ß-glycine from synchrotron are larger than those from sealed tube probably due to different experimental conditions. The lattice vibration frequencies from normal-mode analysis of ADPs and the internal vibration frequencies from ONIOM(B3LYP/6-311+G(2d,p):PM3) calculations agree with those from spectroscopy. Estimation of thermodynamic functions using the vibrational frequencies, the Einstein and Debye models of heat capacity, and the room-temperature compressibility provides C(p), H(vib), and S(vib) that agree with those from calorimetry. The ß-phase with higher H and G is found to be less stable than the α-phase in the temperature range of the experiment.

Betaine 0.77-perhydrate 0.23-hydrate and common structural motifs in crystals of amino acid perhydrates.

The title compound, betaine 0.77-perhydrate 0.23-hydrate, (CH3)3N(+)CH2COO(-)·0.77H2O2·0.23H2O, crystallizes in the orthorhombic noncentrosymmetric space group Pca2(1). Chiral molecules of hydrogen peroxide are positionally disordered with water molecules in a ratio of 0.77:0.23. Betaine, 2-(trimethylazaniumyl)acetate, preserves its zwitterionic state, with a positively charged ammonium group and a negatively charged carboxylate group. The molecular conformation of betaine here differs from the conformations of both anhydrous betaine and its hydrate, mainly in the orientation of the carboxylate group with respect to the C-C-N skeleton. Hydrogen peroxide is linked via two hydrogen bonds to carboxylate groups, forming infinite chains along the crystallographic a axis, which are very similar to those in the crystal structure of betaine hydrate. The present work contributes to the understanding of the structure-forming factors for amino acid perhydrates, which are presently attracting much attention. A correlation is suggested between the ratio of amino acid zwitterions and hydrogen peroxide in the unit cell and the structural motifs present in the crystal structures of all currently known amino acids perhydrates. This can help to classify the crystal structures of amino acid perhydrates and to design new crystal structures.

The effect of partial methylation of the glycine amino group on crystal structure in N,N-dimethylglycine and its hemihydrate.

N,N-Dimethylglycine, C(4)H(9)NO(2), and its hemihydrate, C(4)H(9)NO(2)·0.5H(2)O, are discussed in order to follow the effect of the methylation of the glycine amino group (and thus its ability to form several hydrogen bonds) on crystal structure, in particular on the possibility of the formation of hydrogen-bonded `head-to-tail' chains, which are typical for the crystal structures of amino acids and essential for considering amino acid crystals as mimics of peptide chains. Both compounds crystallize in centrosymmetric space groups (Pbca and C2/c, respectively) and have two N,N-dimethylglycine zwitterions in the asymmetric unit. In the anhydrous compound, there are no head-to-tail chains but the zwitterions form R(4)(4)(20) ring motifs, which are not bonded to each other by any hydrogen bonds. In contrast, in the crystal structure of N,N-dimethylglycinium hemihydrate, the zwitterions are linked to each other by N-H···O hydrogen bonds into infinite C(2)(2)(10) head-to-tail chains, while the water molecules outside the chains provide additional hydrogen bonds to the carboxylate groups.

L-cysteinium semioxalate: a new monoclinic polymorph or a hydrate?

The title compound, C(3)H(8)NO(2)S(+)·C(2)HO(4)(-), (I), crystallizes in the monoclinic C2 space group and is a new form (possibly a hydrate) of L-cysteinium semioxalate with a stoichiometric cation-anion ratio of 1:1. In contrast to the previously known orthorhombic form of L-cysteinium semioxalate, (I) has a layered structure resembling those of monoclinic L-cysteine, as well as of DL-cysteine and its oxalates. The conformations of the cysteinium cation and the oxalate anion in (I) differ substantially from those in the orthorhombic form. The structure of (I) has voids with a size sufficient to incorporate water molecules. The residual density, however, suggests that if water is in fact present in the voids, it is strongly disordered and its amount does not exceed 0.3 molecules per void. The difference in conformation of the cysteinium cations in (I) and in the orthorhombic form is similar to that in DL-cysteine under ambient conditions and in DL-cysteine under high pressure or at low temperature.

Observation of subtle dynamic transitions by a combination of neutron scattering, X-ray diffraction and DSC: a case study of the monoclinic L-cysteine.

The paper illustrates the benefit of combining several experimental techniques (incoherent elastic and inelastic neutron scattering, DSC, and X-ray diffraction) to study subtle dynamic transitions in a biologically important system, probing a broad time (frequency) range of the molecular motions in a wide temperature interval of 2-300K. As a case study the crystalline form (a monoclinic polymorph) of l-cysteine ((+)NH(3)-CH(CH(2)SH)-COO(-)) - an essential amino acid - has been selected. Crystals of amino acids are widely used to mimic important structural and dynamic features of peptides. The conformational lability of cysteine and the dynamics of the thiol-side chains are known to account for various phase transitions in the crystalline state and for the conformational transitions important for the biological function in the peptides. The effect of temperature on the monoclinic polymorph of l-cysteine, metastable at ambient conditions, has been studied for the first time. A dynamical transition at about 150K, marking a crossover of the molecular fluctuations between harmonic and non-harmonic dynamical regimes, was evidenced by evaluating the evolution of the mean-square displacement obtained from the elastic fixed window approach using the backscattering spectrometer IN10 located at the ILL. Although this transition does not manifest itself in the DSC, it was clearly observed by incoherent inelastic neutron scattering. By analyzing the dynamical susceptibility contribution (chi''(omega)) obtained using IN6 also at ILL we were able to evidence another relaxation process at a different time scale. The disordered soft l-cysteine structure has an excess of inelastic scattering at about 3meV, analogous to the "boson peak" observed in glass-like materials and proteins. High-precision X-ray diffraction has revealed an anomaly in the changes of selected unit cell parameters and volume at about 240K.

DL-cysteinium semioxalate.

Two chiral counterparts (L- and D-cysteinium cations related by an inversion centre) are present in the structure of the title compound, C(3)H(8)NO(2)S(+).C(2)HO(4)(-), with a 1:1 cation-anion ratio. The carboxy group of the cysteinium cation is protonated in the trans position relative to the amino group. The crystal structure is built up of double layers, in which dimers of cysteinium cations are connected to each other not directly, but via bridges of twisted semioxalate anions linked to each other via O-H...O hydrogen bonds forming infinite chains. An interesting feature of the crystal structure is the absence of either S-H...S or S-H...O hydrogen bonds.